Rhino 5s1p 4900mAh 20C Lipo Problem

[Mentat]

HI guy, got to take my 8ight-t to the track yesterday for the first time and loved it. I have 2 batteries a Turnigy 5000mAh 25C 4s1p ZERO issues so far. Then there is the Rhino 5s1p 4900mAh 20C pack. after 1 charge/discharge its now showing only 4 cells when i connect to my charger which is a Thunder T6 6 cell charger/balancer (pic below) this charger has worked flawless with my 2 other packs i have which are for my slash they are 2 cell lipos. Is there some test i can do to find out if i really have a bad cell? Currently the cell is discharged i used the castle creations mamba monster ESC with the voltage cut off set correctly to 15v for a 5 cell pack and it stopped running just like my 4 cell did yesterday when it got to low. Not sure what to do at this point. I know the charger has Lipo Balancing/Lipo Charge/Lipo Fast Charge/Lipo Discharge/Lipo Storage settings i have only ever used lipo Balance on any of my packs. There is also a settings that i cant really make sense of, it reads like this (silly chinese manuals) Heading is as follows:

Voltage Balancing and monitoring during the discharge.
The processor monitors the voltage of the individual cells during the "storage-mode" and the "discharge mode" of the lithum battery pack. It tries to normalize the voltages to be equal. For this feature, the individual plug of the battery pack should be connected to the individual port of the charger. (is this talking about the balancing port? any way here is the rest) If the voltage of one or more of the cells varies abnormally during the procedure, it terminates the process forcibly with the error message. If this happens the battery pack contains the bad cell, or the bad connection of the cable or plug. You can easily know which one cell is bad by pressing INC button at time of showing the error message.

This is word for word from the manual. At any rate i always charge my packs at what i believe is 1C which in the case of the 5 cell pack is 4.9 AMPS because its a 4900mAh pack (correct me if im wrong). Do i need to cycle (charge/discharge) a pack before i use it? I had thought Lipo was different. HELP!