Any battery you want to use
If you dig around ts.com, there are some threads/posts about myself using a DK-415 dummy cell! I used 7.2v NIMH RC Batteries with mine, but ANY 7.2v battery type should work! You could make a SMALL 7.2v Lithium setup wired to that and would last for awhile, especially writing to JUST the CF Card
I had Marc Kim aka Leegedy make me a custom DK-415 setup back in 2006 when I bought my SD722, and he put 3x Tamiya connectors at the other end of the dummy cell so that I could hot-swap batteries while the 722 was still recording
So you might want to look into having a similar cable made with whatever battery type you end up using, so that you can hot-swap batteries, if you plan on recording for more than a few hours at a time!