The NiMH batteries are likely in series. They have a capacity of 600mAh. The Lichee Nano that this board is based on has a power consumption (with display of 25

The NiMH batteries are likely in series. They have a capacity of 600mAh. The Lichee Nano that this board is based on has a power consumption (with display of 250mA[0]) [0] https://media.digikey.com/pdf/Data%20Sheets/Seeed%20Technology/102110201_Web.pdf https://media.digikey.com/pdf/Data%20Sheets/Seeed%20Technolo... So even if you can't fully discharge the NiMH batteries you should comfortably get an hour of battery life out of this thing, much more, if you don't use the display.