KWA, as the first airsoft manufacturer to produce Li-Po ready AEGs out of the box, recommend using 11.1V Li-Po batteries in all of our AEGs.
There are plenty of options available, but there are a few specs that should be followed as well. We recommend a voltage of no higher than 11.1V, and an mAh rating of no more than 1600. The discharge rate should remain around 20c. Batteries with higher ratings may cause feeding, skipping, and cycling issues. In some rare cases, using a higher voltage can result in premature wear as well.
Here is a great example of a battery that is extremely inexpensive, and fits all crane-type stocks that KWA manufactures, such as the version found on the KM4, and our PTS Enhanced Polymer Stock.
The Ronin PDW tanker-style stock, which is the same found on the TK.45c uses a smaller compartment, so a stick-type battery may not fit. The specs on the compartment can be viewed HERE.
Likewise, the AKR Series AEGs have a smaller battery compartment when compared to a crane stock. The specs for that battery compartment can be viewed HERE.
*Li-Po batteries require their own type of charger, separate of those found on simple Ni-Mh batteries. It should be noted that a Li-Po battery will run much longer and more efficiently when charged with a smart charger that uses a balancing function.