Laptop pack - how could this happen?

[Fallingwater]

I bought a second-hand Acer notebook for my mom on eBay.
It came with a dead battery, despite the seller's claims that it still had half the original life left... I gloat at the thought of ruining his perfect 480+ 100% positive feedback if he doesn't pay me half the price of the new battery.

Anyway: it could hold *no* charge. The notebook would die the instant I unplugged the power supply. I measured the voltage at the various terminals, and got nothing higher than one volt.
So I figured what the hell, might as well crack it open.
I cut the enclosure with my dremel, cut the various tabs (nickel tabs! Precious nickel tabs to recycle for custom packs! ), cut the wires and separated the eight 1800mah 18650 cells.

I figured they were all dead, but measuring them with the multimeter revealed four cells still alive at 3.90 volts.
Of the other four, two were completely dead at 0 volts, and two were even reversed - one very slightly at -0.2 volts, the other more seriously at -0.6.
I disconnected them and recharged the good ones. The battery must be some three to four years old, possibly more, so a lot of the capacity is gone. I figure they have about 500 to 800mah left... they'll do for small appliances that don't want lots of power.

I let the other four rest a bit. The two dead ones, that measured zero volts flat before, are now showing signs of life with a few tenths of volts. I figure with a VERY careful and slow charge up to 3.something volts I can restore them to the point where my charger will take them again. We'll see.

After resting a bit, even the reversed cells have somewhat recovered. They are still reversed, but the one that read -0.2 volts now only reads -0.003, and the other one -0.12 volts. I'll try recovering those too, after putting them in a metal pot so that should they vent they won't set fire to anything.

Now, the question: how could this happen? The pack was wired with what I assume are balancing taps, with smaller wires going from four (or was it five?) contacts in the series-parallel arrangement to the electronics, so it oughta have prevented total flattening of only four of the eight cells, and reversal of two. I'd expect all eight cells to degrade at the same rate. And yet, this didn't happen. Why?
Considering reversal can lead to fires, this kinda worries me about Acer batteries...

 

[mdocod]

unfortunately, I haven't seen any laptop pack designed correctly for optimum safety, their are usually cells in parallel with nothing to single them out for balance charging, and the protection circuits are usually arranged to handle those pairs of cells, not individual cells.

Every cell in the pack needs to be disposed of in an appropriate manner. The partially *good* cells are probably far below the safety margin.

I wouldn't be surprised if there is an inherent flaw in the design of the Acer laptop packs, brand new they are usually only good for about 30 minutes. (I have one around here and have read about the issue). I wonder if some of the cells on a huge batch were installed wrong, or wired wrong or something, might explain the few cells that were totally dudded out.

 

[koala]

Hi, the balancing tabs are there for charging purposes only. The protection circuit may have drained the cells.

 

[Trashman]

I wouldn't be surprised if there is an inherent flaw in the design of the Acer laptop packs, brand new they are usually only good for about 30 minutes.

The battery in my wife's Acer got at least 1.5 hours, new (actual). Her Acer's battery has never been a problem. There was a problem with bad solder on the HD controller, though, which cost me $150 to repair (and diagnose).

 

[Illum]

look up acer battery recall, you might have a bad cell

 

[Fallingwater]

unfortunately, I haven't seen any laptop pack designed correctly for optimum safety, their are usually cells in parallel with nothing to single them out for balance charging, and the protection circuits are usually arranged to handle those pairs of cells, not individual cells.

That seems pretty stupid to me.
With the money a notebook computer costs you'd think they could give us at least a properly balanced battery...

Every cell in the pack needs to be disposed of in an appropriate manner. The partially *good* cells are probably far below the safety margin.

Two of the good cells have recharged fully. After a day they now read 4.09 volts. Not the 4.2 they should read, but with their age I'm hardly surprised.
During the same day, the third cell has spontaneously recovered to the same 4.09 voltage.
The fourth is charging as I write.

I tried recovering the less damaged of the four "dead" ones. I used my variable power supply to give it 100mah until it got to 3.3 volts (probably too much, actually), then I hooked it up to the charger.
I charged it for the whole afternoon and evening, and the charger didn't show any sign of getting close to completion (it usually blinks slower as the battery fills up).
I finally got tired and disconnected the cell. It reads 4.11 volts now, but it took so damn LONG to get there. I have my doubts it'll hold this voltage though... I'll measure it again tomorrow.
I measured the voltage as it was charging (yes, I did disconnect the charger while measuring), and it got quickly to 3.7v, then slower to 3.8, and finally it crept up to 3.9. This maybe in two hours. And then it took, oh, six more hours to get to where it is now. It would have probably taken the whole night to get to 4.2v, if it ever got there.
It's probably too damaged to ever get to end-of-charge voltage, but at least it can still hold some power.

