Cordless phone and cordless tool battery packs eventually stop holding a full charge or any charge at all. Making your own replace battery pack is much less expensive than purchasing a new one. Batteries secure together with nickel strips to hold them in place and connect them so they operate in a series or parallel. Spot welding the strips on the batteries makes a permanent secure connection between them.
Set the batteries for a battery pack on a solid surface like a table or workbench. Tape the batteries together to stabilise them and to keep them from moving.
Place a nickel strip across the tops of two batteries, so it is touching the steel part of each battery.
Hold one fully-charged welder pen in each hand. Place both welder pen tips on one end of the nickel strip on a battery. Press the foot pedal to weld both areas under the pen at once. Release the foot pedal. Weld the opposite end of the nickel strip to the second battery in the same manner.
Solder wiring with a connector to the negative and positive ends of the batteries to make a battery pack. Shrink wrap a battery pack to keep the batteries in place and aligned to fit a cordless phone, cordless tool or any other battery pack. Flat-top batteries and all supplies to make battery packs are available at electronics stores.