I always wondered about it, too, the replacement batteries are
ridiculous, but from HF they are reasonable, but the plug-in end and
latch is always model-specific. The case of a DeWalt battery comes apart
with a bit of effort, and the cells themselves are D-size ganged up
different ways to produce the various voltages. If we were to buy the HF
battery . . . or: Radio Shack and other sources sell the cells
separately (and cheaper). What stopped me was the soldering required to
re-wire and assemble the new pack because I had heard that heat may
cause them to 'splode. Later, I ran across a low-temp solder kit at HF,
but never tried it yet.