Laptop batteries: when to replace one and how to find the right match

What a laptop battery does

The battery runs the laptop when it is unplugged. Inside are rechargeable lithium-ion or lithium-polymer cells and a small circuit board, sometimes called the gauge or the battery management board, that measures the charge and reports the battery's condition to the laptop. Older laptops use a removable battery that clips into the base. Most recent ones use an internal battery that sits under the bottom cover and is held by screws or a cable connector.

How do you know the battery needs replacing?

Every lithium battery holds a little less charge each month and with every charge cycle. That is normal wear, not a fault. It is time to replace the battery when one of these happens:

• The laptop runs for much less time on battery than it used to, even after a full charge.
• The laptop shuts down suddenly while the battery still shows charge left.
• The operating system warns that the battery should be replaced or that its condition is poor.
• The battery will not charge, or stops charging part of the way through.
• The battery is only detected some of the time, or not at all.
• The bottom cover, keyboard or touchpad is being pushed up from underneath. This is a swollen battery. Stop using and charging the laptop, and replace the battery promptly.

Check the charger before you buy a battery

A battery that will not charge is sometimes a charger or charging-socket fault instead. If the laptop also refuses to run from the mains with the battery removed or disconnected, or the charging light does not come on with a known-good charger, look at the AC adaptor and the power socket first. Most operating systems can also produce a battery report that compares the battery's design capacity with what it holds now, which tells you how worn it really is.

How to choose the right battery

Laptop batteries are made for specific models, and the same model is often sold with more than one battery size. Use these checks in order:

• Read the part number on your old battery's label. There are often several numbers, such as the manufacturer's spare part number, a battery model code and the cell maker's number. Any of them can help us confirm the match.
• If you cannot get to the battery, use the laptop's exact model or product number from the label underneath it, or the machine's serial number.
• Match the shape and the connector. Two batteries for the same series can have different connectors or mounting points.
• Match the voltage printed on the label. A different capacity, shown in watt-hours or milliamp-hours, can be fine if the voltage, shape and connector are the same, and gives a longer or shorter running time.
• Some manufacturers print their own spare numbers. HP labels usually show a spare part number in a form like 123456-001 or L12345-001. Lenovo uses a FRU number, the field-replaceable unit number, which has seven characters on older models, such as 01AB234, and often ten on recent ones, such as 5B10A12345. Dell prints a short part number, often shown after the letters DP/N.

Fitting it safely

• Shut the laptop down fully and unplug it before opening it. Sleep or hibernate is not enough.
• For an internal battery, disconnect the battery cable from the board before touching anything else inside.
• Never puncture, bend or force a battery, and do not use metal tools to lever it out.
• Charge the new battery fully before relying on it, and let the laptop learn its charge level over the first few cycles.
• Take the old battery to a battery recycling point. Lithium batteries must not go into household waste.