My Venus was auto updated this night at 4:10. After several minutes indicated battery voltage (system) dropped from 13V to around 4V. Moreover, the logging interval has decreased suddenly as you can see in the picture. This Venus is powered by this battery so it should have been switched off due to not enough voltage, but it's still working.
I've checked voltage measured by Smart Solar MPPT 75/15 which is connected both to Venus and the same (the only one in the system) batter and it shows the voltage above 12V and the system seems to be still working - the solar panels charge the battery.
How to fix it?
The logging interval is set to 15 minutes.