[sdiy] What do you know about.... eReaders

Vladimir Pantelic vladoman at gmail.com
Sun Jan 24 14:36:19 CET 2021

3.7V is the average voltage, usually LiIon or LiPo cells are run between 3.3V 
(empty) and 4.1/4.2V (full)

so 4.12V is "charged" and I don't think you need to replace it.

the "BMS" attached to the battery is actually a BPS (battery protection system), 
it main function is to prevent a total discharge which would damage the cells. 
as long as you measure 4.1V where the battery leads enter the main boards it's fine.

the ereader itself probably runs at 3.3V plus the voltages that are needed for 
the display...

On 2021-01-24 14:07, Rutger Vlek wrote:
> Hi list,
> As my goto place for electronics knowledge, I hope someone happens to know a thing or two about eReaders and/or battery systems. The thing is, I have a 4 year old Kobo Glo HD, which I have been using frequently until a 2 months ago. When my second child was born, I didn’t get around to reading for a while and didn’t use the eReader for several weeks (5). When I wanted to pick it back up, the battery was empty and I charged it. Somewhere during the night of charging the eReader froze and the screen is locked in the “42% charged” condition. Since then, the device is unresponsive. I’ve tried all the possible tricks in the book for resetting, factory resetting, SUDO factory resetting, but nothing helps. My conclusion was: the microcontroller is simply not running, so not processing any events.
> Somewhere I read that a serious crash can sometimes be resolved by disconnecting the battery, so I opened up the eReader and disconnected the LiPolymer battery pack and reconnected. No luck.
> My next step was to measure battery voltage, thinking that power is the first requirement for a cpu to run.  I measured 4.12v at the testpoint (TP2) near the battery connector and quickly concluded that it was on the low side, and started googling for replacement batteries. When I did so, I learnt that battery of this specific eReader is actually of the 3.7V kind. In other words, it outputs a higher voltage than specified. My experience with LiPolymer batteries is very limited, so I’m not sure if this falls within their normal range, but I think it would certainly be outside the typical power range of a cpu that is intended to run a 3.7V. Would a cpu lock-up in such an overvoltage condition? Or fry? In other words: what are my chances of success when I replace the battery (which is the presumed cause).
> One thing I’m still not certain about is if any voltage conversion takes place in the battery management system, which sits on a tiny separate PCB attached to the battery. I believe that the system has a whole (BMS + pack) is specced to put out 3.7V, but it could still be the case that the battery itself is 3.7V and the BMS pumps it up to 5V required by the eReader.
> What do you think? Shall I attempt to test-run the cpu from my bench supply on 3.7V through the battery connector (of course leaving the battery out)? And is this voltage I’m measuring within normal limits of a LiPolymer battery (+BMS)?
> Cheers,
> Rutger
> ps. By the way: if I connect a USB charger (measuring close to 5V) to the eReader I see the voltage on the testpoint near the battery connector go up slightly further (4.19V). That surprised me as well… though perhaps there’s an explanation.
> ps. I hereby swear my next post will be about synths again :).
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