I just looked and I have 2 Flightmax 1S 138mah batteries, and both died very shortly after use. They are not recommended for the blades if you look at comments on the HK site. Too many people have had problems with them. A lower charge rate is really important in this case. I do have 3 Turnigy 160mah batteries, and those are same size and much better in my MSR and MCX.