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Compass Marine How To | all galleries >> Welcome To >> Alternators & Voltage Sensing - Why It's Important > Voltage Sensing Blunders
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Voltage Sensing Blunders

We've discussed the importance of good voltage sensing, in relation to charging performance, but it is not always as easy as it appears.. Let me try and sum it up in one line..


I know some will still not understand what that means so we are going to delve deeper. Unfortunately I see and correct this blunder far more than I ever should and this one is a battery destroying blunder....

On the typical factory wired boat the alternator output, also called B+, is wired over to the big post on your starter motor. From there it picks up the large wire that comes from the "C" post or "common" post of your 1/2/BOTH battery switch. This is how the alternator gets current to your batteries, by sharing the same wire the starter motor uses. This method of wiring a factory alternator is cheap, easy and saves the builders lots of money. Some builders do a better job and do direct wire the alternator to the house bank but these are usually high quality builders, not your average production builders.

In this overly simplified illustration we simply have the alternator wired direct to the "C" post of the battery switch. It is easy to see how the battery switch becomes a charge directing switch in this scenario.

Set it to #1 and the alternator charges bank #1.

Set it to #2 and the alternator charges #2.

Set it to BOTH and the alternator charges BOTH.

Now what happens if we add a high output alternator, and leave it wired this way? What happens if we leave it wired this way and then run volt sensing to the house bank??

IMPORTANT: Running a high output alternator through a battery switch, and sensing the bank, means your house DC devices will see higher voltage than they should because you are not including the voltage drop between the battery switch "C" post and the battery. Please wire any high performance alternator direct to the house bank and take all DC loads from the house bank. This will prevent overly high voltages on the DC system.

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