When plugged into your Hexoskin shirt, the LED will be:
|LED status||Battery status|
|Solid||More than 70%|
|Short blinks||30% < charge < 70%|
|Slow blinks||Less than 30%|
So basically try to charge your device as soon as the top LED blinks.
You can also take a look at the mobile app in the bottom right hand corner of the home screen: