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Rebuild Ni-Cd Battery Packs for Power Tools

Figure 1 This is the battery pack to be rebuilt. The cells are inside a plastic shell case that attaches to the tool via the rails on top. The red parts are the locking device.
Written by James Lyman

When the Battery No Longer Holds a Charge, Just Replace the Cells

There are myriad Ni-Cd battery-powered tools and devices, but their batteries don’t last forever, and new batteries often cost more than the tools. But don’t pitch that tool! Many battery packs can be revived by replacing the individual battery cells. In this article, James gives step-by-step instructions for rebuilding a battery pack for an electric drill by spot welding metal ribbons to the battery terminals of the new cells.

  • How can I avoid paying for new battery packs for power tools?
  • Can you rebuild a Ni-Cd battery pack?
  • How does a spot welder work?
  • Ni-Cd Batteries
  • sub-C Ni-Cd cells
  • voltmeter

Cordless tools, such as electric drills, are wonderful devices that free you from the constant entanglements of power cords. They give you the freedom to just pick up an electric power tool and start using it, without taking time to run an extension cord, which somehow always get snagged on things. But this freedom and ease is only temporary, as that battery pack is a ticking time bomb of new aggravation. I don’t know how many battery-powered electric tools I’ve had to throw away over the years, because new battery packs were more expensive than buying a new tool.

I somehow managed to acquire three 18V, 3/8” cordless electric drills (two in the shop and one in the lab), plus four battery packs for them. It was great! But last year, one by one, those battery packs started to fade and then fail. Now I’m down to just one, and it has to be recharged constantly, so I’m restricted to doing home projects. I checked to see if replacement packs were still available, but the drill was discontinued, and so were the replacement batteries. I have rebuilt nickel-cadmium (Ni-Cd) battery packs before, and so I decided to see what I could find for new Ni-Cd cells on the Internet, to see if it was worth rebuilding this one.

I quickly found several offers for reasonably priced sub-C Ni-Cd cells. My battery pack uses 15 of these sub-C cells, and one vendor offered a package of 15 cells for $25, a long way from the $3.50 per cell I paid several years ago. The cells come with the metal ribbon tabs installed for interconnecting the cells in a pack.

TESTING THE CELLS

The first step is to check each cell with a voltmeter to make sure the cell is good. You also want to correctly identify the polarity of each cell. Connect the meter’s test leads to each of the battery tabs, and note which battery end is positive and which is negative, and if the voltage is more than a few hundred millivolts. Actually, for fresh, new batteries, the voltage should be near or a little more than the rated voltage of 1.25V. It’s rare, but you may find a dead cell, an out-of-box failure, and it’s best to find that defect before you’ve installed it in the middle of the cluster of batteries. In such cases, rebuilding again can be a real pain!

Note that the only mark of polarity is that the positive ends come partly wrapped in paper, while the negative ends are completely exposed. Building battery packs involves arranging the individual cells and connecting positives to negatives, and a common mistake is to get the battery’s connections turned around. A good practice is to clearly mark the positive end to avoid any confusion or ambiguity. Use some bright, distinct color that stands out from the paper wrap and metal cell, such as a marking made with a red, wide-tip, felt pen.

Once all the cells have tested good and have been marked for polarity, you are left with a set of battery cells almost ready for assembly in the battery pack.

DISASSEMBLING THE OLD BATTERY PACK

When you disassemble the old battery pack, it is important to document how the cells are strung together and the pack is constructed by taking several photos. In Figure 1, we see this battery pack is a plastic case that slides onto rails of the tool and locks on. The tool’s blade contacts slide into slots on top to make electrical contact with the tool’s electrical apparatus, such as a switch and motor. The red parts are the locking device that hold the battery pack securely to the tool when it’s in use.

Figure 1
This is the battery pack to be rebuilt. The cells are inside a plastic shell case that attaches to the tool via the rails on top. The red parts are the locking device.
Figure 1
This is the battery pack to be rebuilt. The cells are inside a plastic shell case that attaches to the tool via the rails on top. The red parts are the locking device.

The first step in disassembly is to test the polarity of the contacts with the voltmeter, so that once the pack is opened, the positive and negative leads can be correctly identified for later attachment to the new battery stack. I clipped the voltmeter’s test leads onto common straight pins used for sewing, and connected these with the pack’s electrical contact to take voltage measurements. Once the polarity was correctly identified, I permanently marked the positive contact with a small spot of Testors enamel model paint, applied with a toothpick.

With the positive contact clearly marked, separate the two case shells by removing four sunken screws from the bottom, and open the case. Take several pictures with your smartphone or digital camera for later reference, so you can reassemble the pack correctly.

