I bought a JRL Viper for my son for christmas and want to buy the battery pack. It uses a 9.6V 700mAH, I can only find 9.6V with 900mAh or more. Does it matter?

Not really. mAH is only capacity, a 900mAH has more capacity than a 700mAH. That translates to longer run times between charges!

The car may require a 700mAH for just that reason though, to keep run times down. It may be a heat issue & running the car for extended periods on high capacity cells may cause too much heat to build up in the motor and speed control. But that can be controlled by running it for a little while and checking the temp (just by feeling if its warm or hot) and stopping if it seems to be really hot. If it's just nicely warm, keep going & have fun.

Thanks for the info. It makes sense, I'll watch the temp the first couple times he runs it. Thanks again