I was charging my 4s this morning, and I noticed that one of the cells had exceeded 4.20V and was 4.21V during charging. I immediately disconnected the battery afterwards. What could have caused this to happen? Bad Battery or could it be my charger's internal balancer? Thanks~

4.21 is fine. Alot of the higher end chargers are now adjustable at their cut off. Some are charging to 4.22 per cell wih no issues.

I agree, totally ok !!

Just having some static electricity in the air can make your voltage creep up that much.

I wouldn't be concerned with that, unless the pack start to puff up! haha

Thanks guys, and its safe to store lipos at 3.90V per cell right?

3.6 to 3.8 is ideal.