I Made My Car Key Fob USB-C Rechargable (because I'm too lazy to swap the button battery)

I’ve never been much of a “car guy,” but now that I drive my Mazda 3 (red, obviously) I have to admit - I love this car! It doesn't have any ultra fancy features, but the original owner did select the wireless key fob option. And it is great. Well, mostly…

Wireless Key Fob Mazda Way

You get used to it so quickly, that you simply have you car key somewhere in you pocket or in you backpack, you sit in your car, and you just press Start. No need to insert the key anywhere. So very convenient. I think this is one of the first generations of the wireless key fob, so I do still have to unlock the car by clicking one of the buttons, but hey, no complains there.

The Problem

What I do have to complain about, is that the whole thing is powered by 2032 button battery. That super slim tiny thing. And I suspect the while I'm sitting in the car, the keyfob is regularly pinging the car (or wise versa) to verify that they are still in the near proximity.

Whatever the exact reason is, this tiny battery gets drained rather fast. Within 2-3 months from replacement, the car starts displaying a warning message, that the battery level is low in the key. And within the next 1-2 months, the experience of using the key goes down, as you have to get really close to the car, or physically touch the start button with the key fob.,

USB-C All the Things!

After replacing the battery about 3 times, I've had enough. I have a bunch of rechargable li-ion batteries salvaged from my friends vapes. (Still can believe they put them even into the disposable vapes, what a waste.)

And I like to always have on hand a few of those USB-C boards (Type-C USB TP4056), which provide terminals to connect a battery, a chargin circuit, over-voltage protection and power output terminals. You can tweak the charge current with the RPROG resistor.

Most boards ship with a 1.2 kΩ resistor, giving about 1 A charging current. You can lower this for better battery life (for example, 5 kΩ ≈ 250 mA), but I didn’t. Because I’m lazy, and it works fine.

I have no sense of safety first. If you try this, you’re doing it at your own risk. You can damage your key, your car, or yourself. I’m not responsible.

So let's do this! I took apart the key fob. The button battery is 3V, fully charged li-ion cell is 4.2V. That smells of magic smoke, if gone wrong. So I inspected all the components on the board and googled the data sheets for them. All of them should support the increased voltage no problem, only the LF Receiver and UHF Transmitter had pretty much no-headroom. Which maybe isn't ideal, but in practice a Li-ion cell only sits at 4.20 V right after charging, and as soon as you put a small load on it, the voltage starts to drop.

I got 2 car key fobs with my car, so worse case scenario, I kill one of them. So let's try this.

The wiring was about as simple as you can imagine. Battery +- to the USB-C board battery terminals, and the output I've soldered directly to the original battery pads. This way, I do not really mess up with the PCB and the changes are even easily reversible, if I wanted to go back to 2032 batteries.

Time for a quick test!

Aaaaaand - it totally works!

I've 3D printed a new top cover for the keyfob, to protect the battery and at least some of the wires, and it was ready for real-life testing.

Results & Long-Term Use

So a little reveal - this all actually happened in December 2023. So I've now been running this key for almost 2 years. You know how it is. A temporary solution becomes permament, if it works well. :)

This works about as well as I could imagine. Even without charging, the key fob simply last so much longer between charges. I've honestly haven't seen the low battery warning ever since. When I rememeber it, e.g. like once every 6 months, I plug in the key into a USB-C charger for a few minutes. And it lasts seamingly forever this way.

The only change I made was to replace the cylindrical vape battery with a flat rectangular one, simply to make the key fob smaller and neater in my pocket.

Was this quicker then replacing the button battery a few times?
No.

Am I glad I did it anyway?
Yes!

This applies to all the things powered by 2032. So I went on a bit of a maniac spree of making some of my tools USB-C rechargable. Once again, some of them probably do not really make sense. But every time they run out and I simply charge them with USB-C I have that sattysfying geek tingle “I hacked this and it's better now”.

Your Turn

Got a pointless-but-awesome USB-C mod of your own? Post it in the comments. I want to see the madness.