Initial Post: April 22, 2021
Topics on “Lithium Ion Batteries” fill textbooks, but I know there is great interest in the boating community. This article is intended as a “brief” survey of the possibilities and challenges associated with Lithium chemistry batteries. My focus throughout is on “electrical system reliability.” The length of this article grew very quickly, so I’ve limited my content to areas that often lack adequate consideration. “How do I make this simple for readers to understand?” That is my primary goal and has been a real struggle. To limit length, I’ve had to assume that readers are already somewhat familiar with the advantages, operating characteristics and risks associated with lithium chemistry batteries. I think that describes boaters actually contemplating adoption of lithium batteries.
A reliable electrical system is essential to crew and vessel safety on any boat. Lithium chemistry battery articles typically focus on lithium technology and its benefits. Such articles rarely discuss the implications to the overall reliability of the vessel’s electrical system. Modern boats are often in immediate peril (physical risk, operator anxiety, inconvenience, lost time, and cost) if any part of the vessel’s electrical system incurs a fault or outage. This article considers the possible – not “inevitable,” perhaps “infrequent,” but certainly “possible” – reliability impacts of lithium chemistry batteries to the “Electrical System” of boats involved in cruising.
Lithium chemistry battery systems involve more technical complexity than their lead-acid cousins. Lithium chemistry requires a “control system” to keep the battery’s internal electro-chemistry within limits that are safe on boats. Design details required of these control systems are maturing rapidly, but in 1Q2021, new requirements are still emerging from the good and bad experience of early adopters and the experience gained from early system implementations rolled out by leading equipment manufacturers. Regarding lithium chemistry batteries, we cruisers are in a period best described as one huge “beta test.” Some designs are proving valuable and others are proving unsuitable.
The Boat “Electrical System”
I have long “preached” that boaters need to think about boat electrical systems “as an integrated system” and not as “individual components” hooked together. Engines with DC fuel solenoids/electronic fuel injection, alternator(s), start batteries, house batteries, battery charger(s), inverter(s), water and waste pump(s), bilge pumps, windlass, thrusters, windshield wipers, horns, DC nav equipment, nav lights, solar panels, DC generators, AC generators, hot water heaters, HVAC equipment, raw-water circulator pumps, watermakers, coffee makers, space lighting, washer/dryers, refrigerators, microwaves and other galley equipment, SOHO routers, computers, entertainment electronics, video cameras, boat monitors; all are “components” of the “electrical system.” While each has unique needs, reliability means all must work cooperatively together, as a whole; never has that been more true than with lithium technology batteries.
Figure 1 is a simplified drawing of a “typical” cruiser’s Electrical System. The AC sub-division is shown in green, and the DC sub-division in red. The interface layer between the AC and DC sub-divisions, in which power flow is “bi-directional,” is blue. This is a “direction-of-power-flow” diagram (source-to-load), not a wiring diagram. In this “model,” the DC sub-division emerges as the “soul” of the boat’s overall electrical system, and the house battery bank is the “beating heart” of the DC sub-division. Most of the “stuff” that comprises the AC sub-division is “optional;” i.e., discretionary to the lifestyle owners want to live while aboard. The “stuff” that comprises the DC sub-division isn’t optional; without that “stuff,” we don’t have a safe, operational vessel.
Above, the DC sub-division “battery bank,” as shown, can be viewed as either 1) a “central” bank that “does everything,” including engine starting, or 2) a “House” bank with an auxiliary “Start” bank “hung off the side.” Both views have at least two battery charging sources: 1) an AC-powered charging source and 2) an engine alternator. There are multiple approaches to battery charging, and many different combinations of charging equipment.
Batteries provide DC power to all of the vessel’s DC loads. Some boats are set up to have a third, entirely separate and independent bank that supports the inverter and its AC loads. However battery charging is done, in the end, all battery banks must be charged. The more banks, the more equipment and monitoring complexity, and maintenance, is involved in managing charging. The current design platform (Unique Figure 1 configuration) of a boat’s “electrical system” will have enormous impacts on how to best adopt a suitable, compatible lithium chemistry solution.
Lithium “Energy” Cells
Lithium battery material is a complex “chemical soup;” in this article, 3.2V cells of “Lithium-Iron-Phosphate,” or “Lithium Ferro-Phosphate,” or “LiFePO4,” or just the “LFP” chemical compound. This lithium technology is the safest of the lithium technologies, but it’s not as safe as lead-acid.
Figure 2 shows the two principle ways that commercial lithium cells are packaged; i.e., “prismatic” and “cylindrical” construction. The word “cell” refers to any package with its own positive and negative electrical terminals. Cylindrical cells are cylindrical in shape, within their own case. Cylinders are a physically strong shape and are relatively easy to temperature regulate. Prismatic cells are composed of thin layers of “soup” folded and sandwiched together and packaged into an aluminum or steel container. Prismatic cells are easy to handle and take less space than packs of cylindrical construction of the same aHr capacity. In packs made up of cylindrical cells, if one cell fails, the pack will continue its useful service, albeit at a lesser reduced capacity. If one cell inside a prismatic package fails, that failed cell may “take out” sibling cells and the entire prismatic package may fail. Cylindrical cells are less expensive to manufacture and are more durable than prismatic cells in some applications. In automotive, RV and marine applications, where the platform is subject to constant mechanical shock and vibration, cylindrical cells are more durable than prismatic cells. Prismatic cells can be utilized effectively in these applications, but measures must be taken to mitigate and absorb mechanical shocks and vibration.
