Nothing really...you just have to be carefull and know what you're doing. NiMh/NiCad batteries aren't necessarily safer....it's more like "it's safer for most people if they use NiMh/NiCad". Or more bluntly...a lot of people are stupid/careless/numb-nuts.
Go to cheapbattery packs...look up a mini 8.4v 1400mah battery in a 3x3x1 rectangle configuration (2 parrallel rows of 3 cells end to end and one accross the top). It should give you the dimensions. That, give or take a little room, is as big a battery that will fit M4/MP5/G36C handguards. That's a "mini" battery made up of 2/3A cells.
Do not get the "butterfly" packs...even though the "specs" are better, they are complete sh*t for anything but low powered stock guns.
If you search around here...there's some 7.4v 2100mah (or 2000 or 2200...can't remember) 15C packs LiPo packs (search for Shelledpants, Tom was selling them at one point...Zippy packs I think). They're about the same size as a mini...probably a little thinner. From what I've seen with them...they're about equivalent to a large 8.4v battery. A little slower ROF...but not much.
The stigma that hooking up a LiPo to your mechbox will destroy it is a bit out of wack. If your mechbox is going to self destruct with a large 8.4v battery or a large 9.6v battery...then it'll break regardless. If it runs well with those and you select your LiPo appropriately...there's nothing different. Voltage is voltage and amperage is amperage. Problems come from when you take a "large" LiPo and hook it up. A 11.1v 4000mah 35C battery will shred pretty much any mechbox pretty quick.
On most rifles that take a mini...you want a small Tamyia female connector. The red (+ve) wire is in the round hole (sometimes they reverse them).
Get a good charger that has balancing ports built in. The B6 Xcharger from airsoftparts.ca is a good one.
Best of luck,
Last edited by m102404; July 7th, 2009 at 11:36..