It us not. I programmed Accu controller. The best way is to track the enery coming in and out. Using only voltage is imprecise bevause the voltage/charge graph

It us not. I programmed Accu controller. The best way is to track the enery coming in and out. Using only voltage is imprecise bevause the voltage/charge graph look a bit logarihmic, so the more charge you have, the more presision you need to have while measuring charge.This graph as well as total possible energy capacity and ability to Charge depend on Temperature. Also you normally have multiple cells that age differently.