Asus laptop battery without charge balancing?

[dietert1]

We have an Asus laptop F756AU where the built-in Lipo battery doesn't work anymore (7.6 V 5 Ah nominal). After power-on W10 would display a "charging" animation, but that would stop after about 20 seconds. Took out the battery pack C21N1515 and looked a bit closer. There is a small board inside that seems to implement the required SMBus queries. The battery itself has two terminals. I connected a power supply and it charged up to about 4.2 or 4.3 V. Later it settled to 4.18 V while disconnected. I assume one of the two cells is dead (short), though no sign of pressure. We will need a replacement.
Wait a minute: Were is the charge balancing circuitry? I thought Asus was kind of a brand.

Regards, Dieter

[ealex]

I don't think I've ever seen one in a laptop battery.

The same in power tools - I have some ~ old metabo and bosch packs that are starting to show their age (5+ years) - all cells are still a few mV apart.

It looks like good quality cells wear out at the same rate, and when one fails the rest are already worn out.

Not the same for some dubious quality ones ... fixed some "china special" packs where at least one of the cells where completely dead, and the others looked OK.
But in those cases, the cost of quality cells alone was near the cost of the entire tool.

[dietert1]

Yes, that's true. On their web site they call laptop batteries "consumables" and that you would need a new one every year in order to stay above 80 % nominal capacity. Meanwhile i found that the C21N1515 battery pack is EOL and the defective battery pack wasn't original either. But that notebook still has an original sticker saying "2.5x extended battery life".
Rather than implementing some fancy SMBus stuff they should have implemented charge balancing. Lipo cells seem to fail after deep discharge resp. reverse charge as it happens without protection. On the circuit board i found two back to back RQE150BN mosfets. So the SMBus processor has the capability to disconnect the load but that didn't prevent the failure. Maybe because that processor itself consumed the remaining charge.

Regards, Dieter

[mikerj]

I have never found charge balancing in any laptop battery I have taken apart.  It would be quite a step up in terms of additional cost and complexity.

[wraper]

Lipo cells seem to fail after deep discharge resp. reverse charge as it happens without protection.

Reverse charge never happens as battery (BMS) disables itself if any cell discharges below certain voltage.

[dietert1]

For a two cell LiPo battery with two terminals there is no way to know the cell voltages. Only the sum appears at the terminals. I assume one of the two cells in the bad battery failed as it maintained 4.18 V the next day after charging it several hours at 2 A, so the other cell has a (near) short.
I remember studying the very same problem with Fluke 8600 meters. The battery pack with four big NiCd cells would not last until i modded a simple monitoring circuit into it. The circuit had one transistor and a resistor per cell plus a mosfet to disconnect the load when any cell went below 0.7 V. I had to buy new batteries only after 10 years or so. We have three of those meters.

Regards, Dieter

[wraper]

For a two cell LiPo battery with two terminals there is no way to know the cell voltages.

Nonsense, any laptop battery BMS senses voltages on each cell. Why do you think there are 2 only terminals and not 3 coming from cells to BMS? If you see only 2 strips or wires, it does not mean there isn't third strip or wire coming to BMS from somewhere else.

[dietert1]

Nonsense, if i write there are only two terminals it's because i tried to fix it and looked at the board in the battery pack in some detail. The Asus battery pack does not have any charge balancing. It does not know the cell voltages.
They implement a lower voltage limit of about 4 or 5 V. It is very unlikely that one cell still has 4.2 V when the other one starts reverse charge. Anyway: Meanwhile i bought a replacement battery pack and everything seems to work, i mean no more errors.

Regards, Dieter

[wraper]

Nonsense, if i write there are only two terminals it's because i tried to fix it and looked at the board in the battery pack in some detail.

And I bet you did not take it apart entirely.

The Asus battery pack does not have any charge balancing. It does not know the cell voltages.

Charge balancing and sensing individual cell voltage are two different things. Balancing is needed for improving battery longevity and normally is not present in laptop batteries, individual cell voltage sensing is needed for basic safety and is a must. Post a photo if you're so sure. What is BMS IC model?

[dietert1]

I understand that you can't believe it. This is why i posted my findings. As someone wrote above this Asus product is on the same simplistic level as a battery screw driver. As i wrote i had a close look at the control board inside the Asus battery pack. Its connection to the LiPo battery has two terminals, while the notebook interface has like 10 pins.
I did not open the LiPo battery. I looked for yet could not find any indications that there is more electronics inside that pack. A power density calculation also indicated that there isn't space for another board. Probably a discharge test could prove there is no lower limit built into the battery itself.

Regards, Dieter

[ealex]

now you made me curious - that should not get out of a "decent" factory.
once they get out of balance that bad, one cell can overcharge until it fails - those pouch type cell will swell (seen multiple notebooks with this problem)
can you take a picture of the BMS ?
is it original or a replacement pack ?

[dietert1]

I discovered the laptop battery problem during preparations for a travel and will post images when i come back. Yes, the battery itself appears like a pouch with aluminum foil outside and the battery pack covers the aluminum leaving the top only covered by some plastic foil, so it can swell a little without developping a lot of pressure. The bad battery was a replacement and it appeared like new. I bought that notebook as new some years ago and rarely used it. The problem was finding the right type of SSD module to use in the socket inside the notebook.
I remember finding in a drawer some old cell phones that had not been used for years. I took them out and looked at their batteries for curiosity. Some of them looked dangerous, almost like an electrolytical cap near pressure failure. Of course all of them bad. Also i remember getting a bunch of failed emergency lamps as present. Those were LED lamps with small Li-Ion batteries inside that would provide illumination for an hour or so during a mains power outage. Since that never happened the batteries would all swell and fail after some time.

Regards, Dieter

[Psi]

a multi-cell (series) lithium powered consumer device without any protection from overcharging one cell (Due to no single cell voltage detection) would not pass safety testing.

So there must be something you missed, perhaps each cell has an internal PCM that prevents overcharge/overdischarge.
It doesn't need a 3rd wire, or cell tap, to stop the battery from getting so imbalanced that it would be dangerous.