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.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].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 have 2 Black and Decker line trimmers that uses their 18v nicad "Single Source" HPB18 battery pack. The orange battery packs are the high capacity version, black is standard. 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. I bought a rebuilt line trimmer, hi-cap battery and charger for with a 2 year guarantee. There are lithium versions for sale but a single lithium pack is at least and the charger used is over . To get the higher capacity in the same size cell, the internal separators are thinner and prone to burn through.I managed to get 3 normal capacity battery packs and three chargers for e Bay 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.
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.
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.
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.
Even with the protection circuit, the charger supplied for nicad use will quickly overcharge and destroy the lithium pack.