battery sudden charge and discharge

[jbeale]

Unlocked Samsung Galaxy-S III (SGH-I747) with Android 4.4.2
This isn't my phone, but I'm playing tech support for it.
When connecting the charge cable, the battery *sometimes* but not always, jumps almost instantly from ~35% to near 100%. When charger disconnected, the battery quickly drops down again. Momentary glitch? Software? Time for a new battery? A new phone?

 

[svim]

It does sound like the battery has failed. The SIII is a six year old phone. If that's the original battery it's surprising at all. Luckily that model has a user-replaceable battery, no tools necessary either. A fingernail is all that's need to remove the back cover.
Pick up a replacement battery from Amazon for under $10. (If they use NFC for anything be sure to get one that includes it. Oddly Samsung opted to put the NFC circuitry into the battery instead of the body of the phone with this model.)

.... since you're the tech support, while the back cover is off that's a good time to tighten up any screws and fasteners you see. One or more tend to loosen a bit when it comes to an old smartphone.)

 

[jbeale]

Thanks for the reply. The annoying thing is this is a replacement battery, only a few months old. But as the OEM battery is no longer available it was a 3rd-party model, and maybe itself old stock.

 

[Tina Nerd]

Oh, the battery draining too fast was my head pain… Guys from https://misterfix.us/shop/iphone-repair/ helped me. They changed battery on my old iphone and now it works almost like a new one (though i quite forgot how it functioned in its ‘young age’). Just look for some small repair shop nearby, i’m sure they can still fix your problem with the old smartphone.