In theory, inflight you should not lose the Battery if the charger fails. As far as I can tell from our manuals, the HOT BAT BUS can be powered by the Battery, the Battery Charger from the R MAIN AC BUS (normal) or via the BAT SWITCH from the L DC BUS if the charger is not doing itīs thing. So you should not lose the HOT BAT BUS.
The Boeing QRH often does not tell you when you need to divert, f.ex. the STANDBY BUS OFF checklist ends up telling you you have got 30-90 minutes of standby bus power, but it does not tell you to divert (or land asap).
I have had an issue with "MAIN BAT CHARGER" STATUS msg on 757s at least twice. In both cases I can recall it was on the ground pre-departure.
In the first case, Mechīs tried swapping APU bat charger (identical) but no change. Turned out MAIN BATTERY was gone bad.
Next time, instead of waiting for Maint to do any swapping I had them look at the battery, same thing. Replace battery. So the status msg may indicate that the charger is unable to charge the battery.