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post #4 of Old 10-12-2012
svzephyr44
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Re: Switching batteries to start the motor.

For people who have matched battery banks (that is, both "A" and "B" are of identical size a frequent strategy is to rename them "odd" and "even" (in your mind.) You use bank "odd" as your house battery (to run lights and stuff) on odd days, "even" on even days. The idea is that this way you distribute usage cycles over both banks evenly. Batteries "age" depending upon the number of times they have been charged and discharged, not how long they have been around.

Why do you care about aging them equally? One reason is that you don't want to discover that your second starting battery (the one you didn't use overnight) is worn out and can't start the engine. But a more important reason is that currently flows from higher voltage to lower voltage. A fully charged battery is at a higher voltage than partially charged battery. When you put your "A" "B" "ALL" switch in the "ALL: position the fully charged battery will start to charge the partially charged battery. Presuming that your alternator is charging this is not a big deal. That is, there is no reason to only charge the partially charged battery until the voltages are the same. But until the alternator charge has brought both batteries up to the same voltage there will be a cross current between the two battery banks that is in effect discharging your fully charged battery. Should you stop the engine and forget to switch back to "A" or "B" you would end up with two partially charged batteries rather than one that is "full."

With respect to which battery to use to start the engine it doesn't matter unless the battery you have used over night is down to less that 50% charge. In general batteries lose more life if they are being used when the voltage is in the 0% to 50% range than when it is in the 50% to 100% charged range. I don't know why but every battery manufacturer suggests you try not to discharge the batteries below 50%. Otherwise you will just use up a little more of the life of the more discharged battery. The next time you do this (on the other battery) things will even out.

Hope this helps.

CAPT Roger J. Jones USN (ret.)
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