# 24 volt battery question

Discussion in 'Boat Electronics' started by John Simpson, Jun 4, 2004.

1. ### John SimpsonGuest

I have two deep cycle batteries that are being used to power a 24 volt Minn
Kota trolling motor.

This spring I replaced one battery which was not able to read higher than
10.40 after charging.

The new battery is happy and reading 12.75.

The old remaining battery reads 12.70 immediately after charging, but seems
to drain down to 12.20. I have been reading on calcification and think that
this battery is starting to loose its ability top hold a charge.

The combined batteries is reading 25.25 volts now.

My question is - do I need to replace the old battery that is not able to
stay above 12.20 or will this combination work? Also, is there anything that
I can do to "pump up" the old battery.

Thanks for the help.

2. ### BobGuest

Your 12.20 reading on this battery is a strong indication of a dying cell
within the battery, as this indicates a charge of only approx. 50%. A
healthy battery will show a reading of approx. 12.60 at full charge, after
allowing to rest approx. 1-2 hours after charging. Also note that best
battery life will occur if you never let a battery fall below 50% of full
charge before recharging. The short and simple answer is to replace both
batteries at the same time, as batteries that are wired together also age
together even though one may last a short time longer than another.

What happens is that any battery system, whether wired in parallel or in
series, cannot perform any better than the worst battery in the battery
bank. This means that you would run out of available power when the worst
battery runs out, not when your best battery runs down. In your case you
would be starting with what amounts to a battery system that is only capable
of supplying 1/2 the power that it should when you are thinking that you are
fully charged up.

When measuring the charge within a battery, it is important to realize that
the readings within a battery are very elastic. What this means is that when
a charge has been applied, your reading will be higher immediately after
charging than after allowing 1-2 hours to pass. The same applies to
discharging, with the battery reading lower immediately after providing
power versus waiting to take your measurement.

This information is basic knowledge within the alternative energy industry
as it applies to powering your home with batteries that are charged by
solar, wind, or hydro, but it is just as important to any boaters that run
multi-battery systems.

One thing that you can try, is to apply an equalizing charge to the battery
if your charger has this setting. This type of charge will run your
batteries up to 15.0 volts, and will then hold them at 15.0 - 15.5 volts for
2-3 hours. This info would be for when you charge them separately as 12 volt
batteries. If your charger allows you to charge them when they are wired
together as a 24 volt system, then your equalizing charge would be to
approx. 30 volts.

Bob