Drill Battery Conversion - NiCd to Lithium

My neighbor's drill battery died and was no longer holding a charge, so I offered to convert it to lithium instead so he could still use the drill in perfect working order.

The drill is made by Einhell but the batteries were made out of NiCd cells which are notorious for losing their charge.

In this Instructable, we will go through the entire process of disassembly of the old battery pack and adding the new batteries in.

The Instructable is sponsored by PCBWay where you can get prototype PCB assembly - 30$ for 20pcs

• 18650 battery cells - https://s.click.aliexpress.com/e/_DlReMG5
• 4S BMS - https://s.click.aliexpress.com/e/_DeUGEbx
• 16.8V charger - https://s.click.aliexpress.com/e/_DBLqFez
• Spot welder - https://s.click.aliexpress.com/e/_DmI7WlJ
• Soldering iron - https://s.click.aliexpress.com/e/_DCdlSuz
• Multimeter - https://s.click.aliexpress.com/e/_DD1fZGD
• Wire snips - https://s.click.aliexpress.com/e/_DBFhBVT
• Wire stripper - https://s.click.aliexpress.com/e/_DcykHKz
• Screwdriver pen - https://s.click.aliexpress.com/e/_DDq1DYh

To start we will need to first disassemble the old battery pack by undoing the 4 screws on the bottom. This will remove the bottom cover from the battery and it will expose the 15 NiCd cells inside that were wired in series.

We can remove them by pulling them out and the last cell will also go out with the connector on the top of the battery pack. It is important that we save the connector, as well as its contact plates as we will re-use them in the upgraded pack.

I sourced the replacement cells from old laptop batteries and had them pre-charged. When making battery packs, it is crucial that all of the battery cells are relatively close to their voltage.

This is a lot more important when connecting batteries in parallel as if you happen to combine a full and an empty cell together, there will be a large current that will flow from one to the other and it can possibly destroy both of the cells.

In my case, the batteries were already in parallel as I did not remove the zink strips connecting them from the original laptop battery pack.

I will be using 4 cells in series to get up to 16.8V so an appropriate BMS is also needed. The BMS will make sure that the pack is not overcharged or over-discharged so it will protect the individual cells.

Before starting with the pack, we need to figure out how the batteries will be placed in the enclosure at the end. This will dictate how we connect things so I used the shell from the old pack to figure out that I will have three of the cell pairs one next to the other and the final one will be placed in front of them.

Once I knew this, I used some packing tape to tie the 3 pairs together so I can start soldering them.

All of the cells need to be connected in series so I started to solder them together with pieces of thick wire. Lithium cells should not be exposed to heat so soldering is not the best option, but since they already had a connecting zink strip that was spot welded, I made sure to only solder on the strip and within as short a time as possible.

For that, I had the biggest tip on the soldering iron so it can hold quite a lot of thermal mass and it soldered them up really nicely.

I first connected the three pairs that we glued together and then I also added the final pair with a bit of a longer wire.

The BMS has a feature to balance charge all of the cells individually at the same charge. This improves the battery life so it is important that the BMS knows the voltage of each battery cell pair.

For that, we need one extra wire from each of the connection points of the batteries, plus one from the negative side of the first battery.

These wires will be later soldered to the BMS so I made sure to orient the BMS initially and leave enough wire so that the solder connection can be made as efficiently as possible.

To prevent any shorts on the battery pack, I used two pieces of cardboard to isolate the additional battery pair and another cardboard piece to isolate the BMS from the rest of the battery.

Everything was then isolated and held together with some more packing tape and electrical tape.

To finalize the electrical connection, I soldered all of the wires coming out from the pack to the appropriate voltage points on the BMS. These are all marked in multiples of 4.2V which is the maximum that the lithium cells will be charged.

So, the wire coming out after the first cell is soldered to the 4.2V pad, from the second at the 8.4V, etc.

Once everything was connected to the BMS, I soldered thicker wires to the connector plates and I then soldered the connector wires to the output pads on the BMS. With that, the soldering was done and it was now time to assemble everything back in the case.

The most important part of assembling the battery pack is to properly set the connector in place and to make sure that we keep the marked polarity on the case. The connector is pushed in the opening as before with the newly soldered wires and to keep it in place, I added one of the NiCd cells in the space to keep it pushed but I first isolated it with electrical tape.

The cells can then be placed inside and the bottom cover can be added back with its 4 securing screws.

The original charger can be modified to charge the new lithium battery and that will probably be for another Instructable later on. For now, I'll charge it with my dedicated lithium charger but you can also order a specific charger online for the voltage of the pack that you want to charge.

These come with a 5.5mm barrel jack that we can also add to the battery so it can be directly plugged in.