Open the battery pack. This is very simply done if the pack has screws that hold the case together. If the pack is a solid, one-piece molded unit, you will need to cut it open.
Attach a chisel tip to the soldering iron, and heat it up. Use the tip to cut an outline along the top of the base of the battery pack. Pull off the top and set it aside. Inside will be a group of C-size rechargeable batteries in a generic cardboard casing. Read the text on them, which will indicate whether the cells are Nickel-Cadmium (Ni-Cad) or Nickel Metal-Hydride (Ni-Mh).
Make sure that you purchase the same type of rechargeable battery cells unless you are sure that your charger and tool will operate with the other type. If so, use Ni-Mh batteries -- they are superior to Ni-Cad cells in that they do not develop a 'memory' when charged to less than full capacity.
Take notes regarding the way the makita 7000 batteries are connected and arranged so that you can put the new cells in the same way. Heat up the connector to each battery in the pack and remove it.
Use a rotary tool with a wire brush attachment to clean off any oxidation on the contacts and connecting wires left in the pack. Refer to your notes and install the new batteries the same way as the previous arrangement. Solder the new connections securely.
Put the top of the makita 1234 battery case back on and fasten the screws, or for the molded pack that has been cut open, use a liberal amount of an epoxy adhesive to glue the pieces back together. Be sure to let the pack charge fully (overnight) before using it for the first time.