The question of having a separate “house” battery and “start” battery or having a combined, dual purpose battery bank is largely a matter of personal preference. Each arrangement has pros and cons, and there is nothing inherently bad about either arrangement.
Modest sized cruising trawlers have limited space for everything, including batteries. Sanctuary had never been used by her previous owners as a cruising boat. Her previous owners may have anchored occasionally, but their preference had been to use marinas. When they did anchor, they were clearly in “camping out” mode, not in “living normally” mode. So when we bought the boat, she was in her OEM configuration, fit with two 8D batteries aboard. One 8D was dedicated to starting the Cummins 4BT-3.9 propulsion engine. The other 8D was used to power house DC loads. In that configuration, the “start” battery was seriously under-utilized; and, the house battery was seriously over-utilized. There was a manual “1-Both-2” switch to combine the batteries, but using it effectively required human awareness and diligent attention. As I “wind down” toward bed time in the evening, I often fail most gloriously in the areas of awareness and diligence. So, Sanctuary’s configuration was a very poor arrangement in terms of commodity utilization and human usability.
Starting the engine requires many amps for a very short period of time; perhaps 500A for 5 seconds or less. The battery needs to supply cranking amps (CA, CCA, MCA, Reserve Capacity) but not amp hour (aHr) capacity. High, short term demand is not the design profile of loading of a deep cycle battery. At the same time, a single 8D was not adequate to handle house DC requirements in cruising service, and we needed more energy capacity for house use (refrigeration, water pumps, space lighting, computer, TV/DVR). However, we did not have the space to install a third 8D battery. It was obvious that combining the two existing 8D batteries into a dual purpose bank was the alternative that returned the best utility for our situation. We ran with the two 8Ds combined into a single bank configuration for several years after that with no problem. Then the day came when that bank went casters-up.
In the summer of 2012, both of the 8D AGM batteries failed, a week apart. We were cruising in New England at that time, not near our home port, familiar marinas or regular marine chandleries. Fortunately, a cruising friend offered us a mooring ball at his yacht club, and the use of his pick-up. I am no longer able to lift and move an 8D battery by myself, so we decided to permanently replace the 8Ds with 6, 6V Golf Cart batteries, wired in a series-parallel configuration, to support our 12V DC system. The yacht club where we were moored allowed me to pull up to their dock (they were closed on Monday), where I loaded the batteries and spent the afternoon fabricating battery cables. I made up 8″ intermediate cables to make the series connections, and buss cables to add what was effectively a third 12V battery into the bank. The resulting bank occupied the same floor footprint as the 2 8Ds had needed.
Following is a diagram of the resulting DC Distribution System aboard Sanctuary”
Anyone who wishes can download an Adobe .pdf copy of this diagram by clicking this link: 20161022_dc_electrical_distribution_system.
A combined battery bank also solved several related problems, including the proper charging and load balancing on the two batteries, while giving us the additional house capacity we needed. So that’s how we have run for the last several years. We think it has been a net gain in several respects. We think there is nothing inherently bad about having a single bank that feeds both the house loads and starts the engine. For a combined battery bank, however, there are two clear cautions: 1) the bank must be big enough so that when it is discharged to the 50% point, it can still easily start the propulsion engine, and 2) there must be a reliable and convenient means to track the state-of-charge of the battery bank to avoid deep discharging to more than 50%. Fortunately, several battery monitoring products are available that enable reliable, accurate battery monitoring. Meeting these two requirements is therefore quite easy and relatively inexpensive.
There are, of course, only two backup options to a severely discharged/dead battery bank. One is to run a genset to re-charge the bank, and the other is good towing insurance. If the genset option fails, the final option is to call for a tow. Our genset does have its own stand-alone start battery. The genset charges its own start battery when it is running. We also have a small, stand-alone external AC charger for the genset start battery. We use it periodically – not continuously – when we are docked for extended periods of time. That keeps the genset start battery conditioned and compensates for self-discharge.