Yesterday I bought a new Varta Powersports 508 012 008 AGM 12V battery. Assuming it was mostly charged I immediately hooked it up to my generators starter motor and it started it without problem. After using it for a couple of seconds in total (starting a couple of times) I decided to hit it with the multimeter to see if it was still fully charged or not - i.e do I keep it hooked up or do I bring it to full charge first. To my surprise it read at 10.8V only. This had me worried that the battery is actually already broken when I bought it.
I used my smart battery charger which also would warn me if the battery is damaged. I set it to AGM mode and it was running for a couple of seconds at 0.2A charge current and then indicated that the battery is fully charged still with a voltage of 10.8V.
Now I was really confused and started googling and the information is inconclusive. It seems pretty consistent that the fully charged voltage of a 12V AGM battery should be between 12.7 and 13.0V max. It is also mentioned there is a difference between under load and open circuit voltage, but nothing that would say that 10.8V is a good voltage.
My battery charger has never steered me wrong in the past and has always done its job so I dont really have any reason to distrust it saying the battery is full. The battery is just barely strong enough to start my generator according to its specs, so I would assume if it were well below 0% charge (which 10.8V would indicate according to all the charts I found) there would be no chance that it would start the generator, but it did and still does...
Does anyone know what is going on here? Batteries are always a bit of a mystery to me :(