My son and I have a couple of cheap 4 wheel drive remote control cars that we love driving around and racing with. We specifically went for cheap cars since he's only young, and there is a significant chance things will get broken, and it's not so much of a problem if your car only cost $20, rather than $200. Of course, being cheap they don't run particularly quickly, and they absolutely chew through AA batteries like they're going out of fashion.

After a few months of replacing batteries, I figured it would be nice if we could use rechargeable lithium ion or lithium polymer batteries instead. Looking at the car, I figured it would be pretty easy to do, and if I was careful, I could still use AA batteries in a pinch. So, I set about modifying the car to take some 2 cell lithium ion batteries I'd made myself (instructable for that coming soon...).

Supplies

Parts required:

Lithium ion (Li-Ion) or Lithium polymer (LiPo) battery
Wire

Tools Required:

Dremel, files, knife or clippers to make any modifications to the car
Soldering iron
Multimeter (optional, but very handy)

Step 1: Pick a New Battery

The first step is to choose what kind of battery to use. Lithium polymer (LiPo) batteries are pretty common for remote control vehicles of all types these days, and can be found quite cheaply, lithium ion (Li-Ion). The main choice is down to battery voltage and capacity. Voltage is the most important choice, as capacity simply determines how long you get to use the car between charges.

Which Voltage?

The voltage needed is going to be determined by how many batteries your car uses. Mine uses 4 AA batteries, so runs on 6V. The closest choice to this is a 2 cell battery, which is 7.2V for Li-Ion or 7.4V for LiPo batteries. If your car only uses 2 AA batteries you can get away with a single cell Li-Ion or LiPo battery. Cars using 3 AA batteries could be problematic, as the voltage from a single cell battery will likely be too low, and I don't know if a 2 cell battery would be too high.

Which Capacity?

Capacity of batteries is measured in milliamp-Hours (mAH), and typical capacities for RC vehicles range from 100 mAH up to 2000 mAH or more. Larger capacity batteries will give longer run times before needing to be recharged. As the capacity increases, so too does the physical size of the battery, so it pays to make sure you know how much space you have, and how big a battery can fit.

Step 2: Modify the Car

Initially I considered modifying the car so the replacement battery could be installed in the existing battery bay. Having thought about it, I ended up deciding not to so I could continue using AA batteries if required. I looked for places to mount the new battery and decided on the back made most sense. I could easily remove the spare tire, and had space for a 2 cell Li-Ion battery pack. I did need to clip out a couple of pieces of the roll cage, but other than that, no major modifications were required.

In the battery bay I needed to be able to connect the new battery to the existing terminals. I ended up cutting two slots in the battery door to run cables out to the battery. I can solder wires onto the appropriate battery terminals without preventing me from installing AA batteries if needed. I used a Dremel to do this, but it could just as easily be done with a knife or small files.

Step 3: Install the New Battery

As mentioned in the previous step, I added two wires that ran to the top of the car. This makes it very easy to change batteries when one goes flat. It is necessary to work out where in the battery bay these wires should be soldered. A multi meter comes in handy here.

There should be diagrams showing how to insert the batteries. One end is the positive, and the other end is the negative. The batteries are typically connected in series, so the negative end of one battery is connected to the positive end of another. Two of the terminals will not be connected to the terminals of other batteries, and this is where we need to solder our wires. Using a multi-meter here makes things much easier, but it should be possible to identify the right terminals visually.

With the wires soldered on you can put the battery door back on, and connect the battery. Ideally the wires from the battery bay should have an appropriate connector for the battery being used. In my case I'd kludged together a battery pack, so I didn't have a connector. I've made something that works, but I definitely recommend using proper connectors to prevent short circuits.

Having installed the battery I tested out the performance of the car. I am pleased to report that the car absolutely flies now. The slightly higher voltage makes the car much faster, and doesn't appear to have had any negative effects on the car's electronics. Whether or not the higher voltage is going to burn out the motors quicker remains to be seen.