New Guy with a New Truck

I was just curious about something. If it was the charger, why would one battery charge fine, and then the other be sa far off. it almost leads me to believe there may be a weak cell in the pack. i know it shouldnt charge over 4.2 to a cell but is it possible it only was a cut off for 8.4V instead of the 4.2V per cell? It has never gone above 8.4v. These packs are rather cheap, so i was thinking that they may be the problem. although they run the truck very nicely.