Spring will be here someday even though we still have snow on the ground.
Some of you may find that you have batteries of doubtful provenance or uncertain age. While it's tempting to just replace questionable batteries out of hand, it's expensive and possibly unnecessary.
The best way to know whether your batteries are still good is to run a drain test on each one. It's simple to do:
1) Charge the battery.
2) Connect a load that draws a known, consistent number of amps.
3) Measure the time until the voltage drops significantly.
By multiplying the amps the load draws by the number of hours the test takes, you can find out the actual amp-hour capacity of the battery. If it's less than around 75% of the rated capacity of the battery, it's time to go shopping.
I made a load test setup with two 12v
50W light bulbs, which draws around 9 amps, because 12 volt
light bulbs draw around 10% more than their rated current at the 12.6 volt discharge voltage. I used a porcelain lampholder and one of those y-shaped adapters to allow two bulbs in a single socket. The parts cost less than $10 at a home center. Not voltage or current measurements are required, since it's obvious when the lights become dim. I put the whole thing in a ceramic dish to protect the floor from heat.
The group 31 starting battery that the PO put in my Cayo camper ran for just under 12 hours, for around 105 amp-hours, better than I expected.