No rechargeables! The M31 can only go up to 3.3v. A 3.7v rechargeable will destroy it (as it says in big red letters on Malkoff's website).
Surefire is now selling a 3.2v rechargeable battery, which should be okay for the M31 and the L4, but I haven't heard of anybody actually getting and testing them yet. Otherwise, use 3v lithium primaries.