Category Archives: Which Batteries Are Best

Which Batteries are Best?

1/13/2013 – Original post
3/20/2023 – editorial updates

On boats, there are two very different sorts of DC electrical loads.  Electronic navigation equipment (VHF Radio, RADAR, Chart Plotter, GPS, AIS), navigation lights and space lighting, water pump(s), refrigeration, computer equipment and entertainment systems require relatively modest amounts of electrical energy over rather a prolonged period of time.  Propulsion and generator engine starter motors, thruster motors and windlass motors require rather large amounts of electrical energy over very short-period bursts.

Lead-acid battery options are manufactured to support the needs of both of these electrical load profiles.  “Traction” batteries are designed to operate in a physically harsh environment and be relatively deeply-discharged many times.  These batteries are intended to supply small-to-moderate amounts of power for long periods of time.  Automotive and light truck “start” batteries are designed to provide very large amounts of energy (amperes) over short periods of time.  They are not designed to be deeply discharged, and can be permanently damaged by deep discharging.  A compromise in manufacturing technique is the “commercial” battery.  These are often found in RV and marine applications to power house DC loads.  They are not a true start battery, but they are also not a true traction battery: they are a compromise between the two.

Battery manufacturers rate traction batteries in Ampere-Hours (aHr), which is a function of lead mass.  Start batteries are rated in terms of Cranking Amps (CA), Cold Cranking Amps (CCA), Marine Cranking Amps (MCA) and Reserve Capacity, a function of lead surface area.  These technical ratings lead to the very different profiles of electrical load the two very different battery-types are intended to support.  These ratings also reflect the very different construction and materials used in these different purpose batteries.

In the US, traction batteries get their aHr rating based on carrying a load from the fully-charged state to the fully-discharged state in a period of 20 hours.  All lead-acid battery manufacturers recommend against discharging traction batteries to more then 50% of the battery’s rated amp-hour capacity.  That means a 200 aHr overnight DC electrical requirement would have to have a minimum of 400 aHr of installed battery capacity.  Batteries that experience a lesser average level-of-discharge will generally return a longer installed service life.  Traction batteries that are discharged at a rate greater than their 20-hour rate are not able to deliver the full 20-hour-rated aHr label capacity.

Traction batteries are built with relatively few, but relatively thick, lead plates.  Start batteries have a relatively greater number of lead plates than traction batteries, but the plates are also relatively thinner.  This design difference gives start batteries a much greater surface area of lead from which chemical energy can be quickly converted into electrical energy.  Traction batteries can be used in starting service, but generally need to be of larger capacity (and therefore, physically larger and heavier) than a made-for-purpose start service battery.

There are two grades of 12 volt traction batteries.  The 4D and 8D batteries commonly used on boats and RVs are “commercial grade” batteries.  They are used in commercial services to start engines on large trucks and in many types of construction equipment.  They tolerate moderate amounts of mechanical vibration and offer large CCA capacities.  They support a reasonable mix of starting and deep-cycle applications.  Sic volt electric car/golf cart batteries are true deep-cycle batteries, again with thicker lead plates and electrolyte reservoirs that are deeper below the plates than their commercial cousins.  Because of plate thickness, they release their energy relatively more slowly, but for much longer periods.  The thickness of the lead plates minimizes sulfation, and improves plate remodeling during the charging process.  Particularly in hard-service applications, these design elements extend their service life.

In RV and boat installations where there are separate battery banks for “start” and “house” applications, start batteries would be most appropriate to power boat engine starter motors, thrusters and windlasses, and traction batteries would be most appropriate to power navigation, computer and entertainment system electronics, refrigeration, water pumps and DC lighting needs.  Battery charging and combining issues often incentivize boat owners to use traction batteries for all DC loads on their boats, which does not result in any significant issues.  Aboard Sanctuary, we have six 6V traction batteries (deep-cycle golf cart batteries) arranged to comprise a single 12VDC bank that powers all of our DC loads, including engine starter motors and our windlass.  Our genset has its own, dedicated 12V “dual-purpose” (start and deep cycle) battery.  That combination gives us greatly more Reserve Capacity and CCA than necessary for engine starting, but very adequate aHr reserves for overnight anchoring in seasons with short hours-of-daylight.  This minimizes our need to run the genset for battery charging.  (Our thruster system is hydraulic, not electric.)

In consideration of the above, the question, “which batteries are best for a boat,” remains a bit of a “personal preference” discussion, like which anchor is best or what’s the best micron size of fuel filters.  To a great extent, the best answer is, “it depends on how you plan to use the batteries and the boat.”

