I have 2 Black and Decker line trimmers that uses their 18v nicad "Single Source" HPB18 battery pack. Inside are 15 sub-c nicads tab welded in series.
The orange battery packs are the high capacity version, black is standard. Stay away from the hi-cap packs. They burn out easily. To get the higher capacity in the same size cell, the internal separators are thinner and prone to burn through.
After both hi-cap packs went bad I opened them both up and salvaged enough cells to build one and that one worked once and it died too.
Black and Decker prices are not rational. I bought a rebuilt line trimmer, hi-cap battery and charger for $28 with a 2 year guarantee. A single battery new is $26. There are lithium versions for sale but a single lithium pack is at least $26 and the charger used is over $32.
I managed to get 3 normal capacity battery packs and three chargers for eBay for a good price, but there are large areas of my lawn that can be line trimmed much easier than mowing, so I needed more packs.
I disassembled the 2 dead packs and removed the nicad clusters.
Then I built a pack of six 18650 sized lithium-ion 3.7v cells in series. The cells are held in two spring-loaded plastic 3-packs that are connected in series with one wire soldered between them [the red wire visible in the second picture]. 6 cells in series at 3.7 is 22.2v The nicad pack when fully charged is 21.75v The lithium cells are protected with a small chipset under the negative end. As such they can be carefully soldered, but not tab welded. The spring-loaded packs eliminate the need to solder any cell and if a cell goes bad makes it very easy to replace the bad cell.
The lithium packs are stuck together with 2-sided tape and then shimmed with foam tape so that it doesn't rattle when the shell is reassembled. Two wires are soldered to the pack and then to the charge terminals inside the pack. The screws holding the shell together are T-10 torx. That size is common on auto headlamp adjustments so you can buy the screwdriver at an auto parts store for cheap if you don't already have one.
Step 1: Fixing the Original Charger for Lithium Cells
Even with the protection circuit, the charger supplied for nicad use will quickly overcharge and destroy the lithium pack. If a 3.7v lithium cell is charged above 4.2v and left that way, metallic lithium will plate out inside the cell and ruin it.
I opened the charger head [also T-10 torx screw] and added cheap 50v rectifier diodes in series with the positive terminal until the finish voltage did not go above 25. Six times 4.2v = 25.2v
It does not harm a lithium cell to undercharge it, it just shortens the runtime. 4.1v for each cell is much better than even one reaching 4.5v. Even with the diodes and protection disk on the cells, it's still a good idea to only charge the pack for an hour or so and then check the voltage. These lithium packs charge much faster than the original nicads did.
the lithium cells on eBay and
the plastic holders.