Tablet does not power on, how to check battery & charge circuit? - Page 1
Forum section: electronics / repair

author: Twiggy
author_rank: Contributor
timestamp: May 01, 2024, 05:32:46 pm
content: Hello,I picked up this old family tablet, a huawei mediapad t3 10 ags-w09, that was sitting dead in some drawer.I've been interested in electronics repair for quite some time, although so far I've only done simple jobs like smartphone screen or swollen capacitors, so nothing that required any kind of diagnostic ability, which is why I write this threadSo this tablet, when I press the power button, vibrates, screen displays some icon indicating a power issue, and turns off.I left it on the charger for several minutes, no change.Charger and cable are good, tested with other devices.I open the tablet, disconnect the battery, and check voltage, it shows 3.4I plug the battery, and check voltage between battery terminals (circled in red in pic) and ground, and now it shows 1.6V. No difference when the charger is plugged in.I conclude from this that the battery is bad, since as soon as it's in circuit its tension collapses, and that it's not charging, since when I plug the charger i would expect to see >4V between battery and ground.But i do not get why it's not coming on when the charger is plugged.Before I try to source a new battery, how could I confirm the problem is not on the charging circuitry? Would it help if I bought some USB power meter?Thanks for any kind of advice

author: CountChocula
author_rank: Unknown
timestamp: May 01, 2024, 07:40:09 pm
content: Have you tried putting some load on the battery, like a resistor (say, 100Ω) to see if its voltage does, indeed, collapse? That should give you an indication whether the problem is in the cell or somewhere in your circuit.

author: Twiggy
author_rank: Contributor
timestamp: May 01, 2024, 10:09:32 pm
content: Hello, I tried what you suggested. With a 100ohm resistor attached to the battery I measured 3.4V.

author: sleemanj
author_rank: Super Contributor
timestamp: May 02, 2024, 01:02:25 am
content: Disconnect battery, get a TP4056 charger, connect battery to charger, connect USB to charger, charge battery fully, then try again.

author: Twiggy
author_rank: Contributor
timestamp: May 12, 2024, 11:07:56 pm
content: External charger revived the battery and I could boot the tablet. However android doesn't detect when I plug the usb cord and I dont measure a different voltage on the battery terminals with or without the usb cord.Attached is a pic of the charging port, sorry it's blurry, i only have a phone and it doesnt focus well when it's so close.Some measurements with usb cord plugged in :Uab = 5V (charger), as expected i supposeUAC= 0, as expected again as both are groundUBC=0.1V, I would have expected to see 0 since both points are supposed to be groundWhat should I check now?

author: Twiggy
author_rank: Contributor
timestamp: May 16, 2024, 11:33:08 pm
content: usb meter says tablet takes 0.0 amps when plugged. if i had some kind of short circuit shouldnt it get hot and take some current?

author: sleemanj
author_rank: Super Contributor
timestamp: May 19, 2024, 07:28:01 am
content: Try a different USB cable. Otherwise socket might be busted.

author: Twiggy
author_rank: Contributor
timestamp: May 21, 2024, 07:34:50 pm
content: socket is indeed busted! after some contact cleaner spraying the tablet did charge depending on the cable angle inside the socket. will order a replacement andmurderfix the board soon!

author: Twiggy
author_rank: Contributor
timestamp: June 04, 2024, 07:20:03 pm
content: first time soldering such a port, had to use my gas torch hot hair. now Uab = Uac = 5v consistently, but tablet still does not charge.If i botched one of the other pins could it explain this behavior? I have a hard time assessing if the port pin contacts the board pads. I tinned slightly the pads then placed the port on top, soldered its legs to the holes then melted everything with the hot hair.