I bought a used Traxxas Slash (completely stock) for my son about a month ago. A friend of mine heard of our recent hobby and said he had an RC car given to him that he never used because it had no battery. I'm trying to get it going it's a Traxxas Slash. It has a brushless Novak 10.5 motor. I don't have any Lipo batteries, I just have NiMH. So I tried a NiMH battery and it doesn't seem to power on (once it powered on, but seemed to power down within 5 seconds). Do I need a LIPO battery? I noticed the plug is a dean plug. I have some adapters that came with one of my batteries and was using that. Could that be the problem? Any insight would be appreciated.

What ESC does the Slash have?
What are you using to charge your batteries?

Using LiPo batteries is not a necessity, but they will offer better performance. Perhaps your ESC is setup to use LiPo packs and the LVC (low volage cutoff) needs to be turned off while using NiMH packs. Weak cells, batteries not fully charged, and bad solder joint on the wiring/adapters could be issue as well.

Download the manual for your ESC and check the LVC setup. That's the first place I'd start checking.