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Batteries
Batteries have three characteristics that need to be considered when building your own battery pack: voltage, amperage and discharge rate. NiCad and Nickle metal hydride (NiMH) batteries have a nominal voltage of 1.2 volts per cell, lithium polymer (LiPo) and lithium ion (Lion) cells have a nominal voltage of 3.7 volts per cell.
The amperage is rated in milliamp hours (mAh). This characteristic is like a gas tank--the larger the tank the longer the engine can run between fillups. The higher the mAh, the longer the battery can be used between recharging. It is common to see batteries rated from 100 to 5,000 mAh.
The discharge rate tells you how quickly the battery pack will deliver its power, it is reported as a C rating. Multiply the mAh by the discharge rate to see how many amps the pack will deliver for example, a 5000 mAh pack with a 20C discharge rate will deliver 100 Amps.
Batteries linked in series increase the voltage of the pack; those linked in parallel increase the amperage of the pack. If you need a pack that delivers approximately 14 volts you can use 12 NiCads or NiMH cells or 4 LiPo cells wired in series.
Assembling the Pack
Some RC vehicles require the battery pack to have a certain shape to fit inside the vehicle. When building your own pack you will need to duplicate the original shape. Once the number of cells and the arrangement of the cells has been determined, you can move forward with assembling the battery pack.
Most packs have the individual cells soldered together, and the finished pack is covered in heat shrink tubing to ensure the pack keeps its shape. Soldering the cells together makes a better electrical connection and reduces voltage loss. Connecting the positive lead of one cell to the negative lead of another is a series connection; connecting negative to negative and positive to positive yields a parallel connection.
When all the cells have been soldered together, there must be a positive and a negative lead that are not connected. This is where you attach the wire harness with its connector.
Things to Consider
It is very important that you look at the amperage rating of your vehicle's speed control and motor before building a pack that differs from the one originally used.
If you look to build a pack with an increased mAh, check the size of the cells; they will be longer in length or larger in diameter and may not fit in your vehicle.
Never run a pack that does not deliver the amperage that the motor will draw. This will damage the batteries and, in the case of LiPo packs, could cause a fire.
Don't build a pack that exceeds the voltage of the motor and speed control; too much voltage will damage the motor and the speed control.
LiPo cells can be damaged by excessive heat and need to be soldered quickly.
Make sure no leads or bare wires remain showing after the pack is complete. Exposed wires and leads can cause short circuits and damage the battery pack.