Figure 2 shows the battery stack and how the cells are linked together. This is the first reference photograph needed to build a new battery pack. Thin metal strips are spot welded to the ends of the cells, thereby electrically stringing them together to give 18V. The cells are tightly packed together, to minimize the size of the pack and to secure the cells inside so they don’t rattle or shift around. Note that both wires (red and black) that connect to the contacts are also spot welded to the ends of cells.

Figure 2 
This is the battery stack. The individual cells are interconnected with thin metal strips welded to the battery terminals.
Figure 2
This is the battery stack. The individual cells are interconnected with thin metal strips welded to the battery terminals.

The four little dimples on the ends of each metal tab strip are the spot welds. They provide both mechanical and electrical contact connections.

PREPARING TO REBUILD THE BATTERY PACK

With battery packs I’ve rebuilt previously, I soldered the metal tabs together to make connections. But the power packs used in power tools can draw enough current to heat and melt the solder and break a connection. I got around that by wrapping the two metal tabs together with AWG #30 bare wire. Then I soldered them together, so when the solder melted, the tabs remained jointed until the solder cooled and solidified again. That worked just great with subsequent battery packs.

When ordering my Ni-Cd cells, I happened upon a device called a portable spot welder (Figure 3), with which you can spot weld the interconnecting metal tabs and avoid soldering altogether. Several spot welders are available on Amazon for about $60 to $80 each. They all have approximately the same capabilities—they are able to spot weld nickel ribbons 0.10mm-0.15mm thick (0.004″ to 0.006”) to cells just by touching a probe to the metal.

Figure 3
This is the portable spot welder machine designed to spot weld the metal tabs onto the battery cell ends and interconnect the cells.
Figure 3
This is the portable spot welder machine designed to spot weld the metal tabs onto the battery cell ends and interconnect the cells.

The one I bought cost close to $60 and has a rated output of 650A at 4.2V. Two short probe leads or pens (the red wires in Figure 3) plug into the welding machine and are used to make the spot welds. To use them, you touch both pen points to the object you wish to spot weld. The instrument senses the contact, and supplies the pens with a short, high-current pulse that heats and melts the metal to form a spot weld. This model has controls and LED indicators for operation, allowing you to set the current level for welding in steps called “gears,” with the number of gears ranging from about four to a dozen or so. As shown in Figure 4, the gears are indicated by the line of six pinpoint red LED lights. Below those, a line of four blue LEDs show the battery charge, next to which are two yellow LEDs that indicate the welding mode (manual or automatic).

Figure 4
I tested the spot welder on the battery cell tabs to determine what gage or power levels to use, and to check the strength of the welds.
Figure 4
I tested the spot welder on the battery cell tabs to determine what gage or power levels to use, and to check the strength of the welds.

The connecting metal strips used on the original battery stack were 0.010” thick, whereas the tabs of the new cells were 0.008” thick. The nickel ribbon that came with the welder was 0.004” thick. This is important to consider in choosing which gear to use.

As an axiom of engineering says, “One experiment is worth a thousand expert opinions.” I needed to do a bit of testing before starting to rebuild my battery pack. First I cut a couple of short lengths of the nickel ribbon (0.004” inches thick) that came with the welder, and started welding using different gear settings. I set the ribbons to overlap by about 30%, then touched the pen points to the top of the two, about 0.1” apart. I tested the strength of the welds by pulling on the ends of the ribbons. At gears 1 and 2 the welds pulled apart easily, but it was significantly harder to pull the ribbons apart at gear 3. At gear 4 I couldn’t pull them apart by hand—this was a good gear for my spot welds.

Recall that the tabs on my battery cells are 0.008” thick, twice the thickness of the test ribbon, so I decided to test weld them as well. As you can see in Figure 4, I overlapped the ends of two batteries by about 0.25” in order to leave a sufficient amount of the tabs after cutting off the test welds. The insert in the lower right corner of Figure 4 (red outline) shows a close-up of the resulting welds. The four dark spots on the ribbon are the actual welds. I found that a gear setting of 5 gave very good results—one step below the maximum gear level. I easily cut out the test weld section using ordinary scissors, and I was now ready to start building.

BUILDING THE NEW STACK OF BATTERY CELLS

Welding the batteries together is just part of the problem. The other part is holding the new cells in exactly the same position as the originals before welding the tabs, so that the stack will fit back into the case. To this end, I made a couple of small wooden holding blocks—nothing extravagant, just a couple pieces of scrap wood with a small stop stud mounted on the end of one block piece (Figure 5). The two battery cells indicate the relative size of the holding blocks, as well as that of the two small bar clamps used to press the blocks against the cells.