First, I’d suggest avoiding all 8D form factor traction batteries, not because they’re inherently bad in and of themselves, but because they are very heavy.  At 160# – 170# or so per battery, more for some, I can not handle 8Ds by myself without risking personal injury.   When they fail – and they do fail – owners need to be able to handle them by themselves, because it won’t happen at the home slip with lots of help available!  Two 6-volt GC2 form factor Electric Car batteries will easily fit in the same foot print as a single 8D, and will have very nearly the same energy capacity (measured in Amp Hours).  Three 6-volt GC2 batteries will “just fit” in most 8D battery boxes, so that would allow six GC2s in the same footprint space as two 8Ds.  If so, that will yield significant additional aHr capacity in the same space two 8Ds require.  For example, most 8D batteries range around 225 – 245 aHr, so for most boats with at least two 8Ds, a capacity of 480 aHr.  A pair of GC2 batteries in series will result in a 12V battery in the range of 215 – 230 aHr.  Two pairs of four 6-volt batteries in a 12V configuration would yield ~450 aHr; three pairs would yield ~675 aHr, that in the same space as two 8Ds, so a significant improvement in energy capacity with the benefit of handling safety (1/2 the weight), all in the same floorspace footprint.

The technology choices for deep cycle “Traction” batteries (“marine”/”commercial”) boils down to the selection of “flooded wet cell,” “AGM” or “Gel” batteries.  All three are lead-acid technology.  Optima spiral-wound batteries are a special – and expensive – case of lead-acid AGM technology.  Firefly Carbon-Foam AGM batteries are another lead-acid technology.  Each of these technologies has pros and cons associated with them.  Some of the pros and cons will carry personal value for some boaters, but not for others.

Flooded lead acid batteries require regular maintenance.  That means periodically checking and adding distilled water, and periodically equalizing them.  Flooded lead acid Electric Car (Golf Cart) batteries are “commodities,” available everywhere in the world; even in the third world.  Flooded batteries have relatively high self-discharge rates.  AGM and Gel batteries may not be either available or affordable in many places, even in the US.  AGM and Gel technologies are “maintenance-free,” meaning electrolyte isn’t lost in normal operation, but also can’t be added.  AGM and Gel batteries generally can’t be equalized. (Some AGMs can, but that is manufacturer-specific, and the general rule is: not.)  AGM and Gel can be mounted in any physical orientation, including standing-on-side and standing-on-end.  AGM and Gel batteries are adversely affected by ambient temperature, Gel more than AGM, so mounting them in the engine room can result in shortened service life in some installations.

Charging traction (deep cycle) lead-acid batteries – regardless of flooded, AGM or Gel – should be done with a modern 3-stage charger set to the correct charging program for the battery technology.  There are lots of technical issues around battery charging which I’m not discussing here (see my separate article, here).  Suffice it to say that a charger with a 100A – 125A DC output for a 675 aHr battery bank is a perfectly acceptable solution; not perfect, but a highly acceptable compromise from a lifetime ROI perspective.

Flooded lead acid batteries are the least expensive traction batteries to buy (lowest capital cost).   The commodity cost of lead has driven battery cost way up in the last few years.  With the Chinese buying the entire world-wide reserves of lead ore, that commodity cost will probably continue to rise.  My summer, 2012, experience was that I bought six, 6V EGC-2 batteries rated at 230 aHr in June/July, 2012, for $92 apiece, from Sam’s Club.  At that time, a single Deka AGM 8D was $600.  So I was able to buy 690 aHr from Sam’s Club for what I would have paid for one 245 aHr AGM commercial battery at Hamilton Marine in Portland, ME.

Now, here following is the “religious” part of the battery topic.  The real issue is, “what’s the best ROI on the battery technology selection you buy?”  And the answer is, again, “it depends.”  From the reading I’ve done and the experience I’ve had aboard Sanctuary since 2004, running 60K miles and 7500 hours engine hours, and anchoring out 1/2 to 2/3rds of the time while traveling,  I am persuaded that if you are an active cruiser –  if you actually use your boat, put many hours on it each year, and anchor out in preference to using marinas – flooded wet cells provide the best ROI.  They are the least expensive to buy, and they return good charge/discharge cycle life.  The Sam’s Club batteries with Duracell branding are manufactured by East Penn in the United States.  They are the same batteries that you’d buy retail, for much more money, over the counter at NAPA or West Marine.  Yes; literally the same batteries, save for the house labels, made by the same people on the same production line in Pennsylvania.

My own personal experience with AGM batteries (Deka, the East Penn brand label) did not meet my desires or expectations.  I have had two sets of Deka AGM batteries fail after only 3 – 4 years in service (less than 300 charge/discharge cycles).  They were not excessively discharged in service.  State-of-charge was monitored with a Xantrex Link20 battery monitor, later upgraded to a Magnum ME-BMK battery monitor.  Both of my battery chargers (Magnum MS2012 inverter/charger on shore power, Balmar 712110 alternator with Balmar MC-614 (earlier, ARS-5) voltage regulator on the main engine) are multistage chargers.  The chargers use the correct AGM charging profiles.  And when these batteries failed, they failed overnight, not slowly; no advance warning. They did not return the charge/discharge cycle life they should have been able to return, and at $600 apiece in 2012, I no longer feel they’re the best choice for our cruising profile for lifetime ROI.

