Battery discharging periodicly

Occationally my Caroo2 will discharge completely when I leave it to charge.   This happens approximately one time out of twenty - which to me means about once a month.   It is very inconvenient to me, as my training is very much depending on the computer.   Often I will have a Garmin Edge 850 or the Caroo1 to turn to as a substitute, but it is annoying to have to spend time on that, when I am all dressed up and ready to go.   I never experienced problems like that with the Caroo1 or the GarminEdge 850.   What to do? 

Ivar