I gave my V18 tools to my son-in-law when I upgraded to M18. He bought a couple of after market lithium batteries and really gave the old tools new life.
I believe he bought the batteries from our local Ellis Battery store.
If nicad 18 can run on V18
and V18 can be made to accept M28 (which is not a dead platform like V18)
then nicad 18 can presumably be made to run on M28