We're all about having fun here at Higherwire. That's why we were interested when a customer asked us to replace the battery in their son's RC car with something more powerful and with more capacity.
This is an inexpensive 1:12 RC car from Amazon that's sold under the Hosim, Bezgar, and other brands. They're surprisingly fast and decently reliable for the price, but they come with an anemic battery from the factory.
This Bezgar came with a 2S lithium-ion battery consisting of a pair of SF18650NR-15 cells. These are INR chemistry, which means they go by lithium nickel, NMC or LiNiMnCoO2. One of the most popular lithium chemistries, NMC is capable of higher drain and are commonly found in electric vehicles, power tools and high-performance applications. These particular cells only provide about 15 minutes of run time, barely enough to get warmed up.
We had numerous LG 18650MH1 cells from various e-bike/scooter batteries we've taken in. While not high-drain per se, they are capable of 10a discharge and rated at 3.2 Ah (see data sheet here). We test every cell that we receive, and the LG MH1 almost universally test over 90% initial capacity. We also look at impedance at 1kHz AC current per industry standard. It's difficult to gauge a "good" impedance simply because manufacturers often state impedance without the PTC (pressure/temperature/current) switch (we'll get into that in a later blog post), whereas we are forced to measure with PTC installed. However, we find that the majority of these cells are still close to or even within the LG spec of 40 mΩ.
Higherwire has numerous manufacturing equipment such as spot welders, heat shrink ovens, analyzers and IR testers, among others. So we have the capability to refurbish, build and test nearly any battery and even assess the relative health of individual cell-strings.
For this application, we built a 7.2 volt, 6 amp-hour (42 watt-hour) 2P2S battery. That's four times the capacity of the battery that came with the truck, good for an hour of run time.
We added two additional 18650 cells as compared to the stock battery, which necessitated a redesign of the battery hold-down. You can see that we offset them slightly, and that's because the frame has a small lip that's just big enough to hold an 18650 (18mm in diameter). It would interfere with the upper row if we put them in a square formation.
Not shown is the hold-down, but we used a piece of plastic screwed into the frame. It has proven to be robust enough through hours of playtime without fail.
As expected, we significantly improved the truck's runtime from 15 minutes to about an hour. A side-effect of all of that is that the components - especially the motor - get HOT! We haven't taken any measurements with our Flir, but even the battery gets very warm to the touch. That said, it has performed flawlessly over the past few months. Keep in mind that this does NOT have a battery management system, so there is the possibility of over-discharging and therefore flat-lining the cells. However, the truck begins to slow noticeably below ~3.5 volts/cell, likely due to the battery beginning to sag under high load from insufficient capacity. Many popular RC chargers show individual cell voltages, and ours shows around 3.3 volts each at the beginning of charge.
While not a big-ticket item or something with a huge impact on the world as a whole, this project is further proof that recycled lithium-ion batteries can still have a productive second life, even in high-drain applications such as this. The best part? It didn't cost nearly as much as a typical lipo. Contact us if you're interested in a battery for your RC, whether it be a cheap one like this or even a Traxxas (we've done those!).
Oh, and the gif above shows the instant before he ran the truck into a brick wall and broke an upper control arm. Fix, race, repeat.