Modders question

Well, I was going to say you could get the Item singleton out using Items[ItemStack.itemID] and ask it if it's instanceof ElectricItem -- if so, as the API says, call charge with ignoreTransferLimit and simulate both set to true... but ElectricItem is final and the things IElectricItem has don't seem to help.

If you're just interested in fullness, you can try to get the damage value -- I believe it's 1 for full items and something like 31 for empties (don't quote me on that, though, I haven't checked recently).

Am I helping you go in the right direction, at least?