Hi, welcome to RCU. Allow me to make a few assumptions because you didn't specify.

(charge at max recommended rate and only doing one pack at a time)


4.4 x 8c = 35 amps and 16.8 volts is around 600 watts. I'd recommend the i-charger 306B , but you'll only get 30 amps from it. Beyond that there's FMA PL6 at 40 amps. Both available at ProgressiveRC http://www.progressiverc.com/chargers

Of course if you aren't in a hurry then numbers can be cut in half , quarters, eights, ect. On a budget maybe one of the many 200 watt 10 amp chargers or one of the many 400 watt 20 amp chargers will suffice.