The accuracy of what the battery meter reports—what percentage of a full charge remains and how long you can use your laptop before you must plug it in—depends on several factors. Most of these factors fall into the following two categories:
What you use the laptop for. Because some activities drain the battery faster than others (for example, watching a DVD consumes more power than reading and writing e‑mail), alternating between activities that have significantly different power requirements changes the rate at which your laptop uses battery power. This can vary the estimate of how much battery charge remains.
Battery hardware and sensor circuitry. Newer, "smart" batteries are equipped with circuitry that calculates the measurements of charge remaining and reports the information to the battery meter. Older batteries use less sophisticated circuitry and might be less accurate.
Most laptops use lithium-ion (Li-ion) batteries. Like most batteries, lithium-ion batteries gradually lose their capacity to store energy as they age, whether or not you charge them. Over time, this change in storage capacity significantly affects the accuracy of the battery meter. If you frequently drain a lithium-ion battery, and then recharge it, it can quickly lose its ability to hold a charge, which affects the accuracy of the battery meter. The practice of completely draining the battery, and then fully charging it, is advice that applied to nickel cadmium (NiCd) batteries and, to a lesser extent, nickel metal hydride (NiMH) batteries.
Lithium-ion batteries last longer if you charge them often, a little at a time, to maintain a minimum charge of about 40 percent capacity.
If your laptop battery can no longer hold a charge, consider replacing the battery.