Saving a broken tablet with a charger hack

I found this Samsung Galaxy Tab S in an e-waste bin. It didn't have any visible damage, but it had been thrown away because it was dead and wouldn't charge or react at all when plugged in. My USB power meter showed it would only pull about 10mA from the USB cable. 

Since this was one of the older designs with the removable back cover, I could easily open it up and directly access the battery connector. By charging the battery from a bench supply, I got it to wake up, and everything seemed to work normally except for charging.  The previous owner had installed some kind of aftermarket OS on it that caused it to get stuck in a boot loop as soon as I tried to do a factory reset, so I had to spend a while figuring out how to use Odin to restore the stock OS. After that diversion it was wiped and working normally, but still unable to charge. 

I couldn't find any visible signs of damage, and I could confirm that the connections to the USB port were still good - in fact the system could still detect 5V when plugged in and indicate that it was charging, but no current was actually going into the battery. It also was still able to accurately report battery charge because it was based on voltage. This meant that if I could add something to charge the cell, the whole thing could be restored without having to dig further into the real source of the failure. 

I prepared a small board to hold a TP4056 charger IC. This was cut from blank flex PCB material since everything needed to stay very low profile to fit in the case. A higher-current charger would have been preferable since this is a 4.8Ah cell, but the only options I had would have required a PCB with a lot more support components and would have ended up being too thick. The TP4056 is set up to charge at 1.2A, which is a slow charge but probably good enough for the use case for a tablet. One other disadvantage is that it will only charge to 4.2V while this is a 4.35V cell, so it only gets to 92% (as estimated by Android). I figured these disadvantages were acceptable tradeoffs for keeping the repair simple and getting it working quickly. 

I probed out the connections for 5V and ground on the USB connector daughter board, and tapped into these with 34AWG magnet wire. 

From there it was very simple to wire in the new charger. I was lucky that system ground was connected to battery negative, so this charger architecture would work. All I had to do was wire 5V and ground to the TP4056, and then connect the output from the charger to a battery+ connection next to the battery connector. 

I then plugged it in, checked that everything was working on the USB meter, and closed up the case. The tablet now works normally and has no visible external changes - it just charges a bit slower and tops out at 92%.