Battery Repair by Converting Ni-Cd to Lithium-Ion (Battery and Charger)

We all have that beloved cordless power tool that ain’t give as much power as at its best days. Most of cases the old technology Ni-CD or Ni-MH batteries became weak or even faulty. The same situation is with this Dewalt cordless drill - the battery does not take charge and does not power the tool anymore. Most likely some of the cells are badly worn out. I like this tool a lot so I will modernize it and do it on a budget by adding lithium-ion cells, a BMS module to protect them, and converting an old charger to charge freshly installed lithium-ion cells.

How I did it - you can check by looking DIY video or you can follow up instructions below.

For this project you will need:

Materials:

6pcs of 18650 Lithium ion cell with at least 20A of continuous discharge current

BMS module with balance function to protect cells ( I used 40A BMS)

Pure nickel tabs to connect cells (used 0.15mm of thickness and 8mm in width)

12.6V lithium-ion battery charger

Solder

3D printed of in any other way made simple 18650 cells holder

Tools:

Screwdriver to open the battery and charger case

Soldering iron

Dremel with cutting disc

Spot welder to spotweld the cells (not recommended, but could be used as a soldering iron too at your own risk)

Voltmeter

Step 1: Inspection

Such a small voltage confirms my guess about the bad state of this battery. 

Step 2: Taking Apart

Let's open the case and take a look inside. There is no visible evidence why this battery failed except for this production date. Still, it is impressive that it worked for more than 10 years.

Step 3: Saving Main Connector

To move forward I need to save this connector by desoldering all wires and grinding spot welds. I’ll use it later on to assemble a new battery pack.

Step 4: New Lithium-ion Cells

As I mentioned before - old Ni-MH cells will be replaced with lithium-ion cells. I’ll use a Samsung INR18650-25R cells rated for 2500mAh of capacity and 20A of continuous discharge current. As you already noticed - they were spot-welded before. I bought them as genuine, reclaimed, non-used cells from a trusted seller. These batteries come from battery packs in which welding errors have been found, cells were disassembled, tested, and listed on a secondary market. The cells may have minimal external damage, but they have never been charged or discharged and internal resistance is according to the datasheet. So we could buy genuine 18650 cells within the specs of the new cells for a solid 30% discount. I paid 3$ per reclaimed cell while the same new one cost 4.5$.

The small issue is leftovers from previous spot welds. Those could be cleaned very fast with a rotary tool and grinding disc.

Step 5: Battery Holder

To keep cells in place I designed and 3D printed a simple battery holder for 6 cells.

I will make a 3s2p battery configuration which means 3 groups of cells connected in series to achieve 12V while each series group will have 2 cells connected in parallel to get a maximum capacity of 5Ah. 

Step 6: Modifying Battery Case

To make it fit inside a small case trimming was needed. 

Such a strange battery shift is needed to bypass the internal battery case structure. On closer inspection looks like the manufacturer has accidentally designed almost the perfect battery case for such a conversion. Cells barely fit and still left some space for connecting tabs.

Step 7: Spot Welding Cells

Before connecting cells, I checked if all were within close voltage. To keep the cells shifted in the needed form I made a simple jig from scrap wood pieces. Added some additional insulators on the positive side and spot welded with 0,15mm of thickness pure nickel strips. First, all parallel cells, after that connected those 3 groups in series.

And if everything was done right - I should get around 11V, due to the cells are not charged.

Step 8: BMS Board

To keep those groups of cells nicely balanced, and ensure proper charge and discharge - I’ll use this 3S 40A BMS module. It is designed for 3S battery configuration and could handle continuous 40A load, with short peaks of 60A. In the market could be found two very similar-looking 3S 40A BMS modules. Take your attention and choose the proper one - with a balancing function, while another one has protection from overcharge and over-discharge only. The best visual difference is this part. On BMS with a balancing feature, you will see 3 big resistors.

Step 9: Connecting BMS Module

One by one soldered battery pack terminals to the BMS board.

Step 10: Soldering Main Battery Connector

Before connecting the output terminals, I’ll soldered pair o thick wires to the saved connector and glued it with 5min epoxy inside the battery case. 5 minutes later both terminals were soldered and the battery could be assembled back.

Step 11: Reassembling the Battery

5 minutes later both terminals were soldered and the battery could be assembled back.

Since a pair of holders were trimmed inside for extra space, I covered those holes by simply gluing shortened bolts with CA glue.

Step 12: Charger Modification

The battery conversion is finished, and it could be charged with any smart charger or a constant 12.6V power supply. But I want to go further and convert the original Dewalt Ni-HM charger into a 12V lithium-ion battery charger.

Step 13: Removing No Needed Components

From all this internal madness I will use only those two connectors while the rest of them could be removed. To make it fast - I used a heat gun.

Step 14: New Power Source

All removed components will be replaced with this lithium battery charger rated for 12.6V and 2A of charging current. Cracked the case and took out the needed electronics.

Step 15: Safety Mod on Old Charger Board

Before working further I cut all circuit traces around the points where new electronics will be connected. Despite a lot of desoldered components there are still left some which may cause some issues if I do not disconnect them permanently.

Step 16: Working to Instal New Charging Circuit

Desoldered old two contacts LED charge indicator which will be replaced with 3 contacts LED from the new charger circuit. For a proper relocation is needed to solder it with 3 extension wires.

With a few drops of CA glue and plastic spacers, the new charger board was glued permanently.

Step 17: Soldering Last Wires and Reassembling

Lastly soldered power IN and OUT terminals. While power IN terminals orientation isn't important, the charging terminals should be connected according to battery polarity. The work is done here, time to assemble back and test it out.

Step 18: Testing the Charger

Its powers up as the green indicator light up, and it charges the battery by indicating with changed LED color. After some time the green indicator tells us that the charge is finished, let's check that. Nice, 12.6V means that the battery is fully charged.

Before the final testing on the drill, a few small touchups were made with proper labeling of the modified battery and the charger.

Step 19: Final Touches

Before the final testing on the drill, a few small touchups were made with proper labeling of the modified battery and the charger.

Step 20: Testing the Freshly Assembled Battery

Ok, let's check out this battery in a real application.

Wood screws - a piece of cake.

M10 drill bit - not a problem too.

M25 Forstner bit - could be gone all the way if not the shavings clogged up the bit.

Step 21: Final Thougts

So, does it worth it? In my case - A strong yes. The drill got back all its power. The battery capacity was increased to 5Ah which is twice that before while at the same time losing 30% of the weight. And what is most important is it all was done on the budget. The lithium battery charger 6$, the 18650 reclaimed cells 18$, and the BMS 6$.