Battery Terminal Voltage and Amp Hour (aHr) Capacity
Figure 3 shows that the more chemical “soup” in a pack of cells, by volume, the more energy that pack can store: i.e., “physically bigger pack, more Amp Hour (aHr) capacity.” LiFePO4 “soup” contains 220 Wh/L of energy storage capacity, so liters equate to stored energy. In describing battery construction, the battery industry describes the number of cells-in-series, followed by the number of cells-in-parallel; i.e., “2S2P.” Four 3.2V packs stacked in series (4S) yield a nominal “12V” battery. Packs can be stacked in series to create 12V, 24V, 48V, and greater nominal voltages, suitable for use in a wide range of applications.
Four 3.2V, 100 aHr cylindrical-cell packs (or prismatic cells) stacked in series (4S) makes a 12V battery of 100 aHr capacity. Eight 3.2V cylindrical-cell packs (or prismatic cells) configured with two in parallel (2P) and four pairs in series (4S) results in a 12V battery of 200 aHr (4S2P). So, in a 4S4P configuration of 100 aHr cells, the result would be a 12V battery of 400 aHr capacity, and in a 4S6P configuration of 100 aHr cells, the result would be a 12V battery of 600 aHr capacity.
Lithium Chemistry Batteries
Compared to lead-acid chemistry batteries, lithium chemistry batteries are:
- more volatile in response to internal temperatures, so are:
- extremely sensitive to high and low ambient temperature conditions,
- more sensitive to rate-of-charge and rate-of-discharge,
- more sensitive to “thermal runaway” events;
- more susceptible to permanent damage caused by a very deep state-of-discharge;
- more subject to permanent damage from a “short circuit” accident;
- more subject to voltage drift between cell-packs of the battery, so
- subject to both High Voltage Events (HVE) and Low Voltage Events (LVE);
- have unique/different charge and discharge characteristics and needs, and are
- more susceptible to physical shock and vibration
All Lithium Ion batteries need a BATTERY MANAGEMENT SYSTEM (BMS) to manage their electrochemical behavior. The BMS is a package of electronic sensors and circuits that ensure the chemical cells remain within safe operating limits. The BMS monitors charge and discharge currents, internal temperature, and voltage variations between packs. The BMS manages voltage equalization between packs. With lithium batteries, owners must be aware of what the BMS is doing, and why it’s needed.
The battery “assembly” shown in Figure 4 is representative of a single, large-capacity, lithium chemistry battery composed of four prismatic cells. This option is practical for cruising boats in 2021. A single 600 aHr capacity lithium battery (cylindrical or prismatic) is approximately equivalent to the usable capacity of a 1000 aHr lead-acid battery bank. Prismatic 3.2V cells are widely-available, commercially-manufactured components. The BMS is available as a commercially-manufactured electronic component, which optionally may come equipped with an Alarm Warning feature. The disconnect solenoid (the BMS Disconnect switch) is a commercially-available component. The battery “case” must provide mechanical protection to the packs, but can be as simple as a plastic milk carton with closed-cell foam cushioning or range to a custom-made fiberglass container.
This overall “solution” amounts to a “kit.” (Yea, “Heathkit!”) The kit’s designer (and general contractor) must specify the bill-of-materials, source the components, order them and install them. The kit installer can be a boat builder, boatyard mechanic, independent consultant or DIY owner.
Pro Tip: When sizing batteries to match the “electrical system” power requirements of a boat, remember that both lead-acid and lithium chemistry batteries lose aHr capacity as they age. That loss can be as much as 20% of the rated capacity of the batteries (0.2C). Plan accordingly.
Balancing the Cell Packs
In the “kit” described above, nominal 3.2V packs are made up of paralleled cells. “New” packs from the supplier can be at a different SOC, so the actual pack voltage can range anywhere along the nominal LFP voltage curve. Pack voltages must be “balanced” before being placed in service, to equalize voltages variations. Packs can be either “Top Balanced” or “Bottom Balanced.”
Figure 5 shows the open-circuit voltage curve of a 3.2V lithium chemistry cell at different SOCs. LFP cell voltage remains very flat from maximum-charge to maximum-discharge, and Peukert’s Factor has little effect at high levels of discharge current. For the “kit” builder, the “out-of-the-box” task is measuring the terminal voltage of each pack when delivered. New cells should be nearly the same voltage; ex: 3.2V ± 100mV.
Because the voltage discharge curve of Lithium chemistry cells is very “flat” from “full” to “empty,” any “brand new pack” could be at 70% SOC and any other “brand new pack” at almost the same voltage could be at 30% SOC. Early in “kit assembly,” the individual packs would not yet have a BMS to protect them. The assembly-time risk is permanent damage to a brand new pack by accidental over-charging or over-discharging. This IS NOT a warranty claim; manufacturer’s consider it “abuse.” It’s a very costly “user error.”
