Battery Voltage (7 and 10 = 12)

[SwiftCloud]

In short: One of my batteries is showing 10 volts, the other 7. When I switch to "both" they show 12 (and can start my F225). How can this be? I'd figure it should show 8 or 9...

Background
My AGM batteries were left on the boat (an oversight by my boat guy) all winter (Boston). They must have been drained during the winterization process (wouldn't be the first time my boat guy left a pump running) because the voltage (as displayed on the Yamaha gauge) was 10 for battery 1 and 7 for battery 2. When I switch to "both" the voltage shows 12 and will start the F225. If i then switch to just battery 2, the engine will run, but quits if I hit the electric trim.

I have not checked the voltage at the batteries, but did check to make sure they are not set up for 24 volt.

Ideas?

 

[Curmudgeon]

:uhm Charge your batteries separately, then see how they act. Might be trashed, might not ...

 

[seasick]

In short: One of my batteries is showing 10 volts, the other 7. When I switch to "both" they show 12 (and can start my F225). How can this be? I'd figure it should show 8 or 9...

Background
My AGM batteries were left on the boat (an oversight by my boat guy) all winter (Boston). They must have been drained during the winterization process (wouldn't be the first time my boat guy left a pump running) because the voltage (as displayed on the Yamaha gauge) was 10 for battery 1 and 7 for battery 2. When I switch to "both" the voltage shows 12 and will start the F225. If i then switch to just battery 2, the engine will run, but quits if I hit the electric trim.

I have not checked the voltage at the batteries, but did check to make sure they are not set up for 24 volt.

Ideas?

Batteries are just about totally discharged at 11.5 volts. Your readings are not the actual battery voltage, you have a bad connection somewhere, probably a ground at one of the batteries. Since the batteries start the motors they are OK.
Also, you can't assume that the voltage of two combined batteries is the average of the individual voltages. In fact, the combined voltage will be nearer to the higher value. The higher voltage battery( the one with the better charge) will 'drive' the lower battery and will in effect charge it as it discharges itself.

Contrary to a lot of folks opinion, leaving batteries in the boat for the off season is fine as ling as they are charged. If there is no drain on them just charging them before storage is OK to last the entire winter usually. If there is drain, for example, bilge pumps cycle or the like, then they have to be watched and charged if they run down. As these batteries discharge, the freezing point temperature of the electrolyte rises. The risk is thast the electrolyte will freeze freezing and that can damage the cells permanently.