Main Batteries Usage Strategies

This Guide gives guidelines for using the three main 12-volt batteries batteries in ways that protect the batteries and the engine's battery charger from harm, showing ways to keep the batteries charged and available for continued needs of cruisers, and helping preserve the batteries' longevity.

Synopsis of Rules:

  R01: NEVER turn all batteries OFF while the engine is running.
  R11: ALWAYS turn all three batteries OFF when leaving the boat.
  R12: NEVER have more than ONE battery ON when engine is not running.
       But allowed when: R12RA: swap charging batteries; 
       R12B: when going power to sail; R12C: when all 3 are near-dead.
  R13: DON'T charge Batts #1/#2 at same time as Batt #3.

Prime Directive:

  R01: NEVER turn all batteries OFF while the engine is running. 

Note: Doing so will likely cause failure of the engine's alternator that charges the batteries.

Important Rules:

  R11: ALWAYS turn all batteries OFF when you leave the boat.

Note: It isn't a disaster if one is left on, as long as all ship's systems are turned OFF; they're turned OFF to prevent electrical fires aboard while the boat is unmanned.

  R12:	NEVER have more than 1 battery ON when the engine isn't running.

Note: Doing so will often cause one battery to discharge into less-charged ones, losing the optimal charge we may need to start the engine, and possibly putting stress on the batteries if one is a lot more charged than another.

Situations where we sometimes DO have two on at once include

R12A: When we want to swap from using, or charging, one battery to using or

charging another, we need to switch between them without breaking rule R01 for a millisecond (oh, I dunno, maybe actually 5, 10, 20 ms?). So to switch between 2 batteries controlled by a single main switch, we merely turn between them by going through the "BOTH" position of the switch.

And if the two batteries are controlled by two different main switches, the only practical thing is to turn the new one ON first, and then as fast as possible (maybe 1 second) turn the other switch's battery OFF.

R12B: A common scanario is when -- after running the engine awhile e.g. while

motoring out from an anchorage or mooring, or when cruising in no wind -- the crew turns off the engine and THEN, some minutes later, realizes they've been charging two batteries. There's no need to panic; both batteries may have been charged sufficiently and somewhat "equalized" that leaving them on together for 5 or 10 or so minutes should do no harm.

R12C: When all 3 batteries have become discharged, one can try turning

two (or maybe even 3) batteries on together in order to crank the engine.

  R13:	Generally, don't charge either of Batteries #1 and #2 at the same time as Battery #3.

The construction of Batteries #1 & #1 differs from that of Battery #3, and we assume that their charging regimens are different.

Notes: Batteries #1 & #2 are "dual-purpose", designed for starting at high amperage AND/OR for deep discharging (at a lower amperage) to run ship's electronic systems and lights. These requirements imply that they're a compromise design for both of these usages, but for starting small engines and powering a modest amount of equipment to power for hours, fitting a pair of these "dual-purpose" batteries is considered cost-effective, even though they are not optimized for either high-current for starting, or for being discharged deeply by lights, instruments, etc.

Battery #3 is designed as a starting battery only; it is designed to withstand high starting currents, and is expected to get re-charged fairly quickly as soon as the engine is running.