Why your battery sometimes doesn't "fully charge"

[anon(10181084)]

So you might have encountered an issue where you battery seemingly gets stuck at 80-90% indefinitely while charging. Recently this started happening on my tablet and, after some extended investigation, I found out that it was indeed fully charging but the fuel gauge wasn't reading the charge level properly. In this case, the ONLY thing preventing you from having a battery fire is basically the protection circuit on the battery itself. To check for this yourself, check the battery voltage and charge current using an app of your choice (Accubattery, DevCheck, etc...). See if the battery current is really low (i.e. 250mA or lower) and whether the voltage is around 4.2 or 4.3 volts. This type of behavior is a problem with failing Texas Instruments fuel gauge chips (TI makes short-lived junk in this department). If you have a Samsung device, see if resetting the fuel guage with the charge still plugged in works (use *#0228# in the dialer and then tap "quick start" and OK when it shows you the warning). In my case it didn't work because the chip itself is failing, but it might work for you if it just glitched out.

 

[Mooncatt]

There's other reasons for this happening as well, so I wouldn't suggest jumping right to a faulty chip in every case. A bad battery with high internal resistance and self discharge will exhibit the same symptom as yours. Some Li-ion batteries are designed to charge to 4.4V, so you need to know your battery's specific limits before assuming a given voltage is fully charged. It's also normal to see such low current when nearly charged as well, so that alone should not be the deciding factor. A simple bad USB cable can cause a similar problem by having excessive voltage drop on a bad connection inside the cable. Something else worth knowing for diagnosing would be if the battery drains quicker.

 

[anon(10181084)]

There's other reasons for this happening as well, so I wouldn't suggest jumping right to a faulty chip in every case. A bad battery with high internal resistance and self discharge will exhibit the same symptom as yours. Some Li-ion batteries are designed to charge to 4.4V, so you need to know your battery's specific limits before assuming a given voltage is fully charged. It's also normal to see such low current when nearly charged as well, so that alone should not be the deciding factor. A simple bad USB cable can cause a similar problem by having excessive voltage drop on a bad connection inside the cable. Something else worth knowing for diagnosing would be if the battery drains quicker.

OK, so yeah the internal resistance is elevated but it is still seemingly working fine for the most par. This battery I've seen reach from little over 4.2 to a little over 4.3 at full charge. When it cuts off it is somewhere between those two voltages and drops a teensy bit if I disconnect the charger. My dad, who is an electrical engineer, also thinks the fuel gauge might be the issue. If it matters, there is (I think) 302 cycles on the battery. The symptoms of high internal resistance I've discussed previously in the help section are super rare but they do happen once in a while. But, I remember my first tablet also got extreme high internal resistance on its battery before I retired but the fuel gauge still worked perfectly (in that case I know for a fact it is one from Maxim, which is known industry-wide for making these chips properly).

Edit: also, I'm using aftermarket cables that have a LOWER voltage drop than the OEM Samsung cables.