I'll try hooking it up to my ferocious little Cree light and see how much it slumps under the 600ma load.

I'm also curious as to how the other three "dead" cells will behave.

 

[DM51]

These cells are all dead, and you should get rid of them immediately. When cells can no longer hold 80% of their rated capacity, they are dead. A sure sign that they have had it is that they take a long time to charge. It would be dangerous to keep using them.

 

[zipplet]

I'll try hooking it up to my ferocious little Cree light and see how much it slumps under the 600ma load.

I'm also curious as to how the other three "dead" cells will behave.

Please, as the others have said trash the cells now before you hurt yourself. Sure, the cells may charge but they have a large potential to explode when actually used. This is a very dangerous thing to do to old/dead li-ions - ask some of the others around here who I am sure can point you at some proof of this.

This is for your own safety... I know you want to save a buck but it's not worth it! Even if it works at first, it may explode/catch fire later on when you least expect it.

See this extract from wikipedia:
If a Li-ion battery is stored with too low a charge, there is a risk of allowing the charge to drop below the battery's low-voltage threshold, resulting in an unrecoverably dead battery. Once the charge has dropped to this level, recharging it can be dangerous. An internal safety circuit will therefore open to prevent charging, and the battery will be for all practical purposes dead.

There is a reason for these safety devices. Just because you can get around them, don't!.

 

[Fallingwater]

Guys, calm down.
I know the risks of running lithium cells in the way I'm doing it, which is why they are always in a metal pot, and also why I always keep a bucket full of sand near my work area.
Should one catch fire I'll just dump the sand in the pot and the problem will be solved.
Well, it's not so much sand as cat litter, actually, but it should work anyway.

I'm doing this out of curiosity. I don't intend to use the dead cells for anything.

On this subject: the recovered cell went below four volts while resting during the night. It read 3.9 today. Hooked to my 9-LED light it slumped down to 3.65 volts under a 200ma draw.
So yeah, it's pretty much dead. I won't try to recover the other three as it seems a waste of time. I'll get rid of them.

As for the other four: they get to full charge, hold 4.1v after resting and don't slump horribly under load. They seem... fine. Old, but fine.

 

[koala]

It's perfectly ok to use old cells as long as you know what you are doing. In fact old cells have less danger compared to new cells as the substance in it is less active. Plus old lithium ion cells probably can't put out more than 1C so they have less risk of short circuit and self destruction.

In a demonstration, I showed several local RC members how to lithium ion safety. I hooked up a fully charged 3yr old 17670 to a piece of wire shorting it. The wire insulation melted nothing happen. Next I proceed with a very new AW 17670, the wire insulation melted and it glow red hot, a safety fuse was installed it blew. Without the fuse the AW 17670 will probably burst in to flame. Which is no good but that was my point. New cells very dangerous. Old cells can't perform but still dangerous. The 3yr old 17670 discharged to about 1300mAh @ 500mA rate. I tried higher discharge rate at 1.2A, it won't budge.

For many years I have been messing with numerous types of cells, I have never seen the "Discard after expiry date" information on any type of cell. I don't know where you guys are getting this information... wiki? is there a conspiracy somewhere?

Cell should be binned when
i) Leaking
ii) Punctured / Out of shape
iii) Stop accepting charge
iv) High self-discharge
v) Deterioration of the shell

If you like to charge dead old cells, charge it slowly at 50-100ma bring it up to a safe voltage point eg. 3.0volts. Then it should be ready to accept higher charge level. Don't exceed 1C of charging rate and it's best to terminate at 4.1V. If it won't accept charge then it's time to bin it. Recovering old cells is a rewarding experience. The cells will not explode for no reason. As long as you are not doing silly things like my demonstration and NOT charging them to more than 4.20v, you are safe.

 

[DM51]

Guys, calm down.

I'm perfectly calm, I assure you. If you understand the risks, that's fine as far as I'm concerned. You sound semi-prepared for a nasty accident, although I hope the metal pot you are using is a solid one and you have taken other precautions too. Let us know what happens, and don't forget to take some photos when it does.

 

[Illum]

Let us know what happens, and don't forget to take some photos when it does.

when in doubt run! photos can wait, saving your life cant:candle:

 

[zipplet]

Fallingwater, as others have said if you are aware of the possible issues and are taking precautions, that's cool. I'll cool off - I just don't want you to end up in a nasty accident - the risk is quite real IMO.

Koala, this "expiry date" that you have heard about is related to the way li-ions degrade irreversably with time - usually the useful life is quoted at 3 years although I am sure people get more life than that from them.

As to why the cells can be so dangerous, one reason I have heard is that metal "dendrites" (?) can form inside the cell when overdischarged, causing overheating and explosion in later use.