Laptop Batteries

The Computer Corner

By Charles Miller

There is more to be said on the subject of laptop batteries than I was able to squeeze into this column last week. I was recently asked, “Why did my laptop battery quit when the meter said I had an hour left?” The reason is something that chemists and engineers understand: that there is no way to measure the amount of electrical energy stored in a battery.

To clarify, there is no way to know in advance how much energy a battery contains. It is possible to test a battery to destruction; when it is dead it will then be known how much electrical energy had been there. But there is no way to know in advance. This is how flashlight batteries are tested—one battery from a batch is destruction-tested, and it is then presumed the others from the same batch should have the same life.

Laptop computers have more sophisticated battery management, and if you look at the connector you will see it has a little comb-like connector with eight or more connectors. This is so the computer is able to charge each individual cell in the battery with just the right amount of charge. Doing this maximizes the amount of battery power but still does not give an accurate prediction of how long your battery charge will last when in use.

So then why do laptop computers even have a battery meter? And what can it tell you? The information reported by the battery meter is what we Information Technology folks call a SWAG (Systematic Wild-A** Guess). There is a logical system for estimating battery life, but in the end the information reported is still only a guess.

 

The way that guess is arrived at is quite interesting. Your laptop keeps track of how long it has been in use since the last time it was fully charged. At the same time, it has also recorded how long the battery lasted the last time it was used until it was completely discharged. Based on these two factors the laptop can extrapolate a SWAG and display this as the amount of battery power you should have available. When you use your laptop until the battery is dead, these factors will be recorded and used to compute the report of the battery meter. That is the point at which the battery meter is really accurate—right after a complete discharge—because the computer knows how long the battery just lasted.

If you read this column last week, you may have already realized the serious flaw in this methodology. The lithium-ion batteries used in modern laptops should not be discharged completely. Doing so is harmful to the battery and by some accounts can reduce the usable lifetime of the battery by half.

So you see, the conundrum is that the SWAG of the battery meter will not be accurate unless from time to time you run your laptop battery completely flat, but discharging the battery until it is dead is not really good at all for the life of the battery.

Charles Miller is a freelance computer consultant, a frequent visitor to San Miguel since 1981 and now practically a full-time resident. He may be contacted at 044 415 101 8528 or email FAQ8 (at) SMAguru.com.

 

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