All of Battlefield Sports' equipment has the possibility of displaying the battery change at startup (this can be hidden by activating the software lock on the device). 

However, during gameplay or on devices that are switched off, the battery charge level is not visualised. This is where the volt meter comes in. Just plug it into the charge port of the device and it immediately tells you the current charge of the battery. 

This will save you both time and money, as devices which are sufficiently charged after gameplay do not need to be charged any further.