We have a (by now) old ASUS Transformer Prime tf201 tablet that has been collecting dust for many months, because it wouldn’t charge. I figured I’d make an effort to revive it, and found it was impossible to turn it on, even when plugged in. Only occasionally I’d see a red battery symbol indicating low or bad battery. Also, the little LED in the power button would do triple blinks rather than stay solid red that would indicate it’s charging. No amount or combination of button-presses would convince the tablet to even attempt to boot up.

Some googling indicated that the triple blink of the LED means that the battery level is so low that the tablet considered it broken and would not charge it.

I assumed I would buy a new battery to replace the broken one, but thought I’d open up the tablet and have a look first. There are youtube videos showing how to open up the tablet, and picking out the battery was fairly easy.

I figured there would be no harm in making an attempt at reviving the battery. That is, there is of course potential harm in messing about with lithium batteries, so do proceed with caution. The battery unit consisted of two battery cells and a logic board. After pulling back some pieces of tape, two flat «wires» to each battery cell were exposed. A multimeter revealed a voltage of about 2.7 volts for each cell, which is certainly on the low side, but also not beyond rescue. The multimeter also identified the positive and negative terminals.

For the kids’ RC cars I have bought an «Imax B6» smart charger. It has modes for charging LiPo and NiCd batteries, among other technologies. I connected this charger to one of the battery cells, using this type of test probe connector:

Note I did this outside, wearing safety glasses and spider-alert-sense on «high».

Firing up (pun intended) the Imax B6 in LiPo mode, unfortunately it refused to try to charge the cell due to «too low voltage». Sigh. Well, a new quizzing of google revealed a trick to use the somewhat dumber NiCd mode to charge the LiPo cell up to a minimal voltage, before proceeding with a LiPo-mode charge. So I set the charger to NiCd and 0.5 amps, and let it run for an hour or two until a voltage of about 3.3 volts was reported, while making sure that the temperature of the cell was not going high, nothing was smoking or turning into a hitherto unknown lithium-based lifeform. Then, switching the Imax charger back to LiPo mode, I’d charge the cell for a few more hours up to some 3.8 volts, and repeat the whole process for the other cell.

After reassembling the tablet, lo and behold the thing booted up happily, reporting about 70% charged. To my considerable surprise, the battery has also been quite good ever since, holding its charge for a very reasonable amount of time. I’m tempted to say it’s good as new. I’m assuming this procedure might work for similar tablets and maybe also telephones.

Finally, when I booted up the tablet the first time, the root of the problem also revealed itself. Trying to charge the tablet by normal means, i.e. with the original charger (the tf201 requires a special 15 volt USB charger…) and cable proved to be very difficult. Turned out it was the USB cable itself that was half-broken, only making a good connection if held in a very carefully selected position. I’ve now bought a new cable, and everything works quite well. (To give you an idea, a good hour of youtube took about 10% battery.)

Again, to perform this procedure you should be very careful, and know your batteries, amps, volts, watts and wattnots.