These packs are built with a lexan plastic shell around the outside of the battery, and then covered in blue heat shrink. The lexan shell is designed to protect the battery cells while in the vehicle, for longer life. Note that due to the lexan shell, sometimes the battery may feel soft to the touch on the outside of the battery; this is normal as there is a small air gap between the lexan shell and the battery cells inside.
Why must Li-Poly receiver packs be used with a voltage regulator?
Although the Li-Poly battery pack’s nominal voltage is 7.4V, when it is fully charged the voltage output is 8.4V. The 8.4V output exceeds the safety level of most receivers and servos on the market, so if you are not using High Voltage servos, you must use a voltage regulator with these Li-Poly receiver packs to decrease the voltage to safe levels that your receiver and servos can accept.