What are the differences between NiMH and Alkaline batteries for the EV3?
NiMH batteries only provide 1.2V per cell whereas Alkaline batteries provide 1.5V (nominal values). This means that new/fully charged batteries will provide a total of 9V for Alkaline and 7.2V for NiMH. The EV3 is programmed to show low battery somewhere around 6.2V and turn itself off around 5.5V. There is no option a available to tell the EV3 the difference between these two types of batteries. The EV3 was designed for Alkaline batteries. You won't get good performance from NiMH with the EV3.

LEGO also makes a rechargeable Li-ion battery pack. Being designed for the EV3, this battery is properly sized for comparable performance to Alkaline batteries. Be aware though, that this battery pack makes the EV3 one LEGO beam width taller, which could require some models to need to be adjusted to accommodate the extra size. Also, when you remove the batteries from the EV3, you will notice a small black circle in the battery compartment. This is actually a button that lets the EV3 know that the battery pack is installed rather than regular Alkaline batteries. This lets it adjust the battery gauge and low battery alarm for the slightly different voltages of the rechargeable battery pack.