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Author Topic:   Battery Charger Wiring
kwik_wurk posted 12-18-2011 04:38 PM ET (US)   Profile for kwik_wurk   Send Email to kwik_wurk  
I have come across OEM installations where the 12-VDC portion of AC battery charger is wired to the battery selector switch. More specifically, the positive charger output is wired to the [battery] side of the switch, and the charger [negative output] is wired to the common ground bus bar. And this is on boats that are only 12 years old.

I thought this was odd, I have always wired battery charegers directly to the battery terminals and installed fuses on both the +12-VDC and +0-VDC. This is based on the premise that chargers are sensitive enough that any resistance in the wiring is enough to affect the charge and trim cycles. (And charger manufacturers seem to state this all the time.)

Blue Sea Systems has a good resource help file on their website and clearly directs the charger output to the terminal.

I don't have ABYC standards to look at, but I am sure there is a standardized installation. So asking the question aloud: wire to the battery terminal or to the 12-VDC selector switch?


jimh posted 12-18-2011 05:26 PM ET (US)     Profile for jimh  Send Email to jimh     
Electrically there is no difference between wiring to the battery terminal or wiring to the battery side of the primary power distribution switch. The only circuit element between those two points is the resistance of the cable between the battery and the primary power distribution switch. That conductor should be sized to handle several hundred amperes of engine starting current without any appreciable voltage drop, and its resistance should be miniscule. It therefore can handle the charging current of a typical charger, say five amperes, without any voltage drop.

In wiring practice it is best to avoid having more than one terminal under a fastener, and particularly at the back of a switch component. This means one ought to avoid attaching multiple leads to the primary power switch. Some batteries have more than one terminal, so you can connect the charger to a secondary terminal at the battery.

I have my AC-powered battery charger output wired directly to the battery terminals. I chose that because there is much more room at the battery to add a second conductor under the battery terminal than there is behind the primary power distribution switch.

In a perfect installation the battery would have only a single conductor attached. That conductor would lead to a high-current bus bar terminal. The charger lead could attach there to a separate terminal. There would be a third terminal for the conductor to attach that feds the primary power distribution switch. However, that is too much complication for a small boat, and I just attach the charger leads to the battery terminal.

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