Phone Won't Charge to 100%

[BigbirdPhila]

Phone is a recently-purchased S3 Mini in "mint" condition. Yes, it's old but it still had some of the original packing tape to prevent scuffs on the metal trim. Looks like it was hardly used.

I charged it with the Samsung wall charger. It will only charge to 95%. From there it will discharge at the normal rate. I compared it to a friend's fully-charged S3 mini, both running only the Google maps app. Both used the battery at the same rate, only mine started at a lower charge.

Occasionally the phone will charge to 100% but it will not stay there. Still plugged in and charging, it will slowly drift back to 95% (even in airplane mode & no apps running). At 100%, if I unplug the charger and immediately plug it back in, I get a popup saying "The battery is fully charged, please disconnect the charger". I don't get that when it shows 95%, indicating that the OS knows the battery is not yet fully charged (and the 95% is not just a calibration error).

The same symptoms occur if I try to charge with an lower-amperage LG charger or my computer's USB port.

Suspecting the battery, I purchased a replacement. The date on the battery was 4/22/15 and the contacts looked brand new. Surprisingly, that battery will only charge to 90% (occasionally to 95%, but then drifting back down as above).

Many Google searches have not turned up anything useful. Any help would be greatly appreciated.

 

[SpookDroid]

Unfortunately that does point to a bad battery. Storing batteries without use for a long period of time will also affect their capacity. Of course, there's also a chance (slim) that the charging circuitry in your phone is bad, so you could try the following:

1) Charge your battery with the phone turned off. Does it charge to 100% (when you turn it on it will drop a couple points...that's normal during booting, but it should go back to 100 after a while if still plugged in).
2) Charge the battery as high as it will go, then let it die out (phone turns off on its own). Then charge fully again.
3) Try charging in a different S3 to rule out the charging circuit.

 

[BigbirdPhila]

Thanks, SpookDroid. Note that I have tried 2 batteries.

To answer your questions:
1. No, with the phone plugged in and turned off it doesn't charge to 100%.

2. I did try to re-calibrate the battery yesterday. Battery went to 0 and phone shut down. Charged all night. 95% this AM.

3. I don't have another S3, but I have tried two wall chargers and two USB cables. Same result. The wall chargers & cables successfully charge an LG phone I have.


I have the "Ampere" app loaded and running now. The phone has gone to 100% charge, plugged into the computer via USB. I notice two things:

1. The charging amperage is 450ma. It has not dropped to trickle charge as I think it should have as it approached 100%. Voltage is 4.366V - 4.367V.

2. If I unplug the charger and re-connect, I am not getting the "Please disconnect the charger" popup.

 

[BigbirdPhila]

More info:

I installed the "Ampere" app. Charging with the wall plug, charging amperage starts out at 900 mA. When the battery approaches 89% full charge (about 4.17V), the charging current drops quickly to 740 mA, 410 mA, and then levels off at 250 mA. Once initiated, this process takes about 5 seconds (you have to be looking at the screen or else you will miss it). Once it's charging at 250 mA, the percentage charge does not increase; it stays stuck at 89%. 250 mA does not appear to be sufficient current to further charge the battery (even in airplane mode, no apps running, and the screen brightness turned all the way down). It appears to me that the phone's charging circuit is decreasing the charging current too early (at too low a voltage - 4.17V) and possibly too quickly once initiated.

The big question: could this be a software issue that could be corrected with a factory reset, or is it probably a hardware issue that probably isn't worth trying to fix? Does anyone have insights on how S3 charging hardware & software are implemented?