I've had my Ryobi ONE+ system for over five years now and the batteries are shot. Let's see what it takes to rebuild one.

Step 1: Tools Needed

Screwdriver
Wire cutter
Soldering iron
Solder

Step 2: Take It Apart

Remove six screws from the bottom. Pull the black and orange sections apart.

Step 3: Identify the Cells

Remove the cluster of batteries and identify the number and type of cells are contained in the pack. In this case, there are 15 Nickel Cadmium C cells.
From here we have some options. Since they're C cells, we can replace them with any popular battery chemistry.
We can use NiCad, NiMh or Lithium Ion.
NiCad or Nickel Cadmium are the least expensive and least powerful at $2.70 for each 2400 mAh cell. This gives us a material cost of $40.50 plus tax and shipping.
NiMh or Nickel Metal hydride is next up in cost and power. At $4.85 for each 5000 mAh cell. Replacement of the cells would be $72.75.
Finally, Lithium ion cells are $10.99 each for 3.6 volt 8500 mAh cells. Granted, you would only need 5 to get to the desired 18 volts, but then they're going to rattle around in the case.

Step 4: Make the Purchase!

With the cost of cells alone exceeding the cost of a replacement battery pack, I think it's time to pack this project up and go buy a battery pack.
Seriously, with brand new Ryobi NiCd packs available for as little as $30 and their new Lithium Ion pack for as little as $40, this is not a worthwhile project.
Don't forget to properly dispose of the old batteries.