While looking for a replacement battery for some older cordless I
stumbled across a person who makes physical adaptors to use the newer
20 volts Li batteries in the older 18volt tools. They are well designed
and very well executed. The plastic is interestingly done by 3D
printing. The kit consists of the plastic and two brass tabs along with
a user supplied old 18 volt. The instructions for your own assembly are
a bit vague and does require some wire and the ability to solder. If
interested go to Craig's list Oshkosh
WI and search for battery adaptor. A bit pricey, but very well satisfied.
Disclaimer: No connection the him other than a satisfied user.
CP