Try discharging the battery completely by running it in your Phantom (with the props and motors off) until your Phantom shuts off. It would be best to fly until the battery is down to at least 30% -- so, it doesn't take so long for the battery to be depleted. Also, it would be a good idea to do this in a cool place and/or point a fan at your Phantom to keep it cool. Once the battery shuts off and cools down, recharge it and check if it's still reporting a bad cell. If so, then it's time to replace it.