Figure 5
Shown are two cells connected together in the holding fixture, to make the cell duets needed to build the battery stack. Note the red circle around one cell duet already assembled.
Figure 5
Shown are two cells connected together in the holding fixture, to make the cell duets needed to build the battery stack. Note the red circle around one cell duet already assembled.

To facilitate clamping cells between the blocks, I cut another two lengths of 0.5” dowel rod, about 3” long, for the blocks to rest on when installing cells; thus, equal amounts of the cells extend beyond the holding blocks, and the ends of the cells are even with one another. I held the cells together with rubber bands before welding.

Building this battery stack presented a challenge, because instead of connections in a straight line, they cross-connect from one side to the center as shown in the top of Figure 2. So first I built up five pairs of duet cells (two-cell pairs), each the same as that shown inside the red circle in Figure 5. These duets connect one cell’s positive end to the negative end of the other via spot welding, while the tabs at the bottom are bent away and in the same direction as in the original battery stack, where the duet will go in the stack. Note the red markings applied earlier to the positive terminals of battery cells.

To build these duets, two cells are placed together with a rubber band to hold them in place, then placed between the clamping blocks which are held firmly with the small bar clamp. Their tabs are cut to length so that they overlap by approximately 0.25″. You must take care that the tabs on the other end do not touch, as this creates a dead short between the two cells. Leave the rubber band on the cell duet until the duets are assembled.

To spot weld, turn the welder on and select the desired gear level, and firmly press either of the pen points down on the overlapping metal ribbons of the tabs, so the two tabs are pressed together. Then, touch the other pen point onto the top ribbon and hold. As I explained, the welder senses the electrical contact and pulses the pens with a high electrical current. There will be a small flash of electrical spark at each pen point, and the ribbon metals will melt at the points of contact until they merge to form two weld joints. Repeat this process to make a number of weld joints spaced about 0.1” apart. Check to make sure the weld joints are solid and holding firmly. Set each duet aside for later assembly, and again, be careful that none of the other free tabs on the bottom touch and short circuit.

Heat-shrink tubing on the battery cells’ tabs is used as insulators to prevent accidental shorting of cells while being handled. These insulators must be removed before welding, but not until you are ready to weld. At first, this may look like a serious problem, but the insulation easily slices open with a new razor knife. Slip a utility knife blade between the metal tab and the covering insulation, then push it along the metal tab, slitting the insulation open so it almost falls off. Remember, the batteries, which have a fresh charge, can easily and instantly heat the metal tabs red hot—and literally brand your hand the same as a branding iron touched to the butt of a cow!

The next step is connecting the cell duets into cell clusters. Figure 6 shows three of the cell duets held into a cluster with the clamps and wooden block holding fixture. Above this is the original battery stack removed from the case. As with the cell duets, rubber bands are placed around the cluster to help hold cells together and in place for clamping in the holding fixture.

Figure 6
This shows the construction of a cell cluster with three cell duets held in a wooden holding fixture to match the section of the original battery stack (above).
Figure 6
This shows the construction of a cell cluster with three cell duets held in a wooden holding fixture to match the section of the original battery stack (above).

With the cells secured in their correct position, overlap the tabs on the end, positive row (red) to negative row (unmarked), ans start welding the tabs on the overlapped areas, usually with six to eight welds each.

Build a second cluster with the two remaining duets. Figure 7 shows this cluster in the holding fixture. As I completeled each sub-assembly, I tested for correct assembly using a voltmeter. The test leads are connected to the ends of each cell cluster to measure the total voltage. The voltage should equal approximately the number of cells in a cluster multiplied by 1.25V, the characteristic no-load voltage of an individual cell:

Figure 7
Here is the construction of a second cluster with the two remaining cell duets, again using the wooden holding fixture and rubber bands.
Figure 7
Here is the construction of a second cluster with the two remaining cell duets, again using the wooden holding fixture and rubber bands.
Total Voltage = 1.25V x number of cells

There are four cells in the second cluster, so the total voltage should be about 5V. The DVM in Figure 8 measures 5.17 volts, indicating the cluster is correctly wired. If a cell were wired in backwards, the voltage would be about 2.5V, and a reading of zero indicates an open circuit in the string of interconnections. This is a quick and easy way to check, and you should check each subassembly before adding any new cells, since it is easier to troubleshoot and rework any mistakes earlier in the process. Nothing is worse than testing after the full stack is assembled, only to have to take it apart to find the trouble and correct it. Also, in pulling the stack apart, you run the risk of breaking the welds of another junction and introducing more problems.

