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How Much Wear Does Your Laptop Battery Have?
So, you are still feeling the love for your fine laptop or netbook, yet it is not reciprocating when it comes to run time. Where before you could use it for five hours at a stretch, now it shuts down after barely two and a half . You diligently charge it up as you always have, and its says 100% when you pull it off charge. Yet it is just not going the distance. What is up with that?
Well, what could be up is a little battery wear. First, a little background about the power system. The information about your battery comes from the Advanced Configuration and Power Interface (ACPI), and it gives you (and your OS) the information about the battery from – the battery itself and some calculations. You can see the numbers on your battery monitor, and the OS uses it to feed the Advanced Power Management (APM), which controls the power alarms and subsequent behavior.
Playing The Numbers
In the case of a battery that is not holding its own anymore, the lifetime is adversely affected and taken into the calculations. Once the OS has a number for run time, it is hard to convince it otherwise. A typical battery lifetime calculation is as follows:
Computer Typer: Asus 901 Netbook
Battery Capacity (B) = 52 whr (Watt Hours)
Average Discharge (D) = 10 whr
Battery Wear (W) = 0%A handy formula to calculate it all:
BatteryLife = ((BxW) – B) / DPlugging it into a formula:
Battery Life = ((52 x 0) – 52) / 10 = 5.2 hours.Now assuming that a battery has 30% wear (W), the calculation becomes:
Battery Life = ((52 X .30) – 52) / 10 = 3.64 hours
So, in the last case, even though the battery shows 100% when we take it off charge, the battery is only going to last 3.64 hours on a good day. If you are a power user, probably somewhat less.
The Good And The Bad
The bad news is that most battery monitors, including that on Windows XP, will not tell you about the battery wear. With Windows 7, it will go so far as to tell you that it is time to change your battery. This advance in handling of laptop battery wear is a very good thing.
But to the poor soul who just upgraded from XP who is now being told that their battery is shot, well they may tend to believe that Windows 7 is not doing them a favor. In reality, it is now reporting on a condition that was previously hidden.
The good news is that finding out your battery wear level is a very simple thing to do. There are a lot of freeware products that will give you this information, and one of my favorites is BatteryBar.
Finding Your Battery Wear
Once BatteryBar (or similar) is downloaded, a popup will show you the current battery state. As shown in the figure, my battery wear on the 901 was up to 45%, which means that I only had 55% of the battery to work with. My capacity was down to 30.761 whr from 55.272, which basically meant that my runtime was cut in half.
For fun, we can plug the figures in and get the total run time that my tired battery would give:
Battery Life = ((55 x .44) – 55) / 10 = 3.3 hours
Down from
Battery Life = ((55 x 0) – 55) / 10 = 5.5 hours
One thing to note on the image is that the full lifetime is being reported differently than we calculate. This is because BatteryBar tries to calculate a true remaining lifetime over usage patterns, and apparently I tend to be a heavy resource user on the machine.
Hopefully by now you realize that battery charge percentage is at best a poor indicator of your remaining run time. Instead, if you take note of your battery’s wear level, then you will have a good ideal of how far the current charge will get you. And, unfortunately, that same knowledge may let you know when it is time to replace your battery. It just goes to show – knowledge can be a costly thing.
