PCB Coin Cell Battery Holder (and Switch)
April 06, 2023
Recently I’ve been experimenting with coin cell batteries and thought it would be fun to make a light-up pick using one with an LED. Two months later, and I have an LED light PCB that holds a CR2016 or CR2032 battery with no extra components, and which can be assembled without solder.
Working on a new PCB design where spring tension arms hold a CR2032 battery in place. Wonder if it will work? pic.twitter.com/RRsPovwnuw— Jeremy Cook 🤖 (@JeremySCook) January 24, 2023
In late January, I tweeted about my PCB pick idea, and was eventually referred to this PCB business card, as well as this one. Both use a clever arrangement of deformable forks to contact the positive and negative pads on a coin cell battery. This allows a single PCB to act as the battery holder with no external components.
Roughly duplicating the first design’s internal fork dimensions on a .8mm PCB, the pick arrived a week or so later, and seemed to hold things in place with heat shrink applied. The pick was designed to complete a circuit using a guitar’s strings via pads at the front of the pick, which worked, but inconsistently. Perhaps I wasn’t always making contact with the strings in the correct place. However, there was another problem.
Better in Theory Than Practice?
Caption: LED-No-Solder tabs are slightly shorter than 19mm
Image Credit: Jeremy Cook
As pointed out by a friend of mine at Tampa Hackerspace, the battery didn’t always make contact with the PCB pads. It seems that there was just enough of an angle that unless there was a light amount of pressure applied to the PCB forks, one couldn’t ensure that electrons could flow. Initially, I shrugged this off as something that you would always do while holding a pick, but what little pressure you apply can be inconsistent.
The other thing at play here is that while the first cards linked featured tabs that are each 19mm long, mine are 17.5mm for the two-tab side, and 18.5mm for the opposite one-tab section. I suspect this change was originally an attempt to squeeze things down into the size of a pick. This may be worth revisiting if you want to make a battery holder with constant electrical contact. However, it worked out quite fortuitously in my case.
A Bug Becomes a Feature
When I ordered a second version of the pick (which I’ve yet to try), I also ordered a small rectangular “non-pick” device that’s actuated only by pressing on the battery pads, not by electrical string contact. This also features a series of through-hole contacts that allow one to weave the LED leads through them so they stay in place and make contact without solder (though solder can be helpful for consistency).
Combining these elements allows the “LED-No-Solder” (design files on GitHub, or available fo purchase on Tindie) to act as a switched light with no solder-on components whatsoever. It should make for a great first circuit project for kids, and a fun key fob for adults. I even included a ruler because… why not?
On a whim, I tried using the .8mm LED-No-Solder PCB as a pick, and–surprise!–it lights up as you strum. It turns out that my picking pressure is just enough to complete the contacts. While it works better on the downstroke than upstroke, for something that’s a bonus feature, I’m thrilled at its operation. In fact, it seems to act as a simple vibration sensor, so perhaps it could be used for other purposes as well.
Bottom Line: Make a Coin Cell Battery Holder From a PCB?
Image Credit: Jeremy Cook
If you need a rock-solid way to ensure your CR2032 or CR2016 battery always makes contact, then my very limited experience would say that you should probably just buy and install an actual coin cell holder. Even if it does make contact, the batteries can eventually slip out, which I somewhat mitigated with heat shrink. At the same time, for applications where thinness and low-cost are key design factors, and where it’s probably OK if it loses power sometimes (e.g. business cards), an in-PCB coin cell holder can be an excellent idea.
In my case, this intermittent disconnection became a huge feature in a gizmo that I hope to enjoy myself and share with others. While there are some limitations, this sort of batter holder is certainly a clever idea to tuck away in your mental toolbox for when it’s appropriate–especially if you have room to fit the full 19mm tabs and time to rev your board as needed!