Recently I wanted to replace a rechargeable battery pack from a RC car. I tried getting a rechargeable battery pack similar to the original one but that didn’t last even half the period the original one had lasted. So I had to find out an economical way of getting batteries replaced.
Instead of taking a chance on another unreliable replacement battery pack, I decided to look inside the existing one. The plastic shell consists of two parts held together with transparent tape that is easily removed with a razor blade. Inside, there are three industrial tabbed cells of the same length and diameter of consumer AAA cells, without the bump on the positive terminal.
Given the identical cell size, it distresses me that the manufacturer didn’t simply mold a AAA battery holder into the handset. This consumer-friendly feature would have allowed the end user to replace the individual cells using off-the-shelf consumer batteries. The idealist assumes this is a safety feature to prevent errant installation of alkaline cells or mixed chemistries that might catch on fire when recharged from the base.
Regardless, the battery pack is simple enough that a hacker with qualified electrical experience and tools could make a replacement. Using a multimeter, I measured the polarity of the cells. Additionally, I examined the flat metal tabs and wires that connect the cells.
Note the thin metal strip is attached with two tiny indentations and no visible solder. Possibly this was accomplished with some form of electrostatic welding. Also worth noting — I am impressed with the attention to insulating material.
Using a fine point marker, I labeled the batteries (as shown in the first photo in this article) in polarity and connectivity order. To avoid damaging the car or causing a fire, it is critical that the battery arrangement is identical to the manufacturer’s pack. Speaking of which, the manufacturer warns me not to do exactly what I’m about to do.
In all seriousness, this article deals with very real fire and chemical hazards. You could burn your house down or cause bodily injury or worse, simply to save a few dollars on a battery pack. Assembling and/or soldering a battery pack must only be attempted by adults with appropriate knowledge, skills, and tools, and who are willing to take full responsibility for any outcome.
Proceed at your own risk.
Buying AAA Batteries
The label on the battery pack indicates that the battery chemistry is NiMH (nickel metal hydride). It is critical to use the same type of battery as the phone charger was designed for.
The advantages of using off-the-shelf consumer batteries are:
- The newer formulations of NiMH batteries are superior in their ability to retain a charge and in the total number of recharge cycles. I can’t be sure what the replacement battery packs use.
- If purchased from a busy store, the batteries are likely to be fresher
- Probably less expensive than the original manufacturer’s replacement pack, depending on whether you count your labor
- If you alter the device to allow easy future replacement, you can continue to use the device long after the manufacturer’s battery is no longer available
The advantages of ready-made battery packs are:
- Appropriate chemistry and orientation
- Commercial joint soldering and insulation
- Not exploding in your face during soldering
All reasons except the first translate into potentially higher safety and reliability, but that’s not guaranteed. There have been examples in the media of OEM battery packs catching fire due to manufacturer’s defects.
Next, I created a prototype battery pack using two pieces of insulated wire (stripped on the ends) and low-adhesion masking tape. Before installing the experimental pack, I confirmed that the output polarity was correct and the voltage (nominal 3.6) matched the original battery pack. If the voltage is less than 3.3 volts, then one or more of the batteries are backwards or miswired.
Here is the finished self made replacement for the battery pack of the RC car. It now back in nick condition.