It happened to me by coincidence, let me explain.
There are few conditions to make it work, first of all you need a diamond drill (didnt try with normal one) and BatPack. Charge them.
Secondly you must have in your inventory electric wrench and electric treetap, charged too.
And this is it, when you mine with DD, BatPAck will discharge, and eventually diamond drill too, but not fully.
Here's the bug, DD will discharge to some point, there'll be a short red stripe under DD, but it won't loose EU fully.
And you can mine with DD whole day without charging BP or DD.
You'll ask why do I need electric treetap and wrench for ? I don't know either, but it works only with those items in inv.
ps. using minecraftforge 1.3.1