One of my batteries has started giving me an error message when I try to use it. It says there is a dead cell. The battery is about a year old, roughly. How long are these supposed to last? Is there a warranty?

I think Dji warrants the battery for 6 months only, could be wrong

Yes, unfortunately warranty only covers batteries for first six months of use. You want to be real careful with your batteries, either preserving them or using them when they're at end of useful life.

Yep, warranty cover is six months or 200 recharge cycles - which ever hits first. Batteries can be dead out of the box or last for literally years. It depends mainly on how they are maintained and used.

I am sorry for the inconvenience. I understand that you are having issues with one of your Mavic Pro's battery. The warranty for DJI intelligent battery is 6 months or 200 charging cycles. If the battery is a year old now the best thing you can do is to purchase a new battery for your Mavic Pro so that you will be able to use your Mavic Pro properly and safely.