My second thought would be the SuperCap issue. It's my understanding the battery power only kicks in when the SuperCap goes "dry". And it only goes "dry" if the solar panel isn't receiving any sunlight for days to recharge it (unless it's gone bad). Sunlight during the day could power the ISS, but if the SuperCap has gone bad, the only power at night would be from the battery, unless the battery has run out of juice because it had to supply power for several nights. If the SuperCap is the problem, replacing the battery may make the message go away for some period of time, but only until that new battery ran down again.