Hi sojudave - There are people using power tool batteries for ebikes - nicad, nimh and lithium. The holder is kind of clunky and inconvenient. If you open it up, you'll probably find a bunch of single cells (likely 'tab batteries') in there wired in series to make 18v. If you have access to discarded battery packs, you might find a bad pack will have one or 2 bad cells and the rest are good. You could scavenge the good cells and solder them together in series to make a battery pack whatever voltage you want, in multiples of 1.2 volts for nicad/nimh, 3.2 for lithium. So 15 nicads make 18 volts. Two 18 volt packs in series would make 36 volts. Additional ones in parallel would increase the amp-hours. If you look on ebay or a place like Only Batteries
The Rechargeable Batteries and Battery Charger Store on the web, you can see tab batteries individually or in packs. There are also Bosch Fat Packs that are compact lithium packs if you want to go that route. I’m thinking about making a few battery packs using regular rechargeable D cells (probably nimh) stacked in a piece of PVC pipe. Ten cells would make a nice 12v pack a little over 2 feet long and I wouldn’t have to worry about soldering anything. I don’t know much about soldering batteries and I think it might be dangerous, especially lithium, so if you are thinking about that, check with somebody who knows that stuff.
I’m not sure what the 18A means but most of the rechargeable cells I have seen are rated in milliamp hours (mAh). I have seen D cells up to 10,000 mAh which is 10 Ah, so using them, each of my 12v tubes would be 10Ah - jd