Low Battery Voltage

[bluedolphin]

1972 22' Aquasport CC with a `82 Mercury V6 200 hp. After charging on land, the house battery reads 13.2v (reading through the fisfinder). After a day, the voltage is down to 12.5v. After another day, it reads 12.4v. I don't know if the other (engine) battery reads the same as the house battery. What's causing this drop in voltage?  Almost dead battery? Something draining the battery? HELP!!!

[JimCt]

You may have a bad diode in the charging circuit or a leak to ground somewhere.

[jdupree]

I don't know if the rectifier is linked into the charging system on a 200 Mercury, but that might fix the tach and the charging problem.  If you will go to themarinedoctor.com it will show you how to check the rectifier to make sure that it is working.

[JimCt]

Have you added anything electrical recently?  If you have, you might want to try disconnecting it to see if that improves the situation.

[warthog5]

Start with probing the batteries with a VOM meter to check their voltage. 12.6V is what your looking for for full charge.

Run the motor on the muff's. Bring the RPM up to 2000RPM. Now prob the batteries again.

You should see aprox 14.2V at 2000RPM.  This will check the charging system.

If that's OK it's time to bring the batteries to full charge and have them load tested.

[John Jones]

you might want to try disconnecting it to see if that improves the situation.

Yep.  Something, either internal or external to the battery is draining it.   With the charger off, it should read about 12.6 like wart said.  It should stay above 12V for weeks with no load.

[bluedolphin]

Thanks, all. Actually, even with the engine running, the voltage reading is showing 12.3 or 12.4 (again, thru the fishfinder). So, like someone suggested, maybe the "rectifier" is not working. I have not had any problem starting the engine, though. Just worried about the low voltage.

[JimCt]

How old is the battery?

[warthog5]

again, thru the fishfinder.

That may be telling you correct, then again it may not. That's why you use a VOM right to the battery terminal's to eliminate this as maybe good maybe not good thru the fishfinder.

[bluedolphin]

Batteries are 2006 (label on the batteries). I will use a VOM to check voltage straight from the batteries. Thanks.

[John Jones]

again, thru the fishfinder.

That may be telling you correct, then again it may not. That's why you use a VOM right to the battery terminal's to eliminate this as maybe good maybe not good thru the fishfinder.

Yep.

[ddd222]

shot in the dark but has anyone suggested a live wire somewhere, maybe in bilge or livewell area?

[JimCt]

ddd's got a good point.  A small crack in wire insulation + saltwater can do it.