ORIGINAL: bgosselin
.............Charging a 6s pack at 7A requires up to 176.4w of power (4.2v/cell * 6cells * 7amps). Again, to account for efficiency losses, add in another 20% for 211.68w. At 12v PS output, it has to supply 17.64A of current.
And it gets worse! Your current demands appear to be loading the supply causing a voltage sag which in turn increases the current load. Using bgosselin's calculations of 211 watts, by the time your voltage has fallen to 10.5v, current requirements are over 20 amps.
Thermaltake claims efficiencies over 80%, even higher if you're on 230v mains (voltage setting on back). What does the spec placard list for 12V1 and 12V2 amperage and wattage - there are several models that are rated at 500W.
Drop your charge rate to 1C and try again.