The continuous discharge rate of a battery is very important because if you choose a battery that cannot discharge at the required current, the robot will either not work properly or not work at all.
You selected four 12V motors for your 4WD outdoor mobile robot. Each motor consumes 1A under normal load, and more in the case of a slope. You decide to choose a 12V, 2Ah NiMh battery pack, not caring about the continuous discharge rate. You discover that your robot stops when it encounters even the slightest obstacle or incline. Why? In this case operating all four motors consumes ~4A while an NiMh pack can only discharge at about 1.2 times the capcity (1.2 x 2Ah = 2.4A). The current draw from the motors is therefore higher than the battery can provide.
You selected two 7.2V DC gear motors which consume 1.5A each under normal load, and up to 2A each under stressful situations. This means that the battery needs to be able to provide at least 3A normally and up to 4A safely. If you choose an NiMh pack it would need to be 4A / 1.2C = 3.3Ah. The alternative would be to choose a LiPo pack because they can often discharge at 5C or higher, meaning you would be able to get away with a 4A / 5C = 0.8Ah pack. Granted the capacity is low, and you may opt for a higher capacity pack.
This tutorial has been borrowed from Coleman Benson's post at Robot Shop