Figure 6 is another view of the pack-to-pack voltage variation, with four nominal 3.2V cells at slightly different SOC voltages. Wired in series while charging or discharging, these unevenly balanced pack voltages would rise and fall by the same proportional amounts, in response to the current flowing through them. The relative voltage differences would remain as pack SOC increased or decreased. Charging, when the lowest voltage cell reached full charge, the three higher voltage cells would be in the over-voltage danger zone (overheating, O2 outgassing). Discharging, when the highest voltage cell reached its discharge low-voltage limit, the lower three cells would be in the under-voltage danger area (cathode disintegration). To avoid over-voltage or under-voltage cell damage, “kit build” packs must be “balanced” during assembly. In consideration of the length of this article, I’ll send readers interested in the details of the balancing process to Youtube. This is a link to a Youtube video I think explains balancing quite well (Russian accent excepted).
The “Dark Ship” Moment
Lithium chemistry batteries are all fit with a BMS, which is intended to protect the battery. If the BMS senses a high or low temperature, a High Voltage Event (HVE), a Low Voltage Event (LVE) or an over-current event, it should ideally emit an Alarm Warning Signal. After a brief delay, the BMS will disconnect the battery from the host DC system. Reliability of the host “electrical system” is critically important! The BMS is there to protect the battery; it does not consider potential adverse impacts to the host vessel in its disconnect decisions. Depending on the configuration of the individual boat’s DC electrical sub-division, a BMS Disconnect can be an “everything just went dark” moment. Knowing the impacts of a BMS Disconnect event to the host DC “electrical system” is the responsibility of the system designer (and boat owner).
Figure 7 depicts the situation where the BMS detected a safety threat to the battery. The BMS initiated a BMS Disconnect event. The Disconnect Solenoid is an internal part of the battery assembly. When it opens, everything powered by the battery simultaneously loses DC power. Many engines will shut down, and their instrument cluster lights and gauges fail! The RADAR, Chart Plotter, Autopilot, Depth Sounder, VHF, SSB, AIS, electronic throttle and steering controls (fly-by-wire) all fail. The horn, foghorn, windshield wipers and inverter are in-op! Space lighting failed. The silence is deafening! The lithium chemistry battery pack is safely protected, but the ship is dead in the water. The rudder position is whatever the rudder position was. “Dark Ship” events are possible, not inevitable! They don’t happen often, but they can and do happen. Good “Electrical System” design is what moderates/prevents them.
Pro Tip: A separate, stand-alone lead-acid bank that provides DC for critical Nav equipment can moderate the “Dark Ship” impact, but also adds complexity to the vessel “electrical system.”
Lithium chemistry batteries made up of ganged prismatic 3.2V packs, including BMS, are usually “repairable” after a failure via parts-replacement.
Pro Tip: “cheap” is not a good word to describe a BMS. Cheap electronic parts are cheap for a reason; they’re made of cheap components. A BMS failure is a big deal. Buy for quality! The longer the warranty, the better. Remember, the BMS is internal to a “drop-in” battery’s case.
“Drop-in” Lithium Chemistry Batteries
Figure 8 shows three individual “drop-in” lithium chemistry battery assemblies wired in parallel to provide DC power to the boat. These “Drop-in” batteries are composed of cylindrical cells made up into 3.2V packs.
This paralleled LFP configuration is analogous to multiple, paralleled, deep cycle lead-acid batteries. Today (1Q2021), commercially-available 12V “drop-in lithium chemistry solutions,” are available in the market. “Drop-in” batteries are made by several companies, in the size and shape (“form factor”) of Group 27, Group 31, and even 8D 12V lead-acid batteries.
Assuming each of these lithium chemistry “drop-in” batteries has a rated capacity of 200 aHr, this bank would have a total capacity of 600 aHr, equivalent to the earlier prismatic example. If one BMS detects a fault, that individual battery would drop itself offline. Depending on the cause of the fault, it may or may not be possible to return that battery assembly to service. The individual components of “drop-in” batteries are not separately serviceable. If internal BMS electronics or other internal battery component fails, that battery assembly would become a toxic waste “brick.”
In preparing this article, several manufacturer websites were queried for information on currently-available “drop-in” batteries and system solutions. Some highlights:
- VictronEnergy® seems by far the most “thorough and complete” of all the manufacturers in lithium battery SYSTEM technology. They provide a complete solution, including the batteries, BMS, monitoring components and battery charging options. In this solution, several monitoring system components are widely distributed throughout the host “electrical system.” These components were not previously necessary in lead-acid systems. Victron® appears compliant with the current content of the ABYC TE-13 Technical Report, and their BMS does provide a pre-alarm prior to BMS shutdown. The pre-alarm is utilized by their own add-on components via a proprietary networking arrangement. The possibility of a “dark ship” event is not eliminated in this system’s solution design. This solution is rolled out as individual parts and is currently in “beta test.”
- Battle Born® (DragonFlyTM) is not TE-13 compliant at this time (1Q2021). They provide only the “drop-in” batteries, and not system-wide monitoring equipment or solutions where multiple batteries would be co-resident. They point users at Victron® accessory components in some areas, but for monitoring, not automatic control of their “drop-in” units. Controls are still manual, the owner/operator’s responsibility. They say they are working towards compliance “in the near future.”
