Laptop batteries display a puzzling behavior: when they are new, a laptop may run for hours on a full charge, but as time passes, the battery doesn’t last as long. Checking the battery charge in Windows may show 100% charged but the laptop only runs for minutes.
Solving this puzzle requires understanding how batteries change as they age. Batteries are like a gas tank that shrinks a little each time its used. Imagine a new car with an 18 gallon gas tank; when filled, it holds 18 gallons and the gauge shows full. If the tank shrank a little each time it was used, the same way a laptop battery does, it would eventually hold only 9 gallons but the gauge would still show full after it was filled.
The best way to evaluate a laptop battery is to use a software diagnostic program designed to check the battery. All laptop batteries include built-in charging circuits that store a history and status for the battery. Hewlett-Packard provides an excellent battery diagnostic program that displays the original and current capacity of a battery, along with the age and number of charge cycles.
Most Lithium Ion batteries begin to lose capacity significantly after 200-300 charge cycles and require replacement when they fail to charge.