AGM 12V battery unreliable power output

Question

In spring I had bought a 12V AGM motorcycle battery for a starter for my generator. I had quite some issues making it work and wanted to return it but eventually it suddenly started working. Anyway during summer I used the generator quite often and there were no issues at all.

I have not used the generator much in the last two or three months and it has been really cold (-10/-15°C) when I needed to use it again. I was not very surprised that it was dead and used the hand start. I took the battery home and put it on the charger in AGM mode.

To my surprise after 10 or 15 minutes the charger said the battery is full. I was skeptical but thought maybe it just had to get warm or something. I went back to my generator and besides the solenoid engaging nothing happened. I took it home, put it on the charger again and after around 2 minutes it said again that it is full. To verify if it is potentially an issue with the starter of my generator, I put a car starter on the battery (not connected to a flywheel) and it would not start the motor, however it was sparking when contacting.

I then put the battery on the charger again out of desparation and same thing again, after some 30 seconds it said it's full. I put it on the starter motor again and it started spinning extremely slow and slowing down even more over time until after a couple of seconds it started spinning up very fast until after approximately 5 seconds it managed to reach the usual speed. Since then the battery is properly outputting power again immediately.

Thinking back this is exactly the same behavior I had when I had initially bought it, I just didn't really do it this "systematically" last time until it finally worked.

Does anyone have any idea what is going on here? Apparently the battery is not really defective, the charger is fine as well. The battery just seems to need some sort of "activation" before it operates normally.

Answer 1

Your battery is sulfated. I work on lawnmowers and I see this pretty often when people leave the mower parked all winter and the battery goes dead. It happens when your battery is too dead for too long. You need a new battery*.

The symptoms of a sulfated battery are that the battery is weak, but the open-circuit voltage is good, which is also why your charger thinks it's fully charged. However, it has an extremely high internal resistance so when you put a load on it, it can't actually deliver any power to speak of.

*There are some chargers that can sort of reverse sulfation. I've used the NOCO Genius10 charger in desulfation mode to repair some batteries. However, that can't recover the battery if it is really bad, and even if it can recover the battery, it won't have like-new capacity and it may fail in a few months (or it might last another couple of years).

Oh and to prevent this from happening again, either buy a trickle charger or make sure to start and run your generator every month or two to keep the battery topped up. For generators it's a good idea to run them regularly anyways. A common recommendation is to run your generator at ~30% load for 30 minutes every month.

Answer 2

If the battery isn't that old (less than a year it sounds like?), I doubt the problem is sulfation. While all batteries suffer from sulfation, it takes some time for it to build up and start causing issues. The problem sounds to me like how AGM batteries sometimes do, and that is they seem as though they are dead and the won't charge. If this is your situation, what I do to overcome the issue is to charge it in parallel with a lead-acid battery which is can be in any state of charge. Then, use the regular charge (not the AGM), on a low charging rate (ie: 2A or less). This will take a while for it to completely charge, but it should allow it to take the charge.

The problem as I see it is not the battery, per se, but the charger not really doing its job. You can at least try this before you go off the deep end and swap it out for a different battery or buy something new as a replacement.