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Cordless Drill Power Adpater

13-14 October 2019

I was able to complete assembly and testing of the packs.  The process was not difficult, but it was time consuming.  Because I wanted to make sure I had a good connection for every solder, I ended up testing for continuty for that connection before moving on to the next.  My belief was that it would be easier to find out if there is a problem right then and fix it than to look for the bad connection when it's all asembled.  The other problem is what to do to help the life of the new battery packs.  The reason the orginal ones died early, was because I didn't use them much after I had left the job that required me to purchase this drill in the first place.  Since NiCd's store better when discharged (see here), I will probably need to make something to safely discharge them if I know I won't be using them for a while.  Or, I could come up with new projects that require a drill. :-)

The braided copper wire I ordered to solder the batteries together.  This wire is tinned, but regular copper probably would have been fine too.
 
A crossection of the braided wire.  When I ordered it, I assumed it would be something like how a belt for pants is braided rather than this tubular arrangement.  That meant is was thicker than I planned, but that design helped when it came to soldering the top battery to the battery pack connector.
 
Two views of the top battery soldered to battery pack connector.  The braided copper wire tube made this process easier because I was able to just slide the end of the braided tube over the metal connector and solder it as a first step.  The next step was to trim it to the correct length.  The braid with fall apart when attempting to cut it to short lengths (e.g. 1/4") prior to soldering.  So, trimming it to length has to be done after enough solder is in place to keep the braid intact.  The third step is to solder it to the battery.  It's easy to wind up with a lot of excess solder during this process or when soldering the batteries together.  A solder sucker is helpful for removing the excess while still maintaining a good connection.
 
The completed botton portion of the battery pack.  Shown here are the connections for the topside.  The remaining tab on the left will connect to a wire going to the negative terminal of the pack's power connector.
 The underside.  To make the connections as thin as possible, there was a lot of mashing down of the edges of the braid with a soldering iron to get them flush after the initial soldering was completed.  Using a solder sucker to remove excess was helpful here too.
 
Two views of the completed pack.  I needed to keep the gasket shown here to prevent a short.  However, I ended up removing the lower one to enable fitting the batteries into the case since the braided wire was thicker than I anticipated.  Forturnately, the case is plastic, so a short on the lower connections isn't a concern.
 
The upper and lower halves on the case.  To fit in the new batteries, it was necessary to grind away some of the plastic on the top and bottom.  I used a 60-grit sanding disk on a Dremmel tool for this.
 
The new battery assemly in its native environment.
 
The other final test: Charging the battery.  Besides checking the output voltage and plugging it into the drill to test it, making sure the charger will accept the new batteries is an essential test.  The red light in the middle indicates the battery is charging.  The green light lit up, indicating a successful charge, after a short time.



15 September 2019

The NiCd batteries I ordered arrived and I set about soldering them together.  One problem I had was soldering the metal tabs that were spot welded to the batteries to other batteries.  My soldering iron isn't designed for that and I don't have a spot welder.  There was also the problem repositioning some of the tabs so the batteries could run in series.  That meant removing them from the batteries and attempting to resolder them in an appropriate orientation which brought me back to my problem of soldering metal tabs.  To solve this problem, I resorted to using 12-gauge copper electrical wire.  That went much easier and I quickly became reasonably proficient at soldering copper wire to batteries.  As one might guess, copper wire is thicker than the metal tabs that are used to connect the batteries in the OEM version.  That created a problem of not enough space the put the batteries back in.  It was obvious that was going to be a problem, but I chose to complete soldering them together anyway mainly for practice and to see if it would work when I was done.  The good news is that it worked.  In series, the cells put out over 19 Volts!  Also, most of the connections were strong.  Moving the batteries in an attempt to break the solder joint didn't work.  I had to remelt the solder instead.  I have some pictures below.  Unfortunately, I forgot to photograph the completed protoype before I disassembled it.

The NiCd batteries I purchased.  Each box has fifteen which also happens to be the number of batteries in each of the battery packs.  1.2 Volts/battery x 15 batteries = 18 Volts
 
The original old batteries removed from the plastic case.  You can see how the metal tabs connecting the batteries are spot welded.  Fortunately, the plastic cases are held together with screws.  That made this project much easier.
 
A close up of the original batteries.  The thing mounted of the top battery is what connects to the contacts inside the drill's handle.  The red and black wires wrapped together with the white insulator go to a temperature sensor that signals to stop the charging process if the batteries get too hot.

To make creating the new pack easier, I traced an outline of it, idicated battery orientation, and showed how it is connect to the battery next to it. I did this for the top and bottom and labeled them appropriately.

 If this project going to be sucessful, I will need to find a way to solder the metal connector to the positive terminal of the battery it goes on top of.


8 September 2019

Some more research into doing this has led me to believe that creating a power adapter is not an ideal way to power my cordless drill.  The reason for that is the amount of current required to make it work makes it impractical.  The average corded household drill runs on 110 Volts and 8 Amps which comes out to around 900 Watts.  A typical cordless drill runs on about half that power (450 Watts).  Since mine runs on 18 Volts, that means it could draw 25 Amps or more.  That means heavy cabling, transformer, and other components that can handle high current draw.  It's possible to design the adapter to limit the current, but then we lose performance causing the drill to bog down during heavy use.  For the trouble, I'm better off just getting a decent corded drill or replacing the battery pack with batteries capable of supplying high current levels (e.g. NiCd or LiPo).  For more information about why using a DC power adapter to replace a battery pack is a suboptimal solution for high current draw devices you can go here or here.

1 September 2019

This is about creating a power adapter for a Sears Craftsman cordless drill.  It is true that making a power adapter defeats the purpose of having a cordless drill. However, both of the NiCd battery packs no longer work and getting replacement packs wasn't an option since I don't use it much and I've no desire of going through the trouble of regularly charging them to make sure they stay good.  Also, sometimes it's not really necessary for the drill to be cordless.  When I purchased this drill, it being cordless was essential.  These days, there is always a power outlet available for the work I would do with it.  Later, I will look into replacing the individual cells or changing the battery type.

 
This is a schematic of the adapter design that includes a transformer has a 6:1 ratio to reduce 120VAC to 20VAC, a bridge recitfier to convert the AC to DC, and a capacitor to help smooth out the DC current.  I assmumed that the resistance of the drill was very low.  This schematic and the simulation below were done with CircuitLab.
 
This is a simulation of the design's performance measured at three different points in the circuit.  120VAC is transformed to 20VAC and then to about 3 to 18 VDC.  The 2.2 mF capacitor helps smooth the voltage out somewhat.  Without it, the voltage continously goes from 0 to 20 VDC at 120Hz rather than from 3 to 18.  The low resistance of the drill also significantly contributes to voltage fluctuation.  Maybe a stable continuous voltage isn't as important for power tools as for electronics.  However, I'm still assuming that more stability is better.
 
My 18 Volt Hammer-Drill

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