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Asus laptop battery without charge balancing?

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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

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.

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

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.


--- Quote from: dietert1 on November 25, 2023, 08:52:04 pm ---Lipo cells seem to fail after deep discharge resp. reverse charge as it happens without protection.

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


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