I am trying to fix my 18v Nicad battery. Hard to get a replacement as it is a Sears and discontinued. Battery is gone so would have to solder 15 cells - not easy. Should I replace with Li cells or any suggestions?

 

Ni-Cad Batteries are becoming virtually non-existent now.

Any other type of Battery will require its own specialized type of Charging-Circuitry.

Other Battery types will likely only be available in slightly differing Voltages.
How sensitive is your "un-named" appliance to Supply-Voltage ?

Space for Batteries is almost always highly restricted.
Exactly how much Battery space do you have available ?

How bad do you want to save your appliance ??
Is it worth saving ??

 

The physical size might be important it has an average number of times you can recharge and the amp/hour rating
If the quality is 6 months, 1 year, 2 years, 3years maybe 2 yrs or 1 yr might be a guess rather than a choice.
I paid $12 for NiCad 18V 1.5Amp Hr battery pack for my cordless drill at harbor freight does everything maybe good for 2 yrs
https://www.harborfreight.com/18v-nicd-replacement-battery-64776.html

 

Sounds just like one of my DeWalt 18V NiCad battery pack. 15 cells. One or more have gone weak or bad. When charging batteries the pack gets warmer than normal. And the cost of a new 18V DeWalt battery is ridiculous.

While looking into a solution for MY issue I came across an adapter that uses the newer 20V DeWalt batteries. It's a cheap solution. If you're looking to fix your DeWalt tools HERE is that adapter I mentioned. If this isn't what you need - then maybe you can find an older battery at a yard sale and scavenge the batteries that may still be good.

Replacing them with Li-Ion type batteries is not recommended because the physical battery will only fit into a charger designed for charging NiCad's. You'll burn up your batteries. And possibly your house or garage.