Figure 8
I test if a cluster of cells is correctly wired together using a voltmeter. The total voltage should equal about 1.25V times the number of cells.
Figure 8
I test if a cluster of cells is correctly wired together using a voltmeter. The total voltage should equal about 1.25V times the number of cells.

This brings up a problem I encountered—the strength of welds. I had zipped through a series of spot welds on a tab, and it looked like I was getting good welds from the top. Then, while handling the cluster, one of the junctions came apart. I discovered the bottom tab had only one small weld dimple and one very light dimple with no real indications of melting. So, I started counting five seconds between welds, to allow time for any charging of capacitors. I also tried soldering the tabs together, but even when applying liquid solder flux (CG Electronics P/N 10-4202), the solder didn’t wet on the tab very well, and didn’t wick in between the tabs.

I think the problem was that most of the welding current traveled across the top tab, while only a small amount of current traveled down and then across the lower tab. I tried to offset the tabs and set one pen point on the exposed edge of the lower tab, and then touched the other pen point on the top tab to force all the current to flow through the weld junction.

Another problem I encountered was that tabs were too short to make a connection, so I had to add a nickel ribbon strip for an extension. As discussed earlier, the nickel ribbon that came with my spot welder was 0.004” thick, whereas the tabs on the new cells were 0.008”. This led to an issue while trying to offset the pen points to weld—the pen point on the thin ribbon blew small holes through the ribbon.

The nickel ribbon that came with the welding machine is wider than the cell tab. Figure 9 shows the cell tab folded along the side of a cell, with the ribbon welded to the tab to extend it past the other end of the cell so that it can be welded to the next cell of the growing stack.

Figure 9
I lengthen the tab of a cell by welding a piece of nickel ribbon onto the tab, so that it extends above the other end of the cell.
Figure 9
I lengthen the tab of a cell by welding a piece of nickel ribbon onto the tab, so that it extends above the other end of the cell.

At this point we are down to installing single cells. You must be careful to correctly place and connect each additional cell. First the two clusters are joined together by welding a single cell between them. Figure 10 shows this setup, with the connecting individual cell on the far right. We again test the ends of the string with a voltmeter to ensure that they are correctly wired in the cluster. For our 13 connected cells, the measured voltage was over 16V—just about what we expect. If you don’t get more than 16V, or maybe a little less, start looking for where the stack is incorrectly wired before continuing.

Figure 10
The two clusters are linked together via a single cell, the tabs of which are each welded to a separate cluster to form a continuous string of 13 cells for 16.25V.
Figure 10
The two clusters are linked together via a single cell, the tabs of which are each welded to a separate cluster to form a continuous string of 13 cells for 16.25V.

Finally, the last two cells are added to complete the full, 15-cell battery stack. These cells are installed by forcing the rubber band outwards so the cells slip in, then turning each cell so its tab connects to the correct tab in the stack. Spot weld the tabs, then repeat for the final cell to complete the stack. As before, we check the voltage—this time for the complete, 18V battery pack. I connected a voltmeter to the tabs to which the battery pack connector wires will eventually connect. Checking the meter, I found a voltage of 19.41V, which means the stack is wired correctly. Note that the rubber band holding the stack tightly together is still in place (Figure 10), and remains in place until the stack is inserted into the lower plastic pack case.

The voltage of 19.41V is so high above the rated battery pack of 18V that it appears that an additional cell is present—but it isn’t. The reason for the excessive voltage is that a brand new, unused, fully-charged cell has a voltage greater than the rated value of 1.25V when not under load drawing any current (DVM is 10MΩ input). The accumulated extra voltage from the 15 individual cells is as if there is an additional, sixteenth cell. But if the stack is correctly wired, there is no reason to be concerned about its excess voltage; it will drop when the cell is under load from a power tool.

INSTALLING THE NEW STACK IN THE CASE

With the new stack completed and testing OK, it’s time to install it into its case. With the rubber band still in place, carefully lift the stack and position it over the bottom half of the battery pack case, then gently press the stack down into the case. Note that the ribbon connecting cells together can be as little as 0.004” thick—about the diameter of a human hair—so it can break rather easily. So take care when handling the stack. Push the cells down to where the rubber band is against the lip of the case, then cut it off. The cells should continue sliding down until they touch the bottom of the case.

