We're using 2 cell Lithium Iron Phosphate (LiFePo4) batteries. They have less power output than Lithium Polymer (LiPo), but are much safer (no fire danger like LiPos), which allows us to charge them inside tanks. These are 3.3v/cell (unlike Lithium Polymer (LiPo) which are 3.7v/cell). At 6.6 v. for a 2 cell pack, (about 7.2v fully charged), they work grea,t and we've been using them in most of our tanks now for about two years.
We buy them from Hobby King and use the 2s
4500 size (cost about 25 bucks each) mostly (due to its ability to fit inside small tanks), but on some tanks (such as heavy battlers with lots of room inside), we use the larger size
8400 which are about double the size of an 4500. The 8400s fit nicely in Tigers and snugly in King Tigers. A typical battleday of 6 hours or so consumes about 2000mha, so we can go for 3 or 4 battledays without a recharge. Plus, they don't lose a charge when stored, unlike a NiMH or NiCad.
As to installation, we use an
adapter to convert the 4mm plugs to a Deans' connector (which we install on all our DMD and MFs) and an
extension to run the balance tap to our charging port location.
You'll need a charger that can do the LiFePo4 battery too. Not all LiPo chargers can. I had to buy a new charger and got a Thunderpower TP-610C and a Cellpro 10XP, which I like.