One of the challenges I talked about a few weeks ago was the high power, and high energy demands of WiFi. If designing for long battery life on a portable WiFi device wasn’t difficult enough, we’ve also thrown in a few more design constraints just to make things worse.
In Situ User Replaceable Batteries
The design of our product is that it can be permanently affixed in its place of use. Essentially it’s a sticker that’s typically mounted on a door, wall, or counter. We want our users to be able to easily replace the batteries without having to go through the trouble of removing and replacing the device.
This eliminated the possibility of a rechargeable battery option. A lithium-ion cell would have been an ideal choice. They have excellent power and energy density, come in a variety of slim geometries, and both the cell and charging logic is relatively cheap. However, forcing the user to tear down the device and plug it in for a few hours (even if just once or twice a year) is a pain in the ass. Enough of a pain that customers may just forgo charging and abandon the service.
If not rechargeable, we could either go with disposable (like the Amazon Dash), or user replaceable. We already have enough waste in the world, and there’s no reason we couldn’t come up with a user replaceable option. Besides, the disposable option would still involve tearing down and reinstalling the device.
A Slim Design
We wanted the batteries to be easily obtainable. My initial thought was to just use standard AAA batteries. Based on the X and Y geometry of our device we could easily pack 4 batteries into our device. With 4 cells we’d have the power and energy required to meet the demands of our WiFi module [link to WiFi module power analysis].
The only real concern was the diameter of the cells, but I assumed at 10.5mm the device would still be reasonably slim. A rough estimate of the stack-up was putting final product thickness somewhere around 13mm. It all seemed reasonable until I made the first mock up. (And by “mock up” I’m not talking about a fancy 3D print. It’s amazing what you can do with scissors, tape, and an old cereal box.)
Wayyyyy too huge. No computer generated model beats holding a physical prototype in your hands. And this prototype was screaming wayyyyy tooooo huuuuuge!
So, cylindrical cells were out. The only real option left was “coin”, or “button” cells. These are the type of battery you’d typically find in a watch. A little light shopping revealed that the most common option appeared to be the CR2032. Locally they’re available at any Walmart, Canadian Tire, Shoppers Drug Mart, or Dollarama. I’d recommend Dollarama as they were by far the cheapest option.
With a thickness of just 3.2mm I was able to estimate a final product thickness closer to 6mm. This felt right.
|Wired Connection (no battery)||
|Lithium-Ion (rectangular cell)||
|AAA (standard cylindrical)||
|CR2032 (coin cell)||
Power Density Versus Energy Density
Perfect, we’ve found a cheap, available, slim battery with amazing energy density! What’s the catch? Power density. While a typical CR2032 cell can have anywhere from 200mAh to 240mAh at 3.0V (energy density), they are severely limited in the rate at which they can deliver that energy (power density).
In order to get anywhere close to their stated capacity they recommend a maximum continuous draw of 2mA, with allowable bursts up to 10mA. By comparison, the datasheet for our WiFi module was stating a base “on” current draw of 40mA, up to 80mA during a transmission, and sharp spikes as high as 500mA. Even with multiple cells in parallel (we can fit 4) we were nowhere near the power density required.
Despite this limitation, these cells still offered an attractive option. Surely I could find some sort of boost circuit to deliver the load. I guess that’s what I’ll be looking at next week.