Jump starting dead laptop battery packs to revive them.

[Refrigerator]

I have this HP ProBook 4510S and when i got it the battery was dead, as in "0%, not charging" kind of dead.
So, you know, sitting here in my uni dorm having nothing better to do i thought why not try revive this battery.
I don't have much with me besides a multimeter and a pair of two cell lithium batteries for my RC car so i had to MacGyver my way out of this.
The laptop pack is 3S and my two series packs make a 4S so i have some headroom in terms of charging voltage.
If i connected both of my packs as-is the BMS would go into current limit and shut everything off so i need to limit the current.
Easiest way to limit the current would be a big fat resistor, problem is i haven't brought any of those.
This is where a pencil came to the rescue. I cut open a pencil i had and about 4cm of the graphite made for about a 4.5 ohm resistor.
I hooked the 4S pack through the resistor to the laptop battery pack and got about 0.65A of current going through everything.
The battery started at 10V and finished charging at 11.2V when my 4S pack died off at about 100mA charging current, with the resistor changed to about 2.3ohms.
And so the time came to test if this worked and worked it did, of course there were some issues.
First one being that either the BMS or the laptop was confused because the battery shows 0%, yet if i pulled the charger out it continued to work.
The other was when i plugged the charger in it showed "0%, charging" but that quickly changed to "0%, not charging".
I then pulled the charger out and plugged it back in until i got "100%, charging", which was a surprise.
The charging quickly changed to not charging but after i pulled the charger out and plugged it back in it showed me "99%, charging".
That then turned into not charging then i repeated the unplug-plug routine and it gave me "98%, charging".
The gaps between plugging in the charger and it going "not charging" get longer every time i do the unplug-plug thing so it think the laptop or the BMS might be re-adjusting to the jump started battery pack.
One interesting thing is that i have a battery pack from an ACER laptop that held for about one hour but once i let drain until the laptop shut down and the battery went "0%, not charging", which makes me think that this might be because one of the cells goes too far out of balance.
The BMS board inside the battery pack has a balancing circuit built in so giving the battery a jump start might give it just enough juice to pass and start charging.
Anyways, the battery still says "98%, charging" and my power brick feels hotter than usual so i say it's a win.

[Halcyon]

Be *very* careful doing this to Lithium battery packs, I think we've all seen what happens in a thermal runaway event.

Yes, back in the days of Nickel Cadmium and to a lesser degree, Nickel-metal Hydride batteries, you could "revive them" with short bursts at relatively high current. Lithium packs are very different.

Also, keep in mind, that even on modern laptops, the battery indicator isn't always accurate and can drift "out of calibration". Many BIOS's have a battery re-calibration routine built-in to reset the battery charge monitor.

[capt bullshot]

Looks like the battery has gone to low voltage, and protection circuit cut off. The protection is designed to re-enable if a charging current is applied, that's what you did. You should use a quite low current for cell recovery from low voltage, less than 0.05C (that's what I remember), increase the current once the voltage has a normal level. Depending on the cells, these may have lost a major part of their energy storage capabilities.
The charge counter inside the battery looks confused, may settle after a few charge / discharge cycles. The charger is usually controlled by total battery pack voltage, not by individual cells. So if one of the cells is damaged, has lost capacity or went high ESR, the internal balancer won't be able to keep up with the rather high charging current and will cut off the pack due to overvoltage, which will in turn confuse the charger / capacity monitor.  The pack may recover a few times, but modern packs have a dual stage safety mechanism with a permanent cut-off to prevent cell overheating in such cases.

[Refrigerator]

@Halcyon
0.65A is no high current and i know these things pretty well by now.
One funny thing i noticed is that my battery health is now apparently 1485.8% 

[TerraHertz]

I wonder if HP and other laptop manufacturers are now doing programmed battery obsolescence, like HP (and others) do with ink jet cartridges and heads?

That is, the embedded battery management software might be deliberately over time understating the battery performance, and/or simply refusing to recharge it after some elapsed time. So you have to buy a new battery from them.

I'm amazed at how well 18650s salvaged from old laptop packs seem to work, once recharged.
It's annoying that every single laptop battery pack I cut open, has the PCB coated in an opaque mask, so you can't trace the circuitry. Does anyone know if there's some legal requirement for that? Safety? In what way does the mask make it safer?

[Halcyon]

One funny thing i noticed is that my battery health is now apparently 1485.8% 

Either the calibration is waaaay out and/or it's time for a new battery. :-)
I always take those software power management and meters with a grain of salt.