battery won't charge when on but charges when unit is off.

[AC Question]

Samsung tab 2 10.1 , has red X over battery icon.

 

[mstrblueskys]

What power brick are you using? It sounds like you might not have a powerful enough power source trying to charge the tablet and it's discharging faster than it's charging when the system is on.

I'd suggest trying a different charger/cable and seeing if that fixes things.