Introduction: Recovering Nicads From Bad Packs
One of my neighbors tossed 2 older nicad-powered Craftsman drills with their cases. The 14.4v version was wet and had no charger, but the 13.2v version was clean with a case and a working charger, but had 2 dead battery packs.
I usually go to Harbor Freight and buy their house brand 18v nicad packs when I need cells to build a pack. They're new and cheaper than ordering sub-C's with solder tabs. I have a tab welder to build custom packs.
After buying one Harbor Freight pack, I decided to salvage the remaining bad packs for enough cells to build a second pack for the 13.2v drill and find another use for the 14.4v drill.
This instructable is a summary of how that is done.
Step 1: Tools You Will Need
Whatever screwdriver will open the pack to be salvaged (the 13.2v pack used a #1 phillips, but the 14.4v used a #15 tamper proof torx. I try to save the shells even for packs I intend to scrap. Batteries can be replaced, but some shells are no longer available.
A hobby knife or blade to cut the paper insulators from the cells
A low voltage, low current, variable voltage dc power supply
At least one pair of wires with alligator clips on each end
A sturdy pair of scissors to cut the solder tabs
A roll of cheap paper masking tape
A wire brush
Step 2: Open the Pack and Stretch It Out
Make sure no cell touches another except by the solder tab. This pack was allowed to dry out because the paper insulators might have shorted the cells across the top if they were wet. If in doubt, cut the paper the long way and pop it off to save for later. Clean around the top with a wire brush if there is rust. The bottom can short to the top where the two meet.
Attach your charging leads, start off on the lowest voltage possible. Measure the string voltage and increase the charging voltage until the pack resists going higher, then back off a bit and charge for 5 minutes.
Check the voltage of each cell in the string and if any are above 1.5v then back off on the charging some more. You don't want to damage cells any more than they already probably are. Charge gently for 1/2 hour.
After 1/2 hour, disconnect the charger and let the pack rest for an hour.
Step 3: Check Each Cell Voltage
The pack MUST rest for a while before checking each cell. Some failing cells will still charge but self-dischage quickly if there is internal damage.
Cutting off the paper also helps. One of the cells in this string had a hole in the side from corrosion. Definitely bad. But others were pitted and still help voltage. Those are not going to be my first choice for building a string in a tool I use a lot. Cells that are corroded and leaking around the top are not grade A, either, even if the voltage is good. A good, properly charged nicad will rest at least 1.28v: brand new 1.4v is possible My 13.2v pack is "full" above 15v.
Cells that show zero volts or discharged during the short rest are duds.
Step 4: Sort and Save
And of course take the dud cells to a recycler. My local Lowes and Home Depot have battery recycle drop offs by the front entrance.
The paper insulators are reattached with masking tape. I also save the mica insulators for rebuilding packs.