How-To - Replace NiCd Battery Cells

How to replace the dead NiCd Battery Cells in a rechargeable cordless tool battery pack.

Step 1: Opening Up the Battery Pack

I bought this second hand Fein cordless screwdriver knowing that the battery packs were dead. The first step was to open each battery pack and inspect the NiCd cells inside. As can be seen, the battery contact tabs are very much corroded with copper sulfate oxidation.

Step 2: Cleaning Off the Corrosion

I removed the cell packs from the battery holders. Since copper sulfate is slightly acidic in water, I made a solution of detergent water which is slightly alkaline. Together, they should neutralize each other.

However, I did wear gloves during the cleaning process. I proceeded to wash the battery compartments and all of the plastic parts.

Step 3: Pre-Charging NiCd Cells

I bought these 1700mAh cells that I found online. If their stated capacity is authentic, they would hold twice the amount of charge as the original Cadnicas. After charging them for 16 hours at 1 tenth C rating, I verified that they all held a similar charge in order build the first battery pack.

I also picked up a pack of battery tabs that needed to be attached to both ends of each cell.

Step 4: Soldering Battery Tabs

My attempt at a makeshift tab welder using a car battery was a dismal failure.
I decided to solder the tabs to the batteries instead.

I sanded each end of the individual cells using an abrasive sand paper in order to give the solder something to attach to. Once this was done, I proceeded to begin soldering the battery tabs to the individual cells.

Step 5: NOTICE: How Not to Attach Battery Tabs!

NOTICE. This is not the correct way to attach battery tabs to batteries. You should use a tab welder if possible as opposed to a hot soldering iron. In the case of NiCd batteries, this is not that critical. However, in the case of LiPo batteries, overheating is a real issue.

Step 6: Soldering the NTC Thermistor / Capacitr Charging Monitor Circuit

After soldering the tabs, I was ready to build the first battery pack. After cleaning the charging tabs, I then soldered them to the first positive cell and the last negative cell.

One of the original battery packs had a charge monitoring circuit consisting of an NTC thermistor and a capacitor.

Step 7: Heat Shrink Tubing

These were soldered together and heat shrink tubbing was used to isolate the leads.
The protection circuit goes in between the negative terminal of the last cell in the series and to the third charing tab which is the monitoring terminal.

Step 8: Reassembly of the Battery Pack

Once this step was complete, it was time to begin reassembling the first battery pack. I trimmed any excess tape and soldered the protection circuit in place. I then reassembled the battery pack. It was now time to test the battery pack and see if the cordless screwdriver would work.

Step 9: Testing the Newly Assembled Battery Pack.

This particular battery pack has a nominal voltage of 9.6V (8 cells in series @ 1.2V each). I verified the voltage on the battery pack and it was fine : It was above the nominal 9.6V of the pack. You should always measure your battery pack before plugging in your tool.

In this case, the battery pack had a voltage of around 10.4 Volts (or around 1.3V / cell). This is good.