Samsung / Kit-Kat battery drain weirdness

I’ve recently acquired an ancient Samsung Young 2 (SM-G130HN) phone running KitKat 4.4.2. The battery never seemed to last more than a few hours (four if I was lucky), despite turning off all the usual suspects – Sync, Location, NFC, Bluetooth, ensuring email was only pulling once every few hours, etc.

Eventually I rooted it and uninstalled all the bloatware, installed a battery monitoring/saving app and now I can get over 18 hours out of it when on standby and pretty much not being touched. Sometimes almost two days. I’m not a big user, it’s just a fallback phone to play with and learn about the underlying Android system.

I’ve noticed that most times it drains massively when it reaches about 40%. It’ll creep down a few percent an hour under light/moderate usage, maybe lose 10% when left on overnight for the first night. By the second night it’d perhaps be on 40-44% before I went to bed, and by the time I awoke, it’d either be:

1. showing around 20% and when I tap in the lock code, it’ll immediately drop to 2% and kick up a battery warning;
2. dead, and require a charge.

Thought it might be the battery — like a cell gone bad or something — so I bought a new one. But it does exactly the same kind of thing. Been trawling forums on sudden battery drain / stuck apps etc and trying all the tips and tricks I can find, to no avail.

Anyway, the phone’s unlocked. Today it was showing 46% battery. I turned it off, swapped the SIM, turned it back on again and the meter showed 28%. What the…?! By the time I’d topped up the SIM card, the battery meter showed 27%. I turned the phone off, put the old SIM back in, powered it on. Battery then showed 19%!

Just for the sake of science, in case turning it on and off was killing the battery, I powered it off and swapped the SIM again. The battery only dropped by one percent to 18%. Cycled the power again: 17%. Used it for a couple of minutes to read a text message and check the battery usage in Settings. It seemed alright for a few minutes longer, then dropped from 15% to 3% and I got a battery critical warning.

Now I understand the meter isn’t super accurate as it’s probably just measuring Coulombs and interpreting the results on a scale. I also appreciate that there are many factors that can affect it — such as ambient temperature, phone temperature and such — but this level of drain is ridiculous. Something’s either misrepresenting the amount of juice left, or KitKat itself (or some app) is chomping through it quick-smart. Since it’s a brand new battery, it ain’t too many charge cycles affecting its life. So my only remaining avenue is that it could be hardware related. But where to start?

The battery usage monitor shows that two-thirds of the usage is split between Device Idle, Android OS, Cell standby and Android system, with the remainder — today at least — being Google Play probably checking for updates (13%), then Screen (6%), then my email app (3%) and a small handful of other sub-3% services. That’s about what I’d expect when the device isn’t being used, as it’s percentages-based after all; if it’s doing nowt, the system services and background tasks will show up as taking the lion’s share of the resources.

What I can’t fathom is why — if it’s doing “nothing” (i.e me barely interacting with it) — it suddenly loses 10-20% of power in the blink of an eye when I do interact with it, or it can’t last an hour or two when it reaches around 40% of the remaining juice. Is the lock screen showing the wrong value and after entering the PIN and tapping a few apps, it checks the battery status and goes “woah! something ain’t right” and updates the value to show the real value remaining? Even so, would that explain why cycling the power twice within a couple of minutes drains 27% of the battery, and yet two more cycles a few minutes later makes almost no difference?

I’m utterly stumped. If anyone has any clues as to what might be causing the random battery drainage, or such battery life misrepresentations by the OS, I’d gratefully appreciate any insight as I can’t figure it out.