June 28, 2007


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Don’t believe the hours
Your laptop can last longer, but how long?
By John “jaQ” Andrews  jandrews@hippopress.com

Anyone who’s ever owned a laptop for more than a few months knows that batteries don’t last forever.

Oh, sure, modern batteries have “no memory effect” like in the bad old days, when your computer would figure the battery was completely charged whenever you unplugged it and never charge it past that point again. And with components engineered to draw less power, batteries do last longer than ever.

When they’re new.

Unfortunately, no matter what the manufacturers tell us, it seems like all laptop batteries gradually last for less and less time. The four hours of runtime out of the box might decline to two after just a year, and older models are lucky to run for 30 minutes on a charge.

You can buy replacements, but they become harder to find the older your computer gets. If they’re not newly manufactured, they might have lost some charge before you even buy them. Even reconditioned or recycled batteries from third parties, though, can come close or even exceed your original battery’s life, but eventually succumb to the same fading away.

For truly long-term unplugged computing, there is a compromise position: external batteries. Rather than a custom battery for your particular make and model of laptop, external batteries fit many different types. They’re a little less portable, but a whole lot more convenient than always sitting by an electrical outlet.

External batteries usually connect to the power input jack on your laptop. Some models come with an array of tips to fit the widest variety of machines, while others require that you buy a tip for your laptop separately. The laptop will then think it’s hooked up to a regular old AC adapter.

That is, if the external battery puts out the right juice. If your laptop model is specifically listed, you’re probably good to go. Otherwise, check the stats on your AC adapter. For example, the power brick for the laptop on which this column is typed says its output is 19 volts at 4.74 amps. Multiplying those two numbers together gets you the wattage of the adapter, 90.06 watts. Any external battery should have the same specs as your AC adapter, though a few milliamps of difference shouldn’t hurt.

I tell you those numbers not to awe you with my power of Googling electrical unit conversion charts, but because they can give you a hint of how long a given battery will last. Most external batteries advertise themselves as “four-hour” or “10-hour extended life” batteries or something like that.

Don’t believe a word of it.

Not only does battery life depend on what model laptop you use, it depends on how you use it. Editing a text file takes a lot less power than burning a DVD and defragmenting your hard drive simultaneously. That 90 watts I mentioned above? That’s the maximum my laptop should ever use. In real life, it usually uses less.

To get a good idea of any battery’s real-world life, take a look at your existing battery. Mine operates at 11.1 volts and has a capacity of 7,200 mAh, or milliamp-hours. Multiply volts by amps and I have a 79.92 watt-hour (Wh) battery. An external 80 Wh battery, or one with a capacity of 4,210 milliamp-hours at 19 volts, would run my laptop for about the same amount of time.

I know that’s a lot of math. When in doubt, go for high Ah or Wh numbers and make sure your laptop is listed as compatible. Those will give you the longest run times.

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