Below is an email I sent to traxxas. I know they don't warranty batteries but I am trying to pinpoint what happened. Let me know if anyone has some insight. The 3000 mah batteries have been fine in both vehicles.

Email:
I recently purchased (2) series 4 NiMh 4200Mah batteries. I originally purchased them for use in an also newly purchased E Max. I charged them using newly purchased TRA2970 chargers according to the instructions provided and used the 1A charge rate. With the cold weather, we were only able to run the 4200 batteries once. I then purchased a R-Evo for my other son and we finally were able to go out and use our new toys. After a few minutes of my son using the 4200 batteries in his R-Evo, first time he was able to use the unit, we heard a pop. I pulled the batteries out and one had popped with black fluid around it. The second battery pack did not pop but there is fluid leakage.
My first question is do you guys cover this under warranty? Secondly, I have read through both manuals and I cannot find anything I may have missed or done incorrectly to cause this. I personally run LiPo batteries so I tend to be overcautious if anything.

Purchase date for batteries is February 10th, 2015 from Amazon.com.

Let me know when you have a moment and thank you for your time.

Tripper

In my experience, the higher capacity NiMh cells are not suited to high current draw - I think your car just pulled too much current from them. That being said, you bought them via Amazon, so raise the issue through their system and you ought to get them replaced or refunded.

Thanks for the input. Traxxas is going to check them out and at least tell me what went wrong if they can. In the meantime, I guess I'll buy the 3000 mah batteries for spares or price out a brushless and lipo setup for each of them.