Main Batteries Usage Strategies

(return to Systems Descriptions 3.3.1)

    'NOTE: The main 12-volt battery configuration has been changed (as of Spring 2010):
    'Battery #3 has been removed, and Main Batt Switch #2 (lower switch) is not wired.

This Guide gives guidelines for using the two 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.

The guidelines are carried out by operating the top main 12-volt switch (switch #1).

Operation of 12-volt Main Switch

The two main 12-volt switches are under the nav-station seat. Only the top one (#1) is used.

Each switch has 4 positions; clockwise from their DOWN positions, the functions of each position are OFF, #1, BOTH, and #2.

The top switch's #1 and #2 positions connect Battery #1 (the aft battery under the nav-station seat) and #2 (forward) respectively to the ship's main power circuits, allowing the battery to power the boat, and the engine to charge the battery.

The bottom switch used to control Battery #3, which has been removed (Spring 2010).

Synopsis of Rules:

  R01: NEVER turn both batteries OFF while the engine is running.
  R11: ALWAYS turn both three batteries OFF when leaving the boat.
  R12: NEVER have more than ONE battery ON when engine is not running.
       But is allowed when: 
              R12A: swap charging batteries; 
              R12B: when going power to sail; 
              R12C: when both are near-dead and engine must be started.

Detailed Rules and Comments

Prime Directive:

  R01: NEVER turn both 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 both 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. However, note that there's no OTHER switch aboard which is guaranteed to turn off all electric circuits.

  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 one, 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.

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 both batteries have become discharged, one can try turning both on together in order to crank the engine. You should crank soon after turning the battery switch to BOTH.

Notes: Batteries #1 & #2 are "dual-purpose", designed for starting at high amperage AND 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 was designed as a starting battery only; this type is designed to withstand high starting currents, and are expected to get re-charged fairly quickly as soon as the engine is running. Nevermore got a new 120-volt battery charger which can only charge two batteries, and the extra complication of the second battery switch also voted for removing battery #3.

Battery Charge/Discharge Strategies

     Below is a question in a draft SOP which triggered the article here:
     HOW DOES ONE KNOW WHICH BATTERY TO SELECT? Is there a rule of thumb, 
     should we run on 1 or 2 or 3 generally when the engine is not running? 
     Or does it vary and how is this determined? (Do we check the battery output 
     de temps en temps to see which is strongest?)**

Possible observations or measurements to make a strategy:

• After starting engine, watch engine-panel ammeter charging rate, and after it indicates near-0 charging (amps) one can switch to another battery and observe ITS charging rate.

• If you suspect that one or both are fairly discharged (e.g. sail all night, then start engine to come to port, etc.) then charge the starting battery awhile, then the other, and then both by switching to BOTH position of switch.

• Observe (over a period of time) what nav panel voltmeter reads when you think the battery is charged (but not right after turning engine off when battery was being charged), and then observe it later. Getting much less than 12-volts indicates it's getting weak. If switching to the other battery doesn't then show considerably more than 12-volts, then save THAT battery by going back to the one you just switched off of (the less charged one.) And (except if racing) then get the engine running and charge the deader one awhile, then both of them together.

Other possible subjects:

• bilge pump is on batt#1, so it may want more (longer) charging if boat hasn't been sailed recently.

• Use of nav-station switch panel voltmeter to indicate battery charging or charged; use of engine control panel ammeter (not nav-station panel ammeter) for monitoring charging but not discharging. The engine-panel meter is especially useful to show when the current "starting battery" has been charged back up, as described previously.