So I have some older Harbor Freight Chicago Electric drill and impact drivers.  I have had them for 8 or so years and I picked them up on clearance for very little.  They have served me well, but the Ni-Cad batteries they came with have completely worn down.  I'm getting about 5 minutes per charge out of each battery.

I searched around for replacement batteries, but unfortunately the world moved past Ni-Cad's years ago.  I tried to find a 3D printed adapter to allow newer batteries to work with my old drill but I came up empty there as well.  I thought about just picking up an 18v Lithium battery and trying to work out my own adapter but I was worried this would be too far of a leap and it would not work for some reason (maybe some communications needed between drill and battery, etc).

After digging around for a while I found some 18v NiMH batteries for $20 on Amazon and decided to take a chance on them. They were for a different drill and did not even pinout the same, so I had to swap the pack into my own case. Fortunately the cells are the same size and in the same configuration, however I needed to exchange the connectors and lengthen the wires so they could reach properly since the pack was wired in a different configuration.

This was a very sketchy project, but in the end it worked.  However these cases are extremely tight and it took some care to squeeze it all back into the case.  I could not find a pack with NiCad batteries so I had to go with the slightly newer NiMH batteries.  However I still have my older NiCad charger and spending another $30 to get a NiMH charger, as well as modifying it to fit my batteries, seems like throwing good money after bad to me.  So I'm going to try and time how long the batteries stay on the charger in order to not overheat them.

What is the final verdict? The battery was able to charge up on my charger, and after 10 minutes of use it is still running much stronger than my older batteries.  It was a relatively small investment at $20, but to be fair I only got one battery.  I really should spend another $20 and get a second battery, but that brings the total investment up to $40.  For $100 I can pick up a name brand drill and driver set with two Lithium batteries and a charger (on a loss leader sale).  That would be a much better investment in the end, I would have batteries that last longer and that hold there charge for weeks on end, and better built tools that have no wear on them.  

I think I will take my win and walk away.  I will use this battery while it lasts and then invest in something more quality in the end.  I hate that perfectly good drills must be tossed because the batteries wore out, but unfortunately that is the world were in right now.