Lithium ion (Li-ion) batteries are the power source of choice for laptop computers, cellphones, and similar electronic devices. Li-ion batteries are manufactured with a circuit that functions as a kind of fuel gauge. The circuit uses the battery voltage to estimate the "state-of-charge," that is, the amount of usable power remaining. Over time the circuit will become inaccurate and must be reset. Otherwise a device like a laptop may shut off unexpectedly even though readouts indicate the battery still has power remaining.
Run the device the lithium ion battery powers until the battery is completely exhausted. This will not harm a lithium ion battery as it will batteries based on some other technologies.
Remove the battery from the device and place it on a charger designed for lithium ion batteries. With the charger turned off, connect the battery to the charger following manufacturer's instructions. For most lithium ion batteries, set the voltage to 4.20 volts.
Turn the charger on and allow the battery to charge to capacity. Normally this takes 2 to 4 hours, depending on the battery. The charger will sense a voltage drop characteristic of a lithium ion battery reaching full charge and automatically halt charging.
Turn the charger off. The state-of-charge circuit is now reset. Disconnect the battery and return it to the device it powers. It's best to use the device immediately for a short period to drain some of the charge. Lithium ion batteries last longer if they are not left at full charge.
Repeat the reset procedure about once a month (every 30 recharges or so) to ensure the state-of-charge circuit is functioning properly.
Never use a charger not specifically designed for li-ion batteries. Other charger types do not have the circuitry to sense the characteristic voltage drop of a fully charged lithium ion battery and will overcharge the battery. This not only can ruin the battery, but also it is a serious fire hazard. For normal charging (rather than a charge intended to reset the state-of-charge circuit) you can set the charging voltage to 4.10 volts. This gives you an 80 per cent charge and will greatly extend the useful life of the battery.