When charging old ie damaged batteries the voltage tends to increase fast, but they fail to hold much charge. Once disconnected from the charger the voltage then falls away, and under load the voltage falls even more. The basic % charge indicators tend to simply measure the voltage at the time, and don't take any thing else into account. So they are not very reliable. It is not easy to get an accurate assessment of % charge without measuring the specific gravity of the battery acid. I have an off grid cabin with solar charging for 12v batteries, and my first battery suffered from my lack of knowledge and understanding about proper charging (ie don't under or overcharge them). Get a good charge controller!
