Laptop batteries, like all rechargeable battery packs, lose their charging capacity over time. Because new laptop batteries are expensive, replacing the individual cells inside the battery casing can save a lot of money. However, rebuilding a battery sometimes does not work as expected, so it's important to be willing to purchase a new laptop battery if the procedure fails.
Allow the laptop battery to run down completely. Turn off your laptop, then remove the battery. Write down the model number located on your battery to help identify the type of Li-ion replacement cells you need to purchase.
Split open a small section of the battery pack (at the seam) with a screwdriver or craft knife. Continue to pry the plastic case loose moving around the outer edge until the entire top is free. This may take a bit of force.
Note the number of cells inside the case (usually four to eight). Write down the part number and voltage information printed on the cell casings. Draw a diagram showing the arrangement of the cells and how they are connected, or take a few photos to help with reassembly.
Purchase the correct number and type replacement batteries from a local or online dealer, based on your battery model and cell specifications. Six Li-ion cells typically cost around £19 to £26.
Carefully remove each cell using a craft knife and wire cutters as necessary. Each battery pack is different, but the cells are usually wrapped in a plastic casing with metal contacts attached to one end. Lay the metal contacts, wires and casing material aside.
Use double-sided tape or a solder gun (only if necessary) to attach the metal contacts to the cells and circuit board. When finished, the old cells should be completely stripped (naked) and the new cells should look like the old ones as much as possible.
Place the new cells inside the case, referring to your diagram or photos if necessary. Glue the battery pack top back on. Charge the new battery for at least 24 hours before using it.
Keep an eye on the new battery while it charges for the first time. If the contacts are not connected properly, the battery could overheat and cause damage to the laptop.