I have a notebook from Asus which is about to make its second year of life. I have absolutely no problems with full battery capacity, which is about 2 and a half hours, only 30 minutes or so less than when it was brand new.
The problem is it shuts down for low battery with more than 30% of battery left, according to Linux and Windows. I can confirm this because, after some minutes turned off, I can still "force" it to turn on and last some minutes before shutting down again. Repeatedly doing this, I managed to get as low as 4% of battery, so I am sure the battery is still lasting as it should, but it's "critical level meter" is faulty.
It is not the system that turns off, as just waiting the battery to waste while watching the BIOS still triggers the shutdown.
What can I possibly do to fix this issue? I am not really into the mood of contacting Asus support, they make me wait a lot (last time, over 2 months) and my warranty is already over.
More info:
Battery is 19V @ 3.42A
Notebook is an ASUS K43BY
The computer just turns off, there is no warning (it's even before the SO warns low battery), it's like it has ran out of power all of a sudden.
Answer
Your symptoms sound like one (or a few) cell(s) in the battery pack is weak. A well designed battery pack would shut down as that cell goes low in voltage, this would be something that no "info" curcuit based on voltage or charge state could predict to occur, so the percentage does not reflect reality.
By continuing to drain the battery it would reset any info to believe that there is a total number of mA. available, in each charge. Not draining it fully it might eventually calculate that it was finished when it was finished anyway. Then the percentage (along with the reduce length of operation) would be reduced to the whole packs reality.
Because the battery (and a cell in it) that is weak could put out some juice , even if it had hit a lower voltage at one time or another, you are able to squeeze a bit more out of it. Similar to how a flashlight with mostly dead batteries could light up for some time again. With li-ion this usually only applies to a very weak cell or set of cells, and normally indicates they are near or at end of life.
My recommendation is to Not keep pushing it after it shuts off the first time, and replace the battery, because Li-ion batteries are only good for so long anyway. Pushing it after the curcuit shut it off could be far worse for it, depending on how the curcuit is designed.
Notes: When one or a few cell in a whole pack are weak, there is no way to replace those few cells, because they will not match well with the others. The age of the other cells are enough to change all cells in the pack. So there is no cheap way out of it.
It also could be something is overheating, and disconnecting, but I suspect that you would know if it was related more to a heat condition.
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