Funny you should say this, the first time I bought GensAce batts I had 2 out of 6 batts totally lose two cells internally, HobbyPartz replaced them immediately no problems. In fact they were 3s and I exchanged them for 4s on the return and they were more than happy to let me substitute. This was on my EF MXS, and after trying the 3s I didn't like the lack of power so I switched to the 4s.The good thing is he has excellent customer service. If that happened with gens ace packs you might have to call it a loss