Batteries not charging

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Veteran Member
Nov 4, 2021
Vessel Name
Vessel Make
Mainship Pilot 30
So I replaced the 8D battery on my boat, with 2 group 31s which I put in parallel. When I put the battery switch on this bank and turn on the charger nothing seems to happen- i.e. no charging. When I turn on the battery switch to both, it appears to be charging both banks and the volt meter indicates such. When I turn my switch to the old bank (the one not new), the volt meter indicates a steady charge 12.8 volts and will remain like that until I put a load on. When I isolate to the new bank the volts will start out high and then begin to slowly decrease even without a load. I did not change any of the hook ups to my charger. Any thoughts on what might be going on ?
Agree on above. Also, if I'm understanding correctly, you replaced the 8D but still have the new batteries hooked up to an old bank? Old/bad batteries will drag down the new batteries if combined.
I guess- the new batteries are in one bank and the older batteries are in another bank- but I think I may have solved my own question- interestingly when I run the boat (engine) on just the new bank I see charging rates of above 14 volts on the old bank about 13.9
12.8V is not charging. It's only reading the surface charge remaining after a charge and will likely decrease over time... faster if a load is present.
V is not an indicator of charge rate. It will take an ammeter to determine Amps flowing. V is an indicator of state of charge and will/should increase as the bank approaches full charge.
A lower V on one bank may be an indication of decreased AH capacity with age. If/when charging in the combined state the old bank will limit the V & SOC obtainable by the new bank... only as good as the weakest link. That's the reason for the recommendation to replace all at same time if they are combined for charging or use.

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