Low battery notification on a device

This topic came up from a conversation we’ve had these past few days in the Domótica Compatible Telegram group. If you’re a user of the Home app and you have devices with batteries, you’ll have noticed that the Home app only notifies us of Low Battery when the device has barely more than 5 minutes of battery life left. Devices that use a rechargeable battery work a bit better, but we still can’t see the battery level either way. Let’s take a look at why this happens and what solutions we have both in HomeKit and Home Assistant. I’m Carlos Sahuquillo and let’s get started!

The problem: measuring battery charge isn’t easy

The Home app can’t put its tongue on the battery terminals to check if it’s still got juice, like we used to do back in the day (by the way, the World Health Organization already stated that this method is unhealthy for a bunch of reasons and can even be dangerous if the battery has leaked, since it may contain heavy metals like mercury, nickel, cadmium or lithium, so better not do that anymore). For a few years there were some Duracell-brand batteries that had a little strip that would show you the charge level if you held the battery touching both terminals… but that didn’t work all that well either. There’s also a little trick where you stand the battery upright, lift it slightly and let it drop… supposedly if it lands standing up and stays that way, it means it still has liquid inside and still has some charge left, but if it tips over or wobbles, it’s dead. This has never worked for me in my life, but hey. Then there are the smart alecks who say: ‘the best thing is to use a multimeter to measure the charge’. I’ve never owned a multimeter in my life, mate, and that’s saying something given how much I’ve tinkered with cables, Shellys and switches. I might have saved myself a spark or two, or a Shelly gone black from touching where I shouldn’t have, I won’t deny it, but it’s not something everyone has at home. The thing is, as a kid I knew the batteries were worn out because my remote-control car, which I used all day long for a gazillion hours (I reckon that car clocked over 100,000 kilometers), would go slow. However, when you put in fresh batteries, it would zoom off and turn and drift beautifully. Well, the Home app does something similar. Since there’s no real way to know the exact percentage of charge left in a conventional battery, it tells you the battery is running low when it’s not delivering the voltage it should… which has a couple of problems: first, when a battery isn’t from a good brand (the typical cheap Chinese batteries we buy in bulk, 400 batteries for 2 euros), it might not meet the quality standards of the more reputable brands and might not deliver the correct voltage. This happens to me with a battery I currently have in a smart button, a button I have in my eldest son’s room because there’s no outlet near his bed for his lamp, so he has a smart button that turns on the lamp sitting on his desk. The thing is, ever since I put that new battery in, it always shows 0.5% charge remaining. It works perfectly and the button’s LED lights up green, if it really had low battery it would light up red… so I haven’t changed it yet. However, when the battery delivers the correct voltage, it then shows 100%… whether it’s freshly changed or not, so it’s possible to see a device showing 100% battery one day and then complaining about low battery at 10% the next, this has happened to me several times. And, what’s the real problem here? Well, that at some point the battery actually runs out and the device goes offline without you even noticing, so sometimes you go crazy trying to figure out why the heck it disconnected when in reality it’s simply out of battery.

Solution: Low battery notification

I have a friend who changes batteries systematically every so often. I do the same thing with the heads of the electric toothbrushes at home, I have an automatic Amazon subscription for that, and every 3 months it sends me Oral-B heads, so when they arrive I swap them all out. A pretty penny really, but I don’t wait around to see if one head is more worn than another because that kid brushed his teeth more or another one skipped it without me noticing. By the way, this kind of time-based notification can also be set up from Controller for HomeKit through Advanced Notifications or with a Shortcut, but in my case I use the iPhone’s Reminders app. Anyway, my friend Luis changes the batteries every whatever-amount of time on all the critical sensors, door openings, smoke detectors, etc, whether they need it or not. Any environmentalist would be against this given how many still-charged batteries this guy throws away, but oh well. In my case, what I’ve done is create a low battery notification that alerts us when the battery isn’t delivering the correct voltage. This can be done in HomeKit and in Home Assistant, though to do it in HomeKit you can’t do it from the Home app… but we can do it very easily with the Controller for HomeKit app, from the Maintenance Menu in the app. Needless to say, this can also be sent to your phone in real time as an Advanced Notification in Controller. Yeah, I know Advanced Notifications are only available in Controller if you pay for the subscription, what can you do. Also, Controller has an interesting little quirk, which is that you can associate the battery model that a given device uses. I don’t know if you listen to Christian Garcia @patuflinx, from the AppsMac en 8 minutos podcast, but the guy has a Notion database for absolutely everything. You can’t imagine the kind of daily-life stuff he keeps in Notion. But anyway, he’s mentioned before that he has a database there for the batteries and light bulbs around his house, so that if a battery dies, he goes to Notion and finds the battery model that device uses. Same for light bulbs, if a bulb burns out at his place he goes straight to Notion, where he’s noted down the bulb model, light temperature, thin or thick base, etc, so he can buy the exact same one that burned out without having to take it apart. It’s been ages since a bulb burned out on me, to be honest, but hey, it could be handy. However, in Controller you can set what type of battery each device uses, and it even lets you buy them right from there! And it sends you to Amazon! It’s true that it includes Controller’s referral link, so it earns a small commission on each sale… which doesn’t bother me at all, it could be a nice little extra for the Controller dev team if people buy their batteries through it, but I think they should disclose it in case you’re not okay with that, especially considering how much a Controller license costs these days, which is anything but cheap (still, I’ll say it again, it’s worth it). In Home Assistant I have it set up as a scheduled notification, every morning at 11 it checks the battery status and sends me a notification of the devices with low battery. The great thing about Home Assistant is that, since you can integrate pretty much any device, not just smart home devices but also phones, tablets, computers, etc, as long as you install the Home Assistant desktop app, you can include those devices in this notification so it alerts you at 11 in the morning if you haven’t plugged in your computer to charge or if your phone is complaining about low battery.