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most chargers will not give you the "real" charge benifit of the A123's although you will see some. I use a mastech Pwr supply at 13 amps, but a lot of guys use 15+, most chargers will charge at 6-10 amps depending on cell count, so check your specs just so you don't get real dissaponted with 20-25 minute charge time instead of 10-15, also the A123's like to be warm or Hot to get 100% of thier potential, so charging at 6-10 amps the cells will actually cool durring charging so you will lack power when flying!
I currently have a 450 with the scorpion 2221-6 motor on 3s and performance is very good, flight times around 5 minutes, much better then with my old jgf 450th and lipo. but shorter flight times, A123's are about 75g each so you can figure weight.
what 550 are you flying? generally you will need to go up 1 or 2 cells from lipo, and/or regear.
on the 450, you will be fine with any charger you have, the 450 sized heli really doesn not require the packs to be warm like a larger heli, i don't see much performane diffrence in warm and cold packs, but in my 500, it hardly flies on a cold pack, but my E620 on 15s HV also doesn't really care if the pack is warm or cold, but at 54v I understand why! but my Hurricane 550 and trex 500 their is a big difference
lastly I am still running the same cells for 2 yrs with no real performace change,i have had 1 or 2 cells I had to change out in 1 or 2 packs though.