6v battery replacement

Jwhy

.
Feb 11, 2013
98
Catalina 320 Kilmarnock
I have four 6 volt batteries purchased at the same time, wired into two circuits. One of the circuits has much less cranking power than the other suggesting at least one weak battery.
Should I replace the single weak battery or should both be replaced?
 

SG

.
Feb 11, 2017
1,670
J/Boat J/160 Annapolis
Jwhy: What type of batteries? (AGM, Gel, Lead Acid Sealed, Lead Acid with "fill-caps"?) How old? The hydrometer might not be the path to a diagnosis.

Have you checked the connections of the batteries? (Voltage is only one of the issues. When a battery is asked to send heavy loads, the voltage you measure at "no- or low-loads" is only a starting point.) You could have an inadequate ground, a bad hot jumper, etc..

When you say "One of the circuits has much less cranking power than the other suggesting at least one weak battery" , how did you determine that? (e.g., the starter just doesn't turn on one bank vs. the other; or, was it an actual load test.)
 

Jwhy

.
Feb 11, 2013
98
Catalina 320 Kilmarnock
Lead acid with fill caps. All about three years old. Using the Perko Switch, one setting cranks the starter instantly and fast. The other setting comes on with a pause and a slow turn that strains to spin fast enough to start the engine. Haven’t used my load tester or hydrometer yet.

I did clean all the battery terminals and wires that attach.

Is there a problem dropping one battery into the set?
 

SG

.
Feb 11, 2017
1,670
J/Boat J/160 Annapolis
Hydrometer is good to start with.

You could exchange one battery in each bank. Th en see what happens. If you are right about one battery being the issue, something will happen. Then depending on what happens, I suspect that you might have exchange one of the batteries form the “good” sEt to find out which one is bad.

However, you might have a bad cable or terminal fitting. If your sure that’s not the issue.
 

DArcy

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Feb 11, 2017
1,691
Islander Freeport 36 Ottawa
Start with measuring each cell with the hydrometer; that's usually the quickest indicator. If there is one battery that is the problem, you could also find the offending one by measuring voltage across one battery at a time while cranking the starter. Keep an eye out for excessive voltage droop on one battery.
 
May 24, 2004
7,129
CC 30 South Florida
I had that same issue in a Golf cart. Six 6V batteries in series producing 36V. The batteries were 2 years old and the cart was sluggish, I checked the voltage individually for all six batteries and determined that they were all within 6.2 and 6.3 Volts (of course they had found equilibrium as they were connected to each other) I then used an automotive type load tester which confirmed they all had acceptable voltage but when a load was applied there was one battery with a large voltage drop. All batteries had been well taken care off and I figured this single battery may have had a manufacturing defect. I could not see replacing 5 batteries which tested good and were only 2 years old so I just replaced the bad one. That was a year ago and the cart is running like a champ. As you have only two batteries to test just charge them and disconnect them and test them individually with a load tester. If one is bad I would go ahead and replace that single one. If both test good then your problem is in the wiring.
 
Jan 18, 2016
782
Catalina 387 Dana Point
GC-6's are pretty inexpensive comparatively. Replace the pair. (Dx advise upthread is all valid)
 
Sep 30, 2013
3,538
1988 Catalina 22 North Florida
:plus: on the hydrometer or you can look into a refractometer tool. They work very nicely.
That's a great tip! I used to use refractometers for testing salinity in aquariums. I never knew they could also do batteries! Seems like a much better tool than a hydrometer. Very accurate, clean, and easy to use. I like it! :thumbup:
 
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