Check the paperwork/instructions to see if that ESC has a LiPo cutoff setting that you can turn off/on. If it does, you cannot run NiMH with the LiPo cutoff turned on. If you are running NiMH just disable the cutoff, but remember to turn it back on when running LiPo. It's a safeguard to keep from running your expensive and volatile LiPos to a dangerously low level.