so i spent 4 hours testing, and there is no way to charge batteries with no input, i can make a batterycharger class, and block, but no matter how i edited the methods, i coldn't get it to charge unless i connected another battery bank, or solarpanel to it. i tried fooling it to think it was powered but could never get it to work.
also this all requires editing the csharp.dll, which would require users have that edit to play on your server, as well as no EAC.
i did create a recipe for a charged battery using a dead one and a bottel of acid, but the result was always a level 1 battery, but that may have been because of my skills, now that i think of that, im sure that was the case.
so best way to solve this issue is a solar panel to charge the batteries,
second best would be generator
third best would be recipe, its not automatic like you want, but quite easy as far as creating a modlet to solve this problem
i have created the modlet, here is the download link
BatteryRefill Modlet
i did test and verify the modlet, as well as intellect level affecting the level of battery crafted