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Everyday after turning off the laptop (not before) I'm unplugging the power supply only from the wall, not from both wall and laptop. But it started to discharge the battery. After unplugging the cable the battery and power indicators on the laptop continue working for 10-20 seconds and on the next plugging the battery is a little bit discharged - still 90%+ but the battery indicator is always flashing red and the battery start to charge for a minute or two.

This is not a big problem for me but it's a little bit irritating and I think it may shorten the battery life.

The laptop is Toshiba Satellite A300-15E, 4+ yeards old. The problem started before few months maybe, not sure. I ignored the problem but now decided to ask for it. I think it's caused by the power adapter but don't have access to another adapter and can't try with different one.

I don't think it may be caused by the OS. Tried with 2 different Windows 7 and Ubuntu 12.04.

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No way a healthy battery can discharge that far in 20 seconds, replace the failing battery with a new one. –  Moab Nov 11 '12 at 15:46

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up vote 6 down vote accepted

This sounds to me more like a dying battery that is no longer able to hold a full charge. Rechargeable batteries do not last forever and after some time will loose capacity.

This can manifest in many ways and I am not surprised by this particular manifestation.

The red flashing light is probably trying to tell you that there is a fault with your battery.

The reason you are likely seeing it quickly go down to 80-90% is probably due to the battery voltage "relaxing" after it has stopped having power applied to it.

4 years is around the right sort of time frame to be looking at getting a replacement.

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No, the red flashing indicator is for discharged battery. When there is a problem the indicators should flash in orange as I remember from the manual. So, red flashing is okay. Is there a way to check if the problem is this which you describe. The laptop is working on battery over 1.5-2 hours and for 4 years old laptop I think it's a good time. Btw I am not using the battery often but it's everytime on the laptop. I know that high temperature is against batteries' health, also. –  enenen Nov 11 '12 at 15:45
    
According to the manual I can find at this link suggests that amber means battery charge is low and doesn't seem to say anything about a red light. To me red is a warning of something going wrong. Your battery lifetime seems reasonable though so I'm not sure... –  Mokubai Nov 11 '12 at 16:04
    
I checked your PDF but the indicator is flashing in red from the first day and I had never seen a amber/orange light for 4+ years. :) No matter, it seems you are right for the battery life after 4+ years. So, dying batteries start to can't hold a full charge and this is the way I can recognize one next time, right? –  enenen Nov 11 '12 at 16:10
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Right, the same goes for mobile phone and other devices with rechargeable batteries, if your usage doesn't change much but your battery is running out sooner then it's time to consider replacing it. –  Mokubai Nov 11 '12 at 16:20
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Your standard-issue laptop power supply brick has some pretty hefty capacitors in them that store a good amount of charge, when you unplug the supply from the wall but not the laptop they have enough juice to keep the charging circuitry in your laptop going for a couple of seconds. If you unplug it from the laptop first then the voltage present on the adaptor is removed immediately by your disconnection, removing the power from the wall means the voltage seen at your laptop will dissipate over a few seconds. To answer your next question, no this will not damage your adaptor either way. –  Mokubai Nov 11 '12 at 18:45

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