It says so right on the screen, but you should have it checked at your earliest convenience. 

My BMW threw a hissy fit last week, and I turned it off and back on again, drove home, took it into the service centre the following morning, they couldn't find a fault, and blamed it on the weather :(

Cheers for the advice mate.Its booked in.I bet they don't find a thing wrong with this one either.

Based on the message, it sounds like an issue with the backup generator. It will likely just mean that it will let you drive with the battery only but won't be able to top up the battery with the generator if it gets low - most i3 owners I know rarely use the generator so this isn't likely to be an issue.