How the power goes. (laptop battery)
by Jill Champion

There's more to testing the life of a laptop battery than turning it on and letting the unit run until the power gives out. To truly test a manufacturer's claim of battery-powered operating time, you have to test the battery as if it were in normal use.

Almost all laptops use rechargeable ni-cad (nickel-cadmium) batteries, with a flat voltage discharge. That simply means that voltage discharges constantly until the battery runs dry. For example, a six-volt ni-cad battery will give a constant six-volt discharge up until the moment it dies, unlike, say, flashlight batteries that gradually lose power. Ni-cad batteries also operate well in low temperatures, unlike their acid-based automobile counterparts that tend to be sluggish on very cold mornings.

Our test lab charges the nicad battery according to the manufacturer's exact instructions. Once the battery is fully charged, the laptop is put to use, and its between-charge lifespan is tested using COMPUTE's battery-testing program. The program monitors the laptop constantly until it completely gives out, so the exact power-up and power-down times can be recorded.

But there's more to it than that. To realistically simulate laptop use, our testing program runs the laptop disk drive in a selected duty cycle. For instance, in a 40-percent duty cycle, the drive runs for four minutes of continuous disk access and then rests for six minutes. The cycle is repeated constantly until the battery dies. The test program's timer constantly saves to disk so when the system is booted up again after recharging or plugging in, the time has been recorded for reference. A stopwatch serves as backup.

If a laptop has an automatic rest mode or screen blanker, someone physically monitors the computer, tapping it every screen cycle to return the screen to normal mode.

Once the battery dies, the lab repeats the test two more times in its entirety, beginning with a battery recharge. After testing each laptop three times, the lab uses a spreadsheet to calculate mean times for each battery.

Most batteries are going to yield about the same amount of running time because they're essentially the same product--rechargeable ni-cad cells. The real difference is in how you use your laptop, what type of microchip runs it, and the machine's built-in power-saving features.