XRAY - Model racing cars

I recently ordered a M18 with a powerpack. A week ago I received the car and yesterday i received the powerpack. I installed everything and I wanted to charge the battery. I have a Robbe Reflex charger, who charges first at a soft rate, then a fast rate (650 mA) until the Delta Peak circuit terminates the process. Then it goes to a topping charge and a maintenace charge. I never had any problems with this charger for all my batterys (transmitter, reciever, starterbox, batterys for toys,... nicd and NiMh) for the last years.
But with the battery from the powerpack i have a problem. I charged it a couple houres in which they should be fully charged. But nothing happend when connected to the car. I measured the pack and i read only +-1.2 V. So i reconnected it to the charger. It made a little crisping sound in the beginning for some minutes but that went away. I measured the pack again after some time and now I measured only 1V. I though I connected the pack wrong to the charger but thats not possible because the charger wont charge if + and - are wronly connected.
I checked with other batteries and they are charging normaly.
So what could be wrong? Are that special batterypack?
(the pack is a 5 cell, 6V 1200mAh pack? correct?)