this post was submitted on 09 Apr 2025
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Yeah. I get it. It's good for old devices to be useful for years to come.
What about the battery though? Having a device with an old battery plugged in the socket 24/7 is a fire hazard.
I would love to use my old nexus 10 with this perfect screen today. There is still no way to use a mobile device without a battery plugged in...
Use a smart power switch.
Battery level drops below say 20% - turn switch on and start charging.
Battery level goes above 80% - turn switch off.
For Android, you could use something like the Tasker app to do the monitoring and switching.
This doesn't address the fire hazard at all.
At all? What do you believe the fire hazard is then?
Adding a high quality smart switch to a 5-year old battery might do some good, of course. But this is a cluster of old batteries, all in bad shape for many years of usage under different circumstances, temperatures, etc. And they're all different models. Each battery will need a different time for going from 20 to 80%... it's wild.
I do this kind of risk assessment on the worst scenario, a fire could turn your home into ashes. The only option to me, being strict, would be removing those batteries (after all, your main goal is to re-use the other hardware.) But that's only possible in a handful of phone models... it's just unfortunate. I guess I should get a FairPhone next time!
The person I responded to only has one device. But even if there were multiple devices, you could just have a smart switch per device.
The fire hazard issue is that holding the battery at 100% increases the internal chemical stress on the battery, and increases the risk of thermal runaway. So keeping it well below that will definitely have significant benefit. Ideally, trickle charging it at around 50% would be best for almost eliminating further strain. (But you'll likely need to charge it from 20-80% occasionally to help the battery management system keep calibrated.)
And things can be done even smarter. Like only charge the battery at the coldest part of the day, typically early morning to reduce heat stress. Or throttle the device if it's temperature reaches a threshold. Etc.
Yes, if you can remove the battery, that's helpful. But keep in mind you lose the benefit of battery backup during mains failure.
Also, mobile phone fires get a lot of media attention because "drama," but they're not the hazard everyone thinks they are. You're far more likely to have a house fire from rodents chewing into electrical cables, clothes dryer lint catching fire, or anything with a heating component. Ebikes and escooters are far more likely to catch fire than phones.
The point is if you're not emptying your dryer lint filter every use; if you're not getting home pest inspections every six months and putting down rodent traps or baits; if you're not getting your heating-element devices professionally checked regularly; if you're not storing your ebike well away from flammables; etc., then worrying about phone battery fires is ridiculously out of proportion.
I've got a colleague who says she'll never buy an electric car because of the fire risk. But then she drives around in her gas car which is 8,000%+ more likely to catch fire.
If you're still paranoid, keep the device in a non-combustable container away from flammables, and near enough to a smoke detector. It's easy to setup something that if the phone catches fire it'll just burn itself out.
Just take out the battery and keep it plugged in?
Unlike laptops, many phones simply won't turn on without a battery connected.
Is that so with new phones? Last I tried with a Samsung S5, it did work.
It's common, and especially so on devices that don't have batteries which are intended to be user-removable - which is pretty much all new phones.
ok thanks for the info
I guess it's better if you set up charge limiting, so that after reaching x percent it continues to run from the wall, but not charge the battery anymore. not all phones support charge limiting though, and even less suports idle charging, some have idle charging buggy and if it doesn't work right after boot you have to reboot it to fix it. charge control almost always requires rooting if android.
but I guess its the same with laptops, except there even the chargers get hot
I used to use old phones (Nexus 6P, Galaxy Nexus, Moto X) as security cameras, stopped for exactly this reason. They all got really warm, even with the screen off, and I was uneasy about it. IIRC my Gnex removable battery started expanding a bit! That's when I stopped using them for it.
You can use a battery powered device without the battery plugged in. I've never built capacitor bank for a more modern mobile device but its probably possible with off the shelf boards. The last one I built was years ago for a dentist office. It was a mp3 player they used for the hold music. Pulled the bad battery and built a fairly small capacitor bank. The only complication was getting the right resister for the third terminal. In the case of the mp3 player it was a temperature sensor. Modern phones use this channel to communicate battery info which might complicate things a bit.