When I try to charge my 500 series roomba, the "clean" light flashes yellow once or twice, then changes to red an the check roomba light flashes alternately. Roomba was run to dead, then left off the charger for several days. The first time I charged it, it ran fine, but the second time, it started doing this. I thought maybe a cell in the batter died, so I just put a new battery in. Same thing. Flashes yellow twice, then red and check unit light is flashing red. Any ideas? I think a cell blew in the old battery (whether that was because of the charge issue or the cause of the charge issue), but like I said, just put in a new one and same problem. I also reset it (took battery out, pushed and held on button for 10 seconds). No change.