I don't know which build introduced these, but I discovered that they only have two redstone behavior modes: "emit if active" and "emit if not active". The non-chargepad versions of these have more modes, and I especially liked the default "Nothing" mode, which allowed me to use comparators to get a redstone output level relative to how full the energy storage was (e.g. 0=empty, 1=barely full, 8=about half full, 15=mostly full). Would it be possible to add the modes from the non-chargepad energy storage blocks to the chargepads?
I'm guessing that's there to make the charge-pad not be superior to the normal energy SUs. Huh. Or maybe TD was just lazy.