- Xantrex® seems to have at least a visual alarm on the “drop-in” battery case, consisting of rapidly blinking LEDs. They have an external monitor that can be attached to multiple Xantrex batteries (doesn’t say how many, but at least two). There is no indication that a battery warning from one battery is available for use in externally connected equipment or that it can be utilized by non-Xantrex equipment.
- Renogy® does not mention any type of warning alarm in advance of a BMS Disconnect. These batteries cannot be connected together in series (two 12V batteries in series to create a 24V system).
For the designer/installer/owner of a lithium chemistry battery, it’s important to realize that one of the most essential components in the entire “electrical system” is the engine’s alternator(s). Alternators do their work through the magic of magnetic fields. The field winding of the alternator is a mechanically-driven, spinning electromagnet. The electromagnet gets its electrical strength from a voltage regulator. As the electromagnet spins, it induces an electric current in the alternator’s stator windings. The induced 3-phase AC voltage is converted to DC output by an internal solid-state rectifier diode pack. The primary electrical load on a working alternator is the battery/bank that the alternator is charging. LFP battery load (charging current) can be very large, so the magnetic field generated in the stator winding is very significant.
Pro Tip: it is always a good idea to install alternators fit with external voltage regulators on boats. This simple configuration alternative is appropriate to both lead-acid and lithium chemistry batteries. External regulators have user-selectable, technology-specific, multi-stage charging programs, and are able to monitor alternator case temperatures. In response to high case temperature, the voltage regulator modulates field drive to protect the alternator from overheating.
Lithium chemistry batteries accept very large charging currents, so “electrical system” designers assume that alternators will be working towards their maximum capacity throughout most of the battery-charging cycle. Lithium chemistry batteries charge in “Bulk” mode to 99% SOC.
Pro Tip: charging programs for shore power chargers, engine alternator voltage regulators and solar controllers must be set so the maximum B+ voltage stays beneath the HVE voltage disconnect specification for attached BMSs. Lithium chemistry (LFP) has a High Voltage Cutoff of ~14.6V. An HVE can occur because of internal pack drift (pack-to-pack voltage variations) or it can be caused by an externally impressed charging voltage coming from an external charging source. Obviously, the charging source itself must not become the cause an HVE disconnect.
Alternators must never be disconnected from their load while they are producing output. Sudden loss of the battery connection to an alternator is usually catastrophic to the alternator. A BMS Disconnect event is exactly that: a “sudden alternator disconnect” in which the alternator output current is suddenly interrupted. This results in the sudden collapse of the magnetic field in the alternator’s stator windings. The collapsing magnetic field results in a huge voltage spike in the alternator windings. Disconnected from its load by the now open BMS Disconnect solenoid, there is no place for the spike energy to go to be safely absorbed. That spike voltage either destroys the alternator’s rectifier diodes or damages the insulation of the alternator’s stator windings, or both. And that’s not all of the bad things that can happen!
In a typical boat “electrical system,” the alternator is one of many device attachments to the B+ buss fed by the battery/bank. Also attached to the B+ buss is all of the rest of the boat’s DC equipment which was simultaneously in use at the moment of BMS Disconnect. Some of that equipment is “delicate electronic equipment” (RADAR, Chart Plotter, Autopilot, depth sounder, VHF, AIS, inverter, solar controller, etc). The spike that kills the alternator is also impressed onto the boat’s B+ Buss, so all equipment attached to that buss is also at risk of damage.
Pro Tip: it is always good to install a surge bypassing diode at any alternator on any boat. Surge Protective Devices (SPD), also called Transient Voltage Suppressors (TVS), are used to shunt high voltage spike energy to ground, thus also protecting equipment connected to the buss. While “good” to do, this strategy is not a 100% reliable damage mitigation solution. Depending on a range of variables, the spike voltage can exceed the capacity of commercial DC SPD products. SPDs that do successfully suppress transient spikes can be left in a partially or totally “damaged” state by the event, and may then not function correctly, or at all, in a future event. So this strategy, while useful, is not “sufficient” to the overall goal of protecting alternators.
Figure 9 is a simplified view of a typical alternator application which includes an SPD. The alternator field coil gets its “exciting” DC voltage from a voltage regulator, shown here as external to the alternator case.
A BMS disconnect scenario in this large-capacity, single battery bank would start with a problem detected by the BMS. The BMS would send out an alarm signal, indicating an imminently pending disconnect event. Good system design would then use that warning alarm signal to shut down the alternator’s field drive by shutting down the voltage regulator. That design removes field drive from the alternator, so it shuts down gracefully, without damage to the alternator, the protective SPD or any sensitive buss-attached equipment.
After a momentary delay, the BMS Disconnect solenoid would open, and the vessel would sustain a “Dark Ship” event. Bad enough, true, but at least the alternator and electronics attached to the B+ buss wouldn’t be damaged as a result of the disconnect event.
A key assumption in this scenario is that the BMS has the alarm warning feature. Not all BMS designs have that capability, so the DIYer specifying a BMS for a “kit build” has to know its available, needed and, indeed, essential to the integrity of the “electrical system.”
