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I have a laptop that's a little over 5 years old, and I've never changed the battery, so I believe Windows 7 when it tells me "there is a problem with your battery", and to consider replacing it.

My question is: how does it detect a dodgy battery? Does the battery not have the same voltage that it used to?

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2 Answers 2

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Laptop batteries have a small chip inside that controls/monitors the charging process and also monitors the number of charge/recharge cycles.

This chip is factory programmed with information how this sort of battery typically degrades over time.
It also can derive information from the charging cycle itself: The time it takes to reach full-charge at a given voltage/current changes when the battery gets worn out.
(Voltage drop during discharge is not reliable as it depends a lot on the amount of current drawn while discharging, so it varies with the use-pattern of the laptop.)

Windows communicates with this chip to get information about the battery health.

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It depends on the laptop but laptop batteries are rather smart. They can calculate how long they can last and how much their capacity is lost because of usage and aging. If your 5 year old laptop just reported that you should consider replacing it, then you are lucky, as a battery's life is 3 years average.

By the way if your computer is 5 years old, I would rather recommend replacing the laptop, instead of just battery. Because 5 years is a long time for computers, and if one other part of your computer does not fail, your HD would likely to fail at the near future.

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Agree about replacing the whole laptop. Unless it was originally a very high-end laptop, that performance-wise still is viable for another 2-3 years, the price of a replacement battery is usually too high to justify the purchase. –  Tonny Jul 15 '13 at 10:57
    
Hah, yeah I don't usually use the laptop any more. It's a 2GHz Core 2 Duo with 2GB RAM, which I replaced about 2 years ago with a fairly high-end (at the time) desktop. I just use the laptop on the couch sometimes for YouTube and reddit etc. –  Cam Jackson Jul 15 '13 at 11:02

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