Red light, alarm, no display / how to charge the battery? 6 hours not sufficient...?

When I power on the infusomat space, I get this horrible alarm sound (every minute or so) and red light. The display does not light on and so I have no error code. It only stops if I stop the device. I guess my battery is depleted. However I read that it can be charged by leaving in 6 hours on. And the 6 hours have elapsed, and nothing changes. Is there any other test I can do? Something to charge the battery? Or is it not the battery? Thanks