The preceding discussion, “Protecting Alternators,” applies equally to “electrical systems” containing solar controllers. Solar controllers receive a high and moment-to-moment variable input voltage from solar panels and convert that input voltage to “battery charging voltage.” In doing so, they act as chargers for attached batteries. A sudden BMS Disconnect disconnects the battery from the B+ buss, but leaves all other DC equipment attached. If the battery – in this case, the load on a charging source – is suddenly disconnected, solar controllers can be damaged in a manner similar to the way alternators are damaged. This can happen in two ways. One, by an alternator spike event. But also, by sudden voltage imbalances within the controller caused by the sudden loss of load. Mitigation of this potential problem is important and needs to be considered in the overall design of the “electrical system.” This is another case where a BMS Alarm Warning system can be used to gracefully shut solar controllers down without damage.
Pro Tip: in the solar industry, the guideline is to cycle lithium batteries between 20% and 80% SOC to improve battery longevity. That is also a useful strategy on boats, which carries a potentially huge hidden benefit. When charging less than max-high, and discharging less than max-low limits, it’s much less likely to encounter an HVE or LVE. Even if one of the packs of the battery (or one of a “drop-in” set) is, or has become, unevenly balanced, its much less likely that condition will result in a HVC or LVC BMS Disconnect. With an adjustable voltage charger, for 12V lithium chemistry batteries, select max-charge and max-discharge voltages to cycle the battery (battery bank) between 20% and 80% (This is also suitable for boats in long-term layup.)
Electrical System Reliability With “Drop-in” Lithium Chemistry Batteries
Figure 10 shows three individual lithium chemistry “drop-in” batteries wired in parallel to form a single bank of suitable capacity to power a cruiser’s boat. This configuration also carries the need to protect the alternator. This multiple-battery configuration may not be able to meet that goal. It does not have the means to shut down the alternator in a graceful and protective way, and there are other concerns with this approach as well.
Individual “drop-in” batteries have their own internal BMS, but today, only Victron Energy® has a “whole system” management solution that controls an entire battery bank of multiple “drop-in” batteries. The “marketplace” for “drop-ins” is “all over the place” in design maturity. Promotional literature is sometimes/often misleading. Buyers MUST do technical “due diligence!”
Considering “system reliability:”
In a 12V system configured as shown in Figure 1, both nav equipment and inverter are powered from the same battery bank. With LFP “drop-ins,” wholesale replacement of lead-acid batteries reprises the “Dark Ship” scenario. Moderating that scenario requires DC sub-system reconfiguration prior to conversion.
In a deeply-discharged 12V configuration, one disconnected “drop-in” battery will increase DC load on its peers and can cause others to disconnect in a cascade-type scenario. Would the owner/operator know that just one paralleled battery had dropped out? How? At a minimum, the bank would lose aHr capacity. What is the system reliability impact of reduced aHr capacity to the system, even over a short time? Many uncertainties and design concerns.
With “drop-in” batteries, the concept of “voltage balancing” still applies. Consider a 24V system with 12V “drop-in” batteries in series. New “drop-in” batteries can vary in SOC. What’s going to happen if series batteries are slightly uneven, or slightly under-capacity for the installation? If just one of several “drop-in” batteries in any configuration were to suffer a BMS disconnect event, there would be some impact to the host “electrical system.”
Cruising boats do not have uncomplicated, fast, simple escape and assistance scenarios. Boat needs must be evaluated based on the nature of boats operating underway, at some distance from land. Today, there aren’t good solution designs for the use of multiple, series-parallel “drop-in” batteries in a large capacity bank that meet the reliability needs of the “electrical system” on cruising boats. The impact to the host “electrical system” of any one individual battery having a BMS disconnect is indeterminate, unpredictable, unique to each boat’s “as-is” wiring configuration. BMS Disconnect events always come associated with risks.
An Alternative Way To Protect the Electrical System
Figure 11 shows a battery bank with a lead-acid chemistry battery wired permanently in parallel with one or more lithium chemistry batteries. These two dissimilar chemistry batteries become one, single bank, charged and discharged as one. They are NEVER separated in service (except for periodic maintenance activity where they need to be physically removed from service). Operationally, they are the right and left hands of the same individual organism.
Good “electrical system” design is that the lead-acid battery remains connected to the B+ buss following any BMS Disconnect to “absorb” any voltage spike. This design actually results in several significant “system reliability” benefits:
- The larger the capacity rating of the lead-acid sibling, the better spike energy would be absorbed to mitigate other severe side effects.
- Even if an attached SPD were driven into conduction, the chances of it surviving to fight another day are improved by the absorption behavior of the lead-acid sibling.
- With a permanently paralleled lead-acid battery as part of the bank, there would be at least some available “DC reserve power” to seamlessly continue powering the vessel’s engine electronics, nav electronics, fly-by-wire controls, lighting and safety equipment following a BMS Disconnect event. That clear benefit does mitigate the negative consequences of a total “Dark Ship” event.
Pro Tip: The “hybrid” schema shown in Figure 11 can be used with either a single, large-capacity lithium chemistry battery or with a battery bank made up of multiple lithium chemistry “drop-in” batteries arranged in parallel. If the boat already has serviceable lead-acid batteries, keep them. Simply install lithium “drop-in” batteries in parallel with the existing lead-acid set.
