How to keep (rooted) Android tablet from ruining battery when permanently connected to power

So I wanted to share a little tip for anyone using a rooted Android tablet as a mounted dashboard. I have three Amazon Fire HD 8 tablets that I managed to root (latest hardware lots aren't rootable) and I installed Lineage OS 14 instead of FireOS. This should work with FireOS, also, as long as you have root.

I was concerned about the battery having less longevity due to being constantly plugged in, at 100% charge. Lithium batteries are happiest at about 40% charge. So I found a tool that keeps the battery charged to whatever range I choose. It's available at XDA, and it's called "Advanced Charging Controller." I found that if I download the Advanced Charging Controller App (AccA) and sideload it, it will install ACC and make it easy to set the charging parameters.

I used a beta version of the app listed at: Advanced Charging Controller (acc) - Post #3379

It could be worth looking at the original post to get an idea of what it does, though the thread is quite long: Advanced Charging Controller (acc)

I didn't install ACC separately, I just installed the App and it takes care of everything once you give it root access. On my Lineage OS Fire HD 8 (2018/karnak), I was able to set the voltage to 3920 mV and the charge thresholds to 43/45%. Now my tablets sit with a very comfortable charge on the batteries, and hopefully they will last a long time. It was too much trouble for me to root those tablets and set up Lineage OS to have the batteries swell and require replacing.

Somewhere I thought I had seen someone using Tasker (I think) to turn an outlet on and off depending on charge, but this is even better, since I can set the desired battery voltage. My tablets are powered by a 12V-5V converter, with the 12V DC all supplied from a power supply adjacent my wired alarm system, which also powers my sirens and wired PIR motion sensors (all connected through konnected). So if I shut that off I'd shut down my sensors and sirens, which would mean I'd need a second power supply. I've put the tablets where my wired keypads used to live, which allowed my to use the wires from those to transmit the 12V DC to the 12V-5V buck converters.

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I am using Tasker but only monitor battery percentage. However, after googling based on your post, I found that Tasker can access a lot of battery parameters using an intent so now I have a new project. Is there a recommended voltage range that battery should be operating or is it device specific? My battery is 75% at 4000mV currently.

4000 mV is a reasonable value for mV. I like being closer to 3920, since I don't need a long battery life off power, only enough for the occasional brief power outage. If I lost power long enough for my tablets to drain from 40%, I'd probably lose my battery backup on the hub and konnected, too.

< 80% charge is much better than 100%, but you may have noticed they typically ship/store lithium batteries at ~40% charge (~3920 mV). It might be the difference between a 5-7 yr life and a 6-8 yr life, vs. potentially < 2-3 yr at 100%.

This is great, thanks for sharing. I just rooted and installed LineageOS on a couple Fire tablets that have been sitting around mostly unused for a while. It occurred to me that keeping them plugged in all the time could have downsides but I wasn't sure what to do about it (I'm not a big fan of the smart outlet solution either). I'm gonna give this a try!