Yeah, a battery that won’t hold a charge is a dead battery. If you really want confirmation, take it to the nearest AutoZone/Advance Auto/O’Reilly’s and ask them to test it. Bet it fails.
So whatever you do, do not waste money on one of those 70-80 Amp Hour “marine” batteries. Lead/acid batteries have a lifespan of 300-500 charge cycles, max, and that assumes you discharge it only 50%. Currently (as of the date this is being written) the way to go is to buy the best built and cheapest LiFePO4 battery on the market which I am seeing for $549 online via Amazon or the WattCycle store: the 314 Amp Hour Mini. Cheaper, but lesser quality, $339 for the Dumfume 300 Amp Hour (rated 4,000 charge cycles). Will Prowse has tested both batteries and torn them apart to examine quality of construction, and he recommends them for those applications needing 12 volts. The WattCycle battery is rated for 6,000 charge cycles, should be charged to 100% capacity and can be drained to 20% (for that max life) which will give you 18 years of daily cycling between 20-100%. You can drain it to only 10% and you will still get lifetime service out of it.
I am in a similar boat and my old 6V batteries are done, they were cheap and I did not get a long life from them...
What are your thoughts of swapping the 2 6V to 2 12V and rearrange the wiring for total 12V, such as the Dumfume 300?
Get a Lithium battery and then forget it and all power issues.
