Elitebook Charging Repair
So my old notebook - an HP Elitebook 840 G5 - did not start charging when I got it out to test something.
Opening it up the battery was discharged to 0% so the BMS turned off the output and the system did not have any voltage on the main supply bus.
This notebook should be able to recover from such a situation on its own but for some reason it didn't.
A hand full of seconds looking around on the mainboard revealed some corrosion around a chip with the markings "Intersil 9241H", which turned out to be a LiIon charge controller. Bingo.
After initially removing the chip from the board, cleaning everything and resoldering it onto the board (which did not improve a thing apart from now looking nice again),
I spent quite a while trying to find a schematic and boardview for this system and about as long looking for the issue.
Turns out: It takes waaaay longer than it should to find schematics for a reasonably modern laptop and finding the actual issue was not as quick as the last sentence makes it look like ^^'
Sidenote: The Elitebook 840 G6 has an identical mainboard and while the G5 schematic seems to be unobtanium, it is quite easy to find it for the G6.
After some probing around, the "Short Circuit Protection" feature of the ISL9142 cought my eye.
According to the datasheet the chip will, before turning on the main DCDC, apply 10mA of current from the internal 5V LDO to the system bus output and wait until it reaches 0.6V.
Well... Apparently that never happens. When I apply 10mA limited to 1V externally it happily turns on and starts charging the battery. So this 10mA source inside the chips seems to be broken.
Thankfully this is an off-the-shelf part that I could easily replace but
- that costs 9€
- takes at least two days for shipping
- and is no fun
So I came up with a surprisingly good and easy solution: A 510Ω resistor in series with a diode from the chip's VDD pin (the 5V LDO's output) to the system bus. This sinks somewhere slightly below 10mA from VDD to the system bus as long as the system bus is below 5V (which never happens as long as the battery isn't empty enough for the battery management system to turn off completely).
