Batteries not charging

[nveater]

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 ?

 

[SteveK]

The batteries are fully charged? Charger shuts off.

 

[Mac2]

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.