How A “Hybrid” Battery Bank Works
Note: Later in this discussion, readers are referred to a Youtube video further describing this “hybrid solution.” To align the two discussions, I’m using “typical” voltages as used in the video; i.e., the “equilibrium state” of a normally charged LFP produces a terminal voltage of ~13.2V, and the “equilibrium state” of a normally charged PbSO4 produces a terminal voltage of ~12.6V.
The “natural resting voltage” of a lead-acid battery is measured 24 hours after the battery has been fully charged and fully disconnected from all load circuits and from its siblings in a bank (four hours for LiFePO4 chemistry batteries). That 24 hour “period of total isolation” gives electrons and ions (charged atomic particles) time to reach equilibrium. The term “natural resting voltage” describes an electrochemical state where the sub-atomic charges in the battery materials have all “found a home” in the atomic structure of their host chemical compounds. At the atomic level, they are “in equilibrium;” “resting,” not floating about, and not “flowing” anywhere.
Observe, then, that lithium chemistry batteries in the normal range of charge have an operational terminal voltage that is very close to the nominal “float voltage” (13.3V) of lead-acid batteries. Once connected together into a bank, absent fault events, a lead-acid sibling rarely has any work to do in the bank. Compared to the lead-acid sibling, the voltage discharge curve of the LFP sibling is flatter and stays at a higher voltage level throughout most of its usable discharge cycle. It simply lives there, symbiotically, neither contributing nor burdening. The lithium chemistry sibling “does the work” of providing DC power to the boat. The lead-acid sibling will enjoy a relatively long service life because to it, there are rarely any discharge cycles, and its LFP sibling(s) continuously maintains it at its “float” voltage.
Pro Tip: this lithium/lead-acid hybrid pair can be charged in the same way – and with the same charging equipment and charging voltage profile – as a lead-acid AGM battery.
Making the Connection
Connecting lead-acid and lithium chemistry batteries together in parallel can be a non-trivial technical chore which requires some explanation.
When paralleling two lead-acid batteries, a small spark may occur when the final connection is made. Fully charged, their terminal voltages and internal charges are nearly identical. In that case, there will be no significant difference in the electrical charge between them. Paralleling fully-charged lithium chemistry batteries is similarly non-dramatic.
When paralleling a LiFePO4 chemistry (13.2V) and a PbSO4 chemistry (12.6V), the difference between the two natural resting voltages isn’t large (600mV), but the amount of electrical charge it takes to “equalize” that difference is actually very significant. Simply connecting them will result in a dramatic, possibly damaging, spark and a prolonged high current flow from lithium to lead-acid. To the lithium sibling, the outrush current looks like a low voltage “short circuit.” The goal here is to get these two dissimilar chemistry batteries paralleled together so that a “surface charge” is built up on the plates of the lead-acid sibling.
“Surface charge” is a very well understood phenomena with lead-acid batteries. With fully-charged lead-acid batteries, “surface charge” occurs because there is no longer an available vacancy for free electrons to occupy within the atomic lattice of the lead plates. When any external charge source is connected to a fully-charged lead-acid battery, a “surface charge” of electrons collects on the surface of the negative plate. “Surface charge,” alters the “natural” resting equilibrium of the fully-charged battery to a new, and “unstable,” state-of-equilibrium, caused by the presence of the externally impressed voltage applied by the charging source. In the automotive world, this is exactly what a portable lithium “jump start” pack does to a “dead” lead-acid car battery. That is, the charging pack delivers electrons to the dead battery faster than the battery can absorb them, and the electrons collect on the surface of the lead plates.
So, when initially connected together, the lithium chemistry “charging source” instantaneously tries to pump a large current into the lead-acid “load.” That “outrush current” flowing from the lithium to the lead-acid battery is seen by the BMS as a very large current, and the BMS may trip “off” to protect the lithium battery from damage. Whence the BMS resets itself, that cycle may repeat. The result is that the two dissimilar batteries couldn’t be successfully paralleled.
Enter now, the humble power resistor, such as this one from Amazon.com.
Think of this resistor as an “installation tool;” a “tool” used at initial setup time. Like all tools, it’s taken out when needed, used, and put away until it’s needed again.
The purpose of the resistor is to limit the “equalizing current” so the BMS doesn’t trip. When initially connected, Lithium at 13.2V gives up electrons to its lead-acid sibling at 12.6V. The resistor is placed “in series” in the conductor connecting the two batteries to “limit” that flow of current to a level acceptable to the lithium electron-donor’s BMS. As arriving electrons build up “surface charge” on the lead-acid plates, the terminal voltage of the lead-acid sibling rises, the voltage difference between the two different chemistry batteries falls, and so the current creating the surface charge in the lead-acid sibling simultaneously falls.
The surface charge on the lead-acid chemistry battery does not dissipate instantly when the charging source is disconnected. Since, at initial connection time, there is no electrical system connected to the bank, there is no path into which “surface charge” is able to “bleed.” So the installer of these connections has enough working time to disconnect and remove the resistor and connect the batteries directly in parallel while that surface charge remains intact. Since the normal terminal voltages of the lithium sibling (~13.2V) and the lead-acid sibling now with its surface charge established (~13.2V) are equalized at the same (or very nearly the same) voltage, the batteries can be connected directly together without drama or fireworks.
