Is the receiver equipped with a BEC? What voltage/amperage is it rated for?
A BEC is used to "step down" battery voltage so the receiver does not send too much voltage to the servos. In the early days of R/C, electric R/C cars used to carry a separate battery for the receiver because plugging the car's "main battery" into the receiver would overload it. The circuitry used to eliminate the need for the receiver battery was known as a "Battery Eliminator Circuit," or BEC. Like most mini receivers, the Traxxas 2.4GHz receiver is not equipped with a BEC. Whatever voltage your receiver battery or speed control puts into the receiver is what the receiver will deliver to the servos. The Traxxas 2.4GHz receiver is designed for use with 6-volt receiver packs (4 alkaline cells or 5 NiMh cells) and any electronic speed control.