In my previous post, I made the
case that old flagship smartphones are seriously underrated as lab nodes.
A Snapdragon 835 from 2016 trailing a Raspberry Pi 5 by only 13-15%
on cryptographic benchmarks, at a fraction of the cost? That’s a compelling
argument for raiding the drawer where old phones go to retire. 📱
The case for mobile hardware has only gotten stronger since then. The AI
boom has quietly reshaped memory manufacturing priorities: chip makers have
pivoted capacity toward High Bandwidth Memory (HBM) to feed the insatiable
appetite of GPU clusters and conventional DRAM and LPDDR supply has paid the
price. Single Board Computers like the Raspberry Pi and its alternatives have
seen noticeable price increases as a knock-on effect, making the
economics of purpose-built hobby hardware shakier than they used to be. An
old flagship sitting in a drawer starts looking less like a compromise and more
like the smart play.
But there’s a catch nobody talks about, what happens to the battery when you leave a phone plugged in permanently as a server?
🔋 The Hidden Cost of “Always Plugged In” Link to heading
Lithium-ion batteries are genuinely remarkable pieces of chemistry, but they have a well-known Achilles’ heel, staying at 100% charge for extended periods degrades them fast. Keep a phone constantly topped up and you’ll watch its battery capacity shrink visibly over months.
This is normally fine for a phone you pocket every day it charges overnight and you use it. But a lab node running Prometheus, Node Exporter, or any other 24/7 service? That phone lives on the charger indefinitely. Without intervention, you’re accelerating battery wear every single day.
The built-in battery is one of the best features of using mobile hardware for a lab (free UPS!), but only if you protect it. So I built Yukti to do exactly that.
🌱 Introducing Yukti Link to heading
Yukti is a small, clean Go application that manages battery charging automatically on rooted Android devices. The name yukti (युक्ति) means clever strategy in Sanskrit - and that’s exactly what it is a clever strategy to make your mobile lab sustainable for the long run.
It runs quietly in the background as a Termux service, monitoring battery capacity every 60 seconds and toggling charging on or off based on configurable thresholds. No UI, no bloat just a focused daemon doing one thing well.
🧠 The Core Idea: Hysteresis Charging Link to heading
The magic behind Yukti is a concept called hysteresis. Rather than switching charging on and off at a single threshold (which would cause constant, rapid toggling), Yukti uses two thresholds with a buffer zone between them:
| Battery Level | Action |
|---|---|
| ≤ 40% | Enable charging |
| 41% – 69% | Maintain current state |
| ≥ 70% | Disable charging |
Think of it like a thermostat, your home’s heating doesn’t kick in the moment the temperature drops by one degree it has a range it’s comfortable with. Yukti applies exactly the same thinking to your battery.
Here’s what an actual charging cycle looks like:
Battery drains → hits 40% → charging enabled ⚡
Battery charges → hits 70% → charging disabled 🔌
Battery drains → hits 40% → charging enabled ⚡
... and so on
The result is that your battery lives comfortably between 40% and 70%, which is the sweet spot for lithium-ion longevity. You get the UPS benefit of a charged battery, without the degradation from perpetually sitting at 100%.
The hysteresis zone (41–69%) is equally important, it prevents the charging circuit from rapidly toggling on and off when the battery hovers near a single boundary, reducing wear on both the battery and the charging hardware.
📸 Seeing It In Action Link to heading
Here’s a snapshot of the periodic automatic recharging cycle and the
corresponding thermal behavior on the Snapdragon 835:
The pattern is exactly what you want, short, bounded charging bursts followed by discharge phases, with thermals staying well within safe operating ranges because charging events are infrequent and controlled.
I have been running Yukti on a Snapdragon 835 lab node which currently
hosts my home’s observability stack.
🏗️ Architecture: Clean Go, No Surprises Link to heading
Yukti is written in idiomatic Go and follows clean architecture principles.
The charging control works by writing to
/sys/class/power_supply/battery/charging_enabled a kernel-exposed sysfs
path available on most Android devices. This is why root access is
required, writing to sysfs nodes under /sys/class/power_supply/ is
restricted to root on Android. Yukti needs to be granted superuser
permissions (via Magisk or similar) to function.
The clean separation between domain logic and the filesystem implementation also means the hysteresis rules are fully unit-tested independent of any hardware you can verify the logic without needing a physical device.
🚀 Getting Started Link to heading
Yukti is built for rooted Android devices running Termux. The recommended deployment is as a persistent Termux service that starts automatically on boot.
Full installation and deployment instructions including cross-compiling the ARM64 binary, setting up the Termux service, and enabling it at startup are covered in the official README on GitHub.
💭 Why This Matters Beyond Battery Health Link to heading
When I argued for mobile SoCs as lab nodes, one of the headline benefits was the built-in battery acting as a free UPS. But a degraded battery is worse than no battery at all it can fail unpredictably and give you false confidence about your uptime story.
Yukti closes that loop. With it running, the battery actually stays healthy as an emergency power buffer, thermals stay predictable because charging bursts are brief and spaced out, and your phone can realistically serve as a reliable lab node for years rather than months.
A 2016-era flagship running modern cloud-native workloads, with a healthy battery, on a few watts of power that’s an outcome worth getting excited about. 🎉
🤝 Open Source & Contributions Link to heading
Yukti is open source under the MIT License included in the repository. Contributions are very much welcome whether that’s supporting additional battery sysfs layouts, adding configurability for the charge thresholds, or improving the Termux service setup experience.
If you’re running mobile hardware in your lab, give Yukti a try and let me know how it goes.
Happy building! 🚀