Many in video production have been doing this for years. Many of the $600 battery packs out there are just a handful of "C" or "D" batteries and the "packs" that are sold routinely go for $200~$800 apiece.

People used to freak out when they were told that my Sony battery and charger system was $4K.....

::::::: No disrespect to Axiom, but my favorite woofer is my yellow lab :::::::