  How we test external laptop batteriesBy Brian Nadel(April 5, 2004) To gauge how long an external battery lasts, we have adapted the CNET Labs laptop-testing protocol to this specific purpose. The test laptop is a Toshiba Satellite Pro 6100 with a 4,000mAh battery that can power the machine for 90 minutes. To start, each external battery is charged completely and checked for the correct voltage. We then connect the external battery to the laptop with the supplied AC power adapter and let the battery discharge by running the Satellite Pro 6100 connected to a Wi-Fi wireless network with Internet Explorer tuned to an Internet radio station. All power conservation software is disabled. We monitor the test's progress with PassMark's BatteryMon program. All tests are repeated three times, and we average the results. When the tests finish, we plug the external battery into an outlet and time how long it takes to recharge. While the best external batteries we tested provide an extra 4 hours of power, your mileage will vary depending on your laptop and hardware and what programs you're running. With a long-lasting, low-power laptop or a tablet PC, such as the Electrovaya Scribbler SC-2000, the external battery may add an extra 10 hours or more of battery life.
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