To initially connect lead-acid batteries into a lithium chemistry bank, the installer will need a large wattage resistor; in the range of 50Ω, 100W. The bigger the resistance, the lower the current (Ohm’s Law), so the longer the equalization process will take, but the “more controlled” it will be. Warning: the resistor may get hot to the touch! The resistor would be needed any time the two dissimilar chemistry batteries need to be connected/reconnected together, so for example:
- when first assembled together, or
- reconnecting them after a BMS disconnect event, or
- when emergency cross-connecting another bank, such as a lead-acid “start” bank, or
- after any maintenance activity on the battery bank, like cell/battery replacement.
For more information on hybrid banks and connecting dissimilar chemistry batteries, review this Youtube video. The presenter talks through the above entire scenario in detail. Clark Willix is a boater with many years of liveaboard experience and an engaging speaking style. He has a 1500 aHr lead-acid bank of L-16s, so way, way bigger than most of us. But that said, he presents sound conceptual descriptions.
Pro Tip: whether the above procedure is necessary at all will depend on the relative charge capacities (aHr) of the lithium and lead-acid siblings. A relatively smaller capacity lithium sibling connecting to a relatively larger capacity lead-acid sibling is more likely to trip the BMS. A relatively larger capacity lithium sibling connecting to a relatively smaller capacity lead-acid sibling could work without significant drama with a suitably-rated battery paralleling switch. Arc suppression at the switch will prolong the service life of the mating surfaces of solenoid or switch contactor points.
Once connected together, there is a permanent, small circulating current (the IFloat current for the lead-acid sibling). The dissimilar natural voltages are “equalized” between the two dissimilar chemistries, and the lead-acid float current load is very low between them. That IFloat current is a “Parasitic Load” to the lithium chemistry battery. Parasitic loads consume power and are oft forgotten. On boats, “always on” equipment can be viewed as a “parasitic load,” such as digital radio station memories, digital clocks, smoke alarms, intrusion alarms, doorbell cameras, CO/CO2 detectors, propane gas detectors, bilge pumps, etc. Parasitic loads can “kill” batteries. Readers are encouraged to view this “float current parasitic load” as a small price to pay in return for a potentially huge reliability benefit (reward!) provided to the boat’s “Electrical System.”
Pro Tip: a useful device to install on all boats is a “boat monitor,” such as the Siren Marine “MTC,” or equivalent. The device notifies boat owners by text message and/or email the moment shore power is lost. They monitor system B+ voltage, and can notify owners of low house battery voltage. These devices can be viewed as “parasitic loads,” because they are “always on,” and their presence forgotten, even when batteries are not being charged. However, these devices can save thousands of dollars in damaged batteries. A text that reports lost shore power indicates there’s a problem at the boat, and a timely phone call to the boatyard or marina can avoid/save great inconvenience and unnecessary costs.
Pro Tip: plan ahead for lengthy dockside stays and long-term boat storage lay-up periods. The lithium industry does not have good solutions for managing “float” charging on shore power. Lithium chemistry batteries do not like to be left fully charged unless that is part of ongoing discharge and charge cycling, and they are subject to self-discharge when chargers are discontinued. Owners of boats hauled and left in storage for some months, or otherwise unattended, must make provisions for the care of lithium chemistry batteries. Lithium solutions in 1Q2021 are for cruisers, not marina mavens. Plan to leave LFPs between 50% and 70% charged, and NOT on a full-voltage trickle charger. With proper equipment (adjustable-voltage charger), one can treat lithium chemistry batteries as in an off-grid solar application, cycling them between 50% and 70% SOC. Maybe with a solar panel and suitably adjusted solar controller…
Pro Tip: a design requirement for any BMS is that the BMS itself must be disconnected from the battery in the case of an LVC event. If the BMS stays connected to the battery AFTER an LVC, the BMS itself becomes a parasitic load. In an unattended situation, like summer layup, that small continuing parasitic load will, itself, lead to permanent damage to (destruction of) the host battery.
Technical Guidance on Lithium Chemistry Batteries
ABYC has published a “Technical Report” entitled “Lithium Ion Batteries” (TE-13, July, 2020). That document is not yet an “ABYC standard” to which boat builders, equipment builders, technicians, and surveyors are expected to adhere, but it’s an important guidance document for those interested in lithium chemistry battery solution design. Some quotes:
13.5.6 If a shutdown condition is approaching, a BMS should notify the operator with a visual and/or audible alarm prior to disconnecting the battery from the DC system.
13.9.4 HVE/HVC/LVE/LVC – A BMS should protect the lithium ion battery cells in response to an HVE and an LVE.
220.127.116.11 Means of protection should not disconnect critical loads without prior warning and should not stop the charging source in a manner that causes damage to the charging device.
EMPHASIS ADDED in 18.104.22.168 is the authors. “How” this good guidance is accomplished today remains at the discretion and responsibility of the designer of the “electrical system,” through specification of equipment in the technical specification of the bill-of-materials.
My interpretation is, if there is an impending High Voltage Event (High Voltage Cutoff) or Low Voltage Event (Low Voltage Cutoff) – or if there were an impending High Temperature Cutoff – the BMS SHOULD provide a warning in advance of a BMS Disconnect. The ABYC document does not specify the lead time in advance of the disconnect or the duration of the pre-alarm signal, but this would be something to look for in the manufacturer’s spec sheet before buying a BMS or before buying any of the commercially available “drop-in” batteries.
