Lipo Balance charging

[PJC]

I am new to Lipo and have two new 4600mAh 45C Lipo batteries.
After running each pack in a five minute race I then balance charged them at 4.6 amps, both needed the same capacity to charge them (approx 1200).
One set took 20 minutes to charge , the other over 90 mniutes. Any reasons why that might be. Does one pack have a problem? Is there a way to test it?

I didn't notice any problems while running the car.