Battery question

[Tom H31]

I have two marine deep cycle batteries wired in parallel through a Guest "off-1-both-2" switch. One battery is used for starting and the other as a house battery. I found that the engine will not start with either battery by itself, only starting with the switch in the "both" position. I checked each battery voltage with no load. One read 12.7 volts and the other 13.1 volts. With the engine running I measured 14.05 volts on each, which I take to mean the alternator is working properly.
My battery charger seems to be working okay as well, so I am thinking that I may have two failing batteries (they are three years old). Any thoughts or opinions?

 

[Waandderer]

Hello Tom, sounds like bad batteries to me. A Battery can lose cranking amps as it ages even though the voltage looks to be OK. I would haul them down to the local auto parts store and have them load tested, or pick up a cheap ($30.00) load tester from northern tool or harbor freight and test them yourself.

 

[Don S/V ILLusion]

I have two marine deep cycle batteries wired in parallel through a Guest "off-1-both-2" switch. ?

If you correctly describe it, both batteries are in parallel and therefore are for all practical purposes, one battery connected to some yet unidentified post on your switch. If so, that's why you perceive a battery problem.

 

[FastOlson]

Before hauling all that lead off somewhere for testing, first remove all the positive AND negative wires on the battery and engine... one at a time... clean (sand or wire brush) each terminal and reattach with proper torque - and a bit of dielectric lube.
This basic maintenance will "fix" a Lot of apparent low-voltage problems.

 

[Captain Larry-DH]

Listen to FastOlsen. If you have a marginal connection it will limit the amps that it can pull, without affecting voltage readings at low current draw. Usually, the issue is on the negative side but you should remove EVERY contact and clean both surfaces before putting them back together.

 

[Waandderer]

That's all good advise...

 

[Davidasailor26]

I agree with FastOlson too. I think it's much more likely that you have a marginal connection than two batteries that hold good voltage and won't put out amps.

 

[RoyS]

Avoid dielectric grease. It is an insulator. I do agree that you probably have poor connections. There is a reason automobiles typically use a high contact lead clamp connection and not a threaded stud with wing nuts.

 

[Tom H31]

Thanks to all for the great advice!

 

[Bill Roosa]

If you want to confirm that corrosion is the problem measure the voltage at the starter terminal on the solenoid (big red wire) as the engine is being started. Bet you will see 9ish volts at the starter when current is flowing. To find the bad actor measure the voltage drop across each connection starting at the battery. You should see no more than 0.2 (0.05 is much more like what you want BTW) volts drop across each connection. You have to have current flowing to do a voltage drop test.