Charging HiPowerRC LiPos.

Our LiPos are currently sourced by ACE Power. Most of their packs including our 1800mah and 4000mah 2S packs should be charged at 1C. This means the following: an 1800mah ( that is 1800 milli-amp- hours) battery pack is also 1.8 Amp Hours. That 1.8Amp figure would be used as the 1C charge rate: (1.8A x 1). Same goes for the 4000mah pack ( 1C charge rate = 4.0A x 1).
We recommend that one charges with a balance type charger. The packs have a balance tap / plug which is standard configuration that is utilized by most lipo chargers. A balance charger will isolate each cell in the pack and make sure that each cell is charged to the correct voltage ( up to 4.2V). By balancing the pack, no cell will be over or undercharged. This is important especially during the charge or discharging of the pack. This will make sure the pack's cells won't be overly discharged or charged, resulting in a much longer lasting life and safety of the battery pack. Never under any circumstance charge with a Nimh charger or use the Nimh charge settings. If your charger has a 1s, 2s, 3s, 4s, etc. manual setting make sure it is set for your battery pack. 1S stands for 1 cell (3.7V) 2S  for 2 cell (7.4V) and so on.
Our own experience with our ACE battery packs is that they can handle somewhat higher charge rates, but for safety sake, 1C is recommended. Always supervise charging of Lipo battery packs and check for damage or "puffing" of the pack indicating damage. Never overstress the battery pack beyond its discharge rating and never fully discharge a Lipo pack. Most newer ESC and all Brushless ESC have lipo voltage cutoff options built in to limit discharge threshold.