I am not familiar with your charger but have been charging batteries packs for a few years. When I buy a new pack I make sure it has sermos ae anderson power pole connectors. Then I slow charge them
at less than 1C rate. That is.. your 2100 mah batteries will be charges at most 200mah.. charge till they are peaked the tricke charge them for a long long time. This will condition the pack and tend to equalize the batteries.

then I will use them and start the quick charging with a good automatic charger.. I have a graupner ultra duo 11 and it handles the batteries with TLC. I have never had the problem you are describing.

Your connectors may be not good enough for the usage you are putting them thru, the batteries nay be not quite up to fast charging and are being stressed, your charger may not be able to do the things needed to be done.

I am not familiar with 2100mah batteries.. are are they rc batteries for power, or rx tx usage, or not nicads, or or or.

You need batteries designed for the usage you have to put them thru, connectors to match and a charger/cycler worthy of both. Anything else leads to frustration and no fun at all. Purchace your equipment to match their usage at a good site.. the ezonemag.con site has many forums you can post your questions/delemas and get answers that make sense.. Just remember to were yout filtering glassed to get thru the bull and locate the meat.

Have fun

Jarvis