With the stack firmly seated in the case, test again for full voltage, to ensure there are no breaks in the connections (which would leave you with 0V). With the battery pack testing for >18V, prepare the top of the case for installation. First, solder the wire leads from the connector plate on the case top, which connects to the power tool. I was fortunate that some of the cell tabs already had through-holes on their ends for soldering wires. So at the start, I set two cells apart with tab holes to connect at each end of the stack string, to give holes for the positive and negative connections. I first painted the end of the tabs and bare wire ends with liquid solder flux, then slipped the ends of the wires through the holes, checking to make sure the positive wires went to the positive tab. If you get the wires mixed up, your power tool may run backwards! Solder the wires with a good-sized bead of solder, to make sure you have a strong mechanical connection.

Once the wires are solidly soldered onto the tabs, test the voltage again. You can’t test too frequently. Something seems to always go wrong when you don’t test enough, and then you have to take things apart to try to find the break. (I am speaking from experience!)

Now set the case top onto the bottom shell, being careful when you fold the wires into the case that you don’t short them to anything. Again, test for full voltage at the contacts of the battery pack to ensure you have voltage and that the polarity is correct. Do you remember that test you did at the very start, where you marked the positive terminal? Make sure the marked end is still really positive.

Finally, reinstall the screws in the bottom of the case to hold the top on tightly, then do a final voltage test. With the battery pack reassembled, it’s time to plug it into its charger and see if it will take a charge. (Each of my battery packs has a different letter written on it, so I can easily keep track of which one needs to charge.)

Once fully charged, the battery is ready to test on the power tool. In my case, the tool was a drill, and I tested it by drilling some holes. Everything worked just great from the start—all for just $25 and a little time. I still have my electric drill that I like so much, and I can use it for many more years to come.

Now you know how to salvage your battery-powered tools by rebuilding old Ni-Cd battery packs, and avoid buying new tools. If you are careful to prevent shorts by working methodically and thinking ahead, you will soon have a good-as-new battery pack at a fraction of the cost of buying a new one, or buying a new tool. Enjoy your new-found freedom! 

Best Charging Practices

You should exercise care whenever you charge or recharge any rechargeable battery. All rechargeable batteries, regardless of technology—whether they are lead acid batteries, nickel metal hydride (Ni-MH) batteries, or lithium ion batteries—can be damaged and their lives shortened if they are charged incorrectly.

Ni-Cd batteries are no exception. For optimal lifespan and retention of a full charge, they must be recharged correctly. Worse, they can catch fire or even explore when charging practices are especially egregious.

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The good news is that the best Ni-Cd charging practices are simple, and there are many quality Ni-Cd charges available on the market. Read further for a few straightforward tips on how to correctly charge and recharge your Ni-Cd batteries. These guidelines will help your batteries last longer, and will help you avoid calamity!

The first charge: To avoid battery degradation in storage, manufacturers don’t fully form Ni-Cd batteries before shipping. So, it’s best to give your new Ni-Cds a slow charge before their first use—and I mean slow. This initial charge usually takes 15-24 hours, but results will vary. Check your Ni-Cd manufacturer’s datasheet for the initial charge time specific to your product.

This process correctly primes each Ni-Cd cell. But also, the cells might have discharged at different rates during shipping, so this slow charge is important to make sure each cell reaches the same level of charge.

Ni-Cd batteries don’t reach optimal performance until they have gone through several charge and discharge cycles. Most Ni-Cd cells need to go through five to ten cycles before reaching this level of performance. Further, in most cases Ni-Cd cells will reach peak performance after approximately 100 charge and discharge cycles, after which point performance will begin to decline.

Of course, that is contingent upon proper care for your Ni-Cd batteries, meaning they are charged in the proper manner.

Ni-Cd charging basics: Ni-Cd batteries need to be charged via a constant current sourcer, unlike other battery and cell types such as lead acid batteries. If instead constant voltage is used, due to their internal resistance they would draw currents large enough to damage the cells.

Cells are usually charged at a rate of C/10, where C is the battery capacity. Not all energy entering the cell gets stored as electrical energy, which means charge times typically take longer than 10 hours. However, the charging process is nearly 100% efficient up to 70% of a Ni-Cd battery’s full charge. This efficiency falls afte the 70% mark.

Cell voltage increases rapidly during the initial phase of charging, slows during the next phase, and spikes again when full charge is reached. This last increase in voltage is pronounced and hence easy to detect. Additional clues that a full charge has been reached are that the battery temperature increases significantly—this happens because energy is now lost as heat in the cell—and there is a spike in pressure inside the cell. One may perhaps now see why incorrect charging can be hazardous. So, avoid over-charging your Ni-Cd cells.

PUBLISHED IN CIRCUIT CELLAR MAGAZINE • NOVEMBER 2022 #388– Get a PDF of the issue

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Rebuild Ni-Cd Battery Packs for Power Tools

by James Lyman time to read: 19 min