That said, for people who don’t use their boats much, for those who use marinas rather than anchoring out, etc., the AGM and Gel maintenance free batteries may be a good choice.  These owners will get several years of service from the batteries.  In these applications, it may not matter that they haven’t returned their rated cycle life.


One final “religious” argument comes up with comparing Sam’s Club batteries to high-end Trojans, Rolls Surette, Lifeline, Optima, or other “high-end” battery brands.  Again, “it depends.”  There is no denying those brands are excellent, well made batteries.  They are also expensive to buy.  If two boats have equivalent aHr battery banks, equivalent loads, and equivalent usage profiles, it is likely that these premium brands will, indeed, outlast my Sam’s Club batteries.  However, the real question is, will they outlast them by the ratio of their purchase price to mine?  If mine last 5 years, will theirs last 15 years?  Unlikely, I think.  So, my Sam’s Club batteries may well offer better ROI and lesser total-cost-of-ownership than those elegant premium brands.  My trade-off opinion then is, for coastal cruising in the US, the Bahamas and Canada, Sam’s Club batteries are just fine.  If you’re a blue water cruiser who’s going to circumnavigate and travel to places like Figi or the Marquesas, or cruise south of 60º S latitude in the Channel Islands or Straights-of-Magellan of Chili, well, maybe in that case the premiums offer more comfort and re-assurance to their owners.  But I’m a mid-size trawler cruiser.  Most single engine trawler owners are not going to circumnavigate, cross the Atlantic, or cruise the Windwards.  So for me, Sam’s Club batteries are just fine for coastal and near-coastal cruising in the US, the Bahamas and Canada.

Optima batteries are a variation of AGM technology called “thin plate pure lead” (TPPL).  They return lots of aHrs per unit of weight and space, and are relatively more efficient (lower Peukert’s exponent) to charge than other AGM batteries.  They are expensive to purchase, per aHr.  The technology has great potential, but not for me at current consumer price levels. In automotive start service batteries, Sears Diehards are of this “spiral wound” TPPL design.

LiON batteries are an entirely new and “emerging” battery technology.  They deliver several times the amount of energy at 1/4 the weight compared to their lead-acid counterparts.  LiON batteries (the lithium, iron, phosphate variant) are stable and safe in operation.  Some early adopters have installed LiON battery systems and are getting very encouraging results.  That said, in 2013/2014, I consider LiON systems as an emerging technology.  The technology requires very different charging and battery monitoring equipment which is not compatible or interchangeable with lead-acid charging equipment or systems.  LiON charging and monitoring equipment is currently (2014, 2015) only available from hobbyist and custom-developer sources.  It is not yet available as a commercial product from a reliable equipment manufacturer with stated length-of-life expectations, reliability and quality standards and consumer product warranties.  LiON batteries are currently quite expensive.  Supplier sources and product availability are highly limited.  My net is, for the average boat owner/cruiser, from the perspective of fit-up cost, lifetime ROI and in-service maintenance, I consider conversion from lead-acid to LiON technologies to be impractical at this time.  Yes, it works.  Yes, it is highly efficient.  Yes, piece parts are available from specialty sources.  But, significant technical knowledge is required to install and manage these systems.  If an outage occurs, the necessary service knowledge is not yet generally available in the marine service industry.   This technology undoubtedly represents the future, but not yet the present.

For those interested, following is the chemistry of a flooded wet cell during discharge.  When the lead/acid galvanic cell discharges into an electrical load, the following reactions occur:

Anode half-cell reaction:
Pb(s) + HSO4(aq) + H2O(l) → 2e + PbSO4(s) + H+(g) + H2O(aq)

Cathode half-cell reaction:
PbO2(s)+ HSO4(aq) + 3H3O+(aq) + 2e  → PbSO4(s) +5H2

Add the two half-cell reactions together, the full-cell discharge reaction is:
Pb(s) + PbO2(s) + 2H2SO4(aq) → 2PbSO4(s) + 2H2O(aq)

During discharge of a lead/acid cell:

  • Notations: (s)=solid, (l)=liquid, (g)=gas, (aq)=aqueous solution
  • PbSO4 (lead sulfate) precipitates out and deposits on BOTH the anode and the cathode.
  • Free hydrogen (H+) from the sulfuric acid electrolyte (H2SO4(aq)) produces water (H2O(aq)) at the cathode.
  • The concentration of free hydrogen (H+) decreases over time.
  • The concentration of sulfuric acid (H2SO(aq)) decreases over time.
  • The pH of the electrolyte (H2SO4(aq)) increases over time.
  • Two electrons are transferred in the overall reaction.
  • Both half-cell reactions go from left to right when load is applied to the battery.
  • The half-cell reactions are different processes.
  • For each mole of lead sulfate produced, two moles of electrons travel through the external circuit.
  • During discharge, the “-” plate is the “anode” (since the “-” plate material is being oxidized),
  • During discharge, the “+” plate is the “cathode” (since the “+” plate material is being reduced).