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post #9 of Old 10-12-2012
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Re: Switching batteries to start the motor.

MBB, I assume you mean make before break - it makes the next connection before it breaks the old connection. This prevents a voltage spike and frying of diodes.

Some older switches didn't do that.

a sick (sulphated I think) battery can THINK it's full and hold 12.6v as long and nothing is drawing from it, but in fact be unable to hold any useable amp hours. My AGM's showed 12.6 - 13.1v until a load was placed on them when they dropped to 7v - 10.2v (the range is because I had a bank of 3 and all were different).
Thus, as soon as I turned the key I got one "rrr" then not even a clicking solenoid.

Hindsight being 20/20, I think I had a dead starter battery - and had the switch in 'both' so the house bank continuously drained to the start. Meanwhile the house never got fully charged (which AGM's require) - and that in turn killed them.
They were so dead a honda 2000 couldn't get me started even after 4 hours of 'trying' to charge a single battery.
Fortunately the dinghy is a pull start - and a open marina willing to sell overpriced batteries was near by.

Lessons learned are opportunities earned.
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