Plug it in to a charger made for that battery's voltage

A lithium-ion battery charges through a charger that matches its voltage rating. The charger converts wall power to the right voltage and current for your specific battery — a phone charger for a phone battery, a laptop charger for a laptop battery, a drill charger for a drill battery. The charger itself does most of the work: it knows when to stop, how fast to push power in, and when the battery is full.

You do not need to do anything special while charging. Plug the charger into the wall, connect it to the battery or the device containing the battery, and leave it alone. The charger will turn off or switch to a trickle charge when the battery reaches full capacity. Most lithium-ion batteries reach 80 percent charge in 30 minutes to two hours, depending on the charger's power rating and the battery's size.

Key Takeaways

  • Use only a charger rated for your battery's voltage — using the wrong charger can damage the battery or create a fire risk.
  • Lithium-ion batteries do not need to fully discharge before recharging, and partial charges are actually better for battery lifespan.
  • Charging speed depends on the charger's wattage rating, not the device — a higher-wattage charger will charge faster if your device supports it.
  • Heat is the main enemy of lithium-ion batteries, so avoid charging in hot environments or leaving a charged battery in direct sunlight.
  • Most modern chargers have built-in safety circuits that stop charging when the battery is full, so leaving a device plugged in overnight will not overcharge it.

Match the charger voltage to your battery

Every lithium-ion battery has a voltage rating printed on it or on the device manual. Common ratings are 3.7V for single-cell batteries (like in phones), 7.4V for two-cell batteries (like in some cameras), and 11.1V or higher for larger packs (like in power tools). A charger rated for 5V will not charge a 12V battery, and trying to use the wrong charger can damage the battery's internal chemistry or cause it to overheat.

The safest approach is to use the charger that came with your device. If you need a replacement, buy one from the device manufacturer or a seller who lists the exact voltage and current rating. The charger label will show "Input" (wall power) and "Output" (what goes to the battery) — match the output voltage to your battery's voltage rating.

Understand why you should not wait for a full discharge

Older battery types like nickel-cadmium needed to fully discharge before recharging, or they would "forget" their capacity. Lithium-ion batteries work the opposite way. Fully discharging a lithium-ion battery actually stresses it and shortens its lifespan. Charging from 20 percent to 80 percent is gentler on the battery than charging from 0 to 100 percent.

This means you can plug in your phone, laptop, or power tool whenever you want — you do not need to wait until the battery is dead. In fact, if you use a device daily, letting it drop to 20 percent before charging and unplugging at 80 percent will keep the battery healthier longer than fully draining and fully charging it every time.

Know what affects charging speed

Charging speed depends on the charger's wattage, not on how long you leave it plugged in. A 5W charger will take much longer than a 20W charger to fill the same battery. The charger's wattage is listed on its label (often shown as watts, or as volts × amps — for example, 5V × 2A = 10W).

Your device will only accept as much power as it is designed to handle. A phone rated for 20W charging will not charge faster if you plug it into a 65W laptop charger — it will still charge at 20W. However, if your device supports fast charging and you use a charger with higher wattage, it will charge faster. Check your device manual to see what charging speeds it supports.

The battery itself also affects speed. A larger battery (measured in milliamp-hours or mAh) takes longer to charge than a smaller one, even with the same charger. A 5,000 mAh phone battery will charge faster than a 10,000 mAh battery with the same charger.

Keep the battery cool while charging

Heat damages lithium-ion batteries and reduces their lifespan. Avoid charging in direct sunlight, in a hot car, or in a room above 85°F if possible. It is normal for a battery to warm up slightly while charging — that is the chemical reaction inside — but it should not be too hot to touch.

If your device gets very hot while charging, unplug it and let it cool down. Repeated overheating can permanently reduce the battery's capacity. Some devices have built-in temperature sensors that slow charging or stop it entirely if the battery gets too warm — this is a safety feature, not a malfunction.

Recognize when a charger has failed

A charger that no longer works will show clear signs. The battery will not charge at all, or it will charge very slowly even after several hours. The charger cable may be damaged — look for cracks, exposed wires, or bent connectors. The charging port on your device may also be damaged, which looks like bent pins, corrosion, or a loose connection.

If the charger is the problem, replace it with one rated for your battery's voltage and current. If the port is damaged, the device may need repair. Do not try to use a damaged charger — a broken charger can deliver the wrong voltage and damage the battery or create a fire risk.

Store batteries safely when not in use

If you are storing a lithium-ion battery for weeks or months, charge it to about 50 percent before putting it away. A battery stored at full charge or completely empty will lose capacity over time. Keep it in a cool, dry place — room temperature is ideal, and avoid direct sunlight.

If you store a device with a built-in battery (like a laptop or phone), the same rule applies: charge it to 50 percent before storing it. Some devices have a storage mode in their settings that you can enable to help preserve the battery during long periods of non-use.

Frequently Asked Questions

Is it bad to leave a lithium-ion battery plugged in overnight?

No. Modern chargers have circuits that stop charging when the battery reaches full capacity and switch to a trickle charge to maintain it. Leaving your phone or laptop plugged in overnight will not overcharge it or damage the battery. However, keeping a battery at 100 percent for extended periods does slightly reduce its lifespan, so if you charge overnight regularly, unplugging it in the morning is slightly better for long-term health.

Can I use a phone charger to charge a power tool battery?

Only if the voltage matches exactly. A phone charger is usually 5V, while most power tool batteries are 12V, 18V, or 20V. Using the wrong voltage will not charge the battery and may damage it. Always use the charger designed for your specific tool and battery model.

Why does my battery charge slowly on some days?

Cold temperatures slow charging. Lithium-ion batteries charge more slowly below 50°F and may not charge at all below freezing. If your device is cold, bring it to room temperature and try again. A slow charger, a damaged cable, or a failing battery can also cause slow charging — try a different charger if you have one available.

What should I do if my battery gets very hot while charging?

Unplug the charger immediately and let the battery cool down. Repeated overheating damages the battery permanently. If the battery continues to overheat with a new charger, the battery itself may be failing and should be replaced. Do not use a charger that causes overheating.

Do I need a special charger for fast charging?

Only if your device supports it. Check your device manual to see if it lists a fast-charging wattage. If it does, a charger rated for that wattage will charge faster. If your device does not support fast charging, a higher-wattage charger will not make it charge any faster — it will just charge at the device's maximum supported speed.