The EW-DX series transmitters can be used with either 2x AA batteries (Alkaline 1.5V) or with 1x Sennheiser BA 70 battery.

The battery monitoring within the transmitter is matched to these two battery types to ensure the correct battery life time calculation is shown. 

It is not advisable to use the EW-DX with NiMH rechargeable batteries. The voltage of these batteries is too low - each cell is only 1.2V, so with 2x 1.2V you only get a total voltage of 2.4V. Whilst the transmitter will likely turn on and work for a while, as the battery charge drops, the transmitter will soon show low battery and shut down.