Problem solved !
To answer my own question, I trickle charged the pack twice (approx. 50 mA). I dissconnected it after 48 hours. Discharged at 10 amps. I've also mounted a thicker wire between the powersupply and the charger.
I've used them several times by now, no false peaking anymore. Unfortunately I trickle charged and changed wiring at the same time, so I can't tell which part is most important. But hopefully somebody can use this.