There is no specific guidance in the ABYC document on a need for a single “Operation Over-Watch” or “Operation Overlord” BMS intended to protect lithium chemistry systems made up of multiple individual 12V/24V “drop-in” units. But, here is what the document does say (again, emphasis is mine):
3.5.1 All lithium ion battery systems should have a BMS installed to prevent damage to the battery and provide for battery shutoff if potentially dangerous conditions exist.
NOTE: BMS can be external or internal to the battery.
Clearly, a battery bank composed of multiple “drop-in” LFP batteries does constitute a “lithium ion battery system.” In 1Q2021, the BMS component(s) needed to accomplish 13.5.1 with a bank of “drop-in” batteries, and the engineering of the “drop-in” batteries themselves, is/are not widely available in the marine market as commercially-manufactured offerings.
Lithium-Iron-Phosphate battery alternatives (using prismatic cells) are practical on boats in 2021. “Drop-in” LFP alternatives exist, but systems to control them are immature and incomplete.
This article has focused on boats used for long-distance cruising, characterized by frequent, repetitive battery cycling profiles (charge/discharge cycles) and a very high dependency on reliable, always-available “electrical systems.” The complexity of LFP conversion and operation isn’t technically justified in 2021 for those who’s preferred lifestyle involves traveling from marina to marine or anchors out only occasionally. The industry is still trying to figure out the best way to handle extended time on shore power; i.e., boats in infrequent, day-use or prolonged periods at dockside on shore power (boats used as seasonal condos).
Readers that have reached this summary have seen that lithium chemistry battery installations are complex and do have some potentially negative technical impacts. The net is, as of 1Q2021, there is not yet “expert consensus” on the “optimum design” of a lithium-based SYSTEM; that is, configuration variants that need to be supported and how those variants can, should and need to be controlled and protected for their safety and for the overall SAFETY and RELIABILITY of the host vessel and its crew. A “dark ship” event is a super big deal, even on a sunny afternoon in uncrowded waters. It would be a particularly unwelcome event coming into a lock on the Tenn-Tom Waterway or the Waterford Flight on the Erie Canal, or coming into Barnegat Inlet or Ft. Pierce inlet at max ebb (running from a thunderstorm), or crossing the Columbia River bar any time! We boaters do not yet have the control systems and equipment that will be the “necessary, normal, safe and reliable” platform in just 5 – 10 years time.
Lithium chemistry systems are, in 1Q2021, an “emerging technology;” NOT YET an “install and forget” platform. Engineers, manufacturers, boatyards and electrical technicians continue to learn daily from the failure events experienced by “early adopters.” There are still no accepted industry standards for lithium chemistry “systems,” and the existing UL Standards for lithium battery cells refer to batteries with very small amounts of lithium metal (<5 grams), as found in cell phones and tablet PCs. There is still way too much “technical stuff” for individual boat owners to have to know. There are still too few technicians trained to work on lithium systems. Costs are still very high. Mistakes are very expensive. Lithium batteries are subject to permanent damage if “abused.” Long service lifetime claims – although better than lead-acid batteries – are not borne out in “real life” by “early adopters.” LFPs should not be installed in engine rooms (heat). These systems will probably not fail where they were built and installed, so it’s likely the owners, or someone the owner has to locate and hire, will have to deal with any future failure event without benefit of the original builder’s design assumptions, drawings, documentation or assistance; a scenario which leads to “system re-design on the fly… …at lots of additional cost…”
A quote from an experienced friend who’s made this transition: “1) Don’t do it just because you can, and 2) if you do, be damn sure you understand how to do it right.”
The net is, it’s on you as the buyer/owner/operator to ensure your lithium chemistry platform is as reliable from a design point-of-view as it can be before accepting it from the builder. That means you need to know a lot about lithium technology and “electrical systems.” It’s also on you to have current electrical diagrams and manufacturer names, part numbers, user manuals and contact information, on-board and available, for all components of the entire system. If that system needs “repair” at some future time in some exotic cruising locale, you’ll at least have the reference documentation you’ll need to get help.
For the Serious DIYer
Serious DIYers interested in comparing lithium batteries to lead-acid batteries should view this analysis. A very clear lesson that emerges from the data presented here is the cost effectiveness of flooded wet cells compared to AGMs.
For those who are truly interested in lithium chemistry batteries, and also have SIGNIFICANT DIY ELECTRICAL SKILLS, TECHNICAL CURIOSITY, TOLERANCE FOR POSSIBLE DISRUPTION, A CRUISING BOAT USE PROFILE and at least SEMI-DEEP POCKETS, here is a reasonable 1Q2021 lithium chemistry platform option for you. In my opinion, this is the most cost-effective way to move to lithium technology in 1Q2021. I would only undertake this conversion if I planned to own the boat for 10 – 12 more years. In less time, there is essentially no opportunity for ROI, and the remaining advantages of lithium chemistry over lead-acid are less appealing without ROI as a reward. Lithium technology has great potential; but that potential is still a few year away for the needs/wants/desires of the “install and forget” buyer/owner.