batteries

Lead acid batteries in a stable open circuit condition will be around 12.6-12.8v when fully charged. The 13.1V indicates that either the battery is charging or has recently been charged. An hour or two at open circuit is sufficient for the battery to stabilize. So your voltage greadings are normal. That doesn't really tell you much about the battery capacity though. Capacity is measured in amp hours. To test that you need to leave a known load (in amps) on the battery for a specified time (in hours). Amps x hours = amp hours. Take out something like 50 amp hours to get a good capacity reading. You'll need a timer and an ammeter. If your battery gets below 12V then stop. There are inexpensive DC clamp on multimeters on Amazon.

When you test right after turning power off, you are getting a surface charge reading. I think this will be confirmed, but it is recommended to wait before testing. The 12.88 is likely the true state of charge reading since the battery had time to even out.

Good question. After my personak experience with 16+V from my WFCO, I wouldn't. That would cook 'em. I'd consider getting a Progressive Dynamics charger future proofed with a lithium setting in case you want to change over later.

A little battery 101.

Flooded batteries need higher charge voltages occasionally. Flooded batteries tend to stratify, with the heavier sulfuric acid (lead acid battery electrolyte is a combination of water and sulfuric acid) settling to the bottom leaving lighter water on top. Eventually this can cause damage to the battery.   Once a battery is fully charged additional charging results in electrolysis of the water, creating hydrogen and oxygen. The bubbles this forms mixes the electrolyte and counteracts the stratification. This process is called equalization, and good chargers have an equalization cycle at intervals if you select the flooded battery option.

AGM and gelled electrolyte batteries (generically called valve regulated lead acid batteries) fix the electrolyte in place either by gelling it or absorbing it into glass matt's so they don't stratify and don't need equalization charging.

However, they also have less electrolyte and no way to add more. They are aren't really sealed, just have valves that regulate the internal pressure, hence the name VRLA. The valves are needed to vent the gasses if they do get overcharged. Otherwise they will try to recombine (burn) the hydrogen and oxygen internally, overheat and can explode. Not good. So be sure you do not charge AGMs or gell batts using the flooded battery setting on your charger.

So the pros of AGMs are no stratification and no need to equalize or add water regularly. The cons are high cost and lack of a way to add water if they are overcharged. Not much if any difference in capacity or cycle life.

Hope that helps.

Not to confuse things, but I read somewhere that one should not charge AGM batteries with chargers for Lead- Acid batteries.  Could that be part of the problem?