Step 1: Disassemble
Step 2: Solder Wires to Battery Terminals
Step 3: Reassemble and Choose Battery
Rechargeable batteries vary in voltage, current, type and size.
This particular RC truck used 6 AA batteries which means that they control board has voltage regulation and circuitry engineered for 9V. The servo and motors are also designed for 9V. AA batteries are 1.5V. 1.5V x 6 = 9V. A 8V NiMH was chosen for this application.