You can work around that limitation by hooking the battery into itself via an induction motor and induction generator feeding into a loop. You can't feed it in directly since batteries can only accept one input at a time, similar to induction motors. The loss won't be apparent unless you use the debug tab to see the exact how much power is within the battery.
If you want to also add in power while discharging, and you can be sure it will have a consistent speed input, you can use gearboxes and combine both the incoming power and the looping power with a shaft junction, that way you can get some crazy efficient power drains while still charging it up at full speed.
Or, you know, the bug could be fixed where batteries will always draw 100% of their potential all of the time. I know that ElectriCraft runs on a request system, so it's not completely out of the question to do it this way.