Battery Powered Drills.


Having built our own house without being connected to the power grid, I have some appreciation for battery powered drills, and quite a collection of those who "did their job well".

For most people, when their battery powered drills "dies" they just go and buy a new one - which is fair enough if you have had a few years use out of it.  But when you are using your battery powered drill to do everything, including puting in hundreds of Tek screws in a day and drilling 12mm holes through steel, and so on, the time between drills can be more like months than years.

So, to keep the cost down, and a number of drills "at the ready", I have formulated from experience the following hints at how to keep some battery powered drills going after they have been given up as "dead" by other people.  These can usually be found at Trash and Treasure sales and the like, or passed on by friends.

Some General Comments.
There is no doubt in my mind, that drills with high/low gear shift are the more useful type. You use the high gear for starting Tek screws and drilling small holes, and then use the low gear for putting the screws in tight, and for the bigger drill sizes.  But having said that, they are not all that easy to find second hand.

A chuck that you hand tighten is the most convenient.  However, if you come across an old drill that has a key operated chuck, don't ignore it.  It can be allocated to a specific task - like putting in Tek screws - and you won't have to open the chuck all that often.


Usually, the first thing to stop working with a battery powered drill, is the battery.  I have found that buying a replacement battery pack costs the same or more than buying a new drill, so that is not a good solution.

I have tried a number of times to pick the best batteries out of two packs to make one good one, and have had some success, but too many "rebuilt" packs have had a short life.  To put it another way, it is not a good use of time to rebuild battery packs - it is not an easy job - and the chance of success is limited.  I have never tried buying a complete set of new NiCad's and starting from scratch, but it could be an option worth considering.

My preferred option these days is to go straight to a 7 amp/hour gell cell.  They are larger than the original battery pack, and a bit heavier, but they do deliver lots of power.  Obviously you can buy them new - which I have never done - but you can also pick them up second hand.  They are the type of battery  used in a lot of UPS's (Uninterruptible  Power Supply) which are used to keep computers running in a blackout, and the batteries should be changed on a regular basis. If you know someone who works in a place where the computers have to be kept going 24/7, you can ask about the batteries they take out when their maintenance falls due. Some battery dealers may also sell them second hand. A friend picked up a new 5.4 aH gell cell off eBay for around A$20 (plus A$10 postage), and you could expect it  (looked after) to last around 10 years. It is also a bit lighter than the 7 aH.

My second option is to run a lead from the drill to a larger 12 volt gell cell  - 30 to 40 amp hour. Again, these are used in larger UPS applications, and can be picked up second hand from battery dealers.  Companies that repair wheel chairs usually have second hand batteries this size for sale as well. This system is suited to a work bench situation. Sure, the cord gets in the way at times, but you can soon train yourself to put it to one side as soon as you finish with the drill. I have put on a roof using a 200 aH gell cell on the ground and house wiring type cable (low resistance) going up to the drill.

You will need some way of charging 12 volt gell cells.  A battery charger for wheel-chair batteries is ideal.  A normal car battery charger will also do the job, as long as you don't let the voltage get over 14 volts.  I have made a simple charger out of chip and a transistor.  I will try to include the circuit at the bottom.

Some may be wondering why I use a 12 volt battery for drills that may have had an initial voltage anywhere between 9 and 20 volts.  This is possible because DC motors are very robust and can handle a range of voltages - but there as some effects.  If the drill was rated below 12 volts, then it will run a little faster.  If it was rated over 12 volts then it will run a little slower.  However, and this is a big plus, the torque is still useful. I have a 20.5 volt drill running on 12 volts, and you can't stop the chuck turning with your hand. That is enough power to do an awful lot of work, even though the top speed is a little slower than the original.

A comment about battery types.  The normal cheap car battery is what I call a wet cell, and you have to add distilled water as the water is converted to hydrogen gas. These batteries are suited to a car application, as they need to be charged as soon as you use them. If used and then left uncharged, they fairly quickly self destruct due to a process they call "sulfonation".  The plates resist taking in a new charge.  In an emergency you could use a "wet cell" type of battery for a drill, as long as you recharged it as soon as you have finished using it.  The gell cells that I use are able to survive longer without being charged. That said, all batteries do better if they are kept fully charged.  There is another type of "wet cell" called "deep cycle", and these behave much like gell cells except you have to add clean (rain water) water from time to time. The water should be "soft" - no dissolved limestone.

Connecting the Batteries.
Obviously a square battery does not have any way of being clipped onto the drill handle. The way I overcome this is by using large hose clamps.  You can buy the clamping strip by the metre, and just clamp on the screw part at one end - cut to length - and you have a clamp big enough to go right around the battery.  How you attach the clamp to the drill handle will depend on the type of drill.  The easiest way is to drill a slot (or hole) right through the handle, and just run the clamp through it.  For others with a big flair, I have bolted a small bracket to the flat part and run the clamp over it (it has a lip to keep the clamp in place). With this method you will probably need two clamps (front and back). If the end of the handle is larger than the battery, then cutting a slot into the handle will give more stability.

For the smaller 5.4 aH batteries, it should be possible to cut off the bottom part of an old battery pack and attach that to the battery, instead of putting the battery direct onto the drill. This way the battery can still be clipped in and out of the drill, and may make storage and charging easier.

To make the electrical connection you will need some terminals suitable for the battery and some wire (like 240 volt figure eight flex), and solder a join onto the original wires.  It is best to keep the polarity the same, in case the switch is sensitive to the polarity. If you don't have different colour terminals, wrap some different colour insulating tape around the wires. Remember, an electric motor under full load can pull quite a few amps, so the wiring and connections need to be reasonably  solid.  If the handle is flat with the battery, you might have to drill a hole in the handle to get the wires out that come from the motor, to connect to the battery.  I prefer to keep the battery connections to the rear as this stops the wires getting caught with your fingers.  If you have to have them in front, then just cover the wires with packing tape.

Having breathed new life into a drill that has worn out the battery pack, you are off and running - that is until the switch gives up.  A lot of switches these days are the "soft start" or the type that also controls the speed.  At this stage, I haven't a way to replace the switch with those characteristics. However, just because the switch goes, doesn't mean that the drill is not useful.

I have tried a number of switch replacements that have worked with reasonable success. If you have a solid (remember it will have to take over 5 amps) push-on switch then give it a go.  You may have to modify the handle, or make a plate to go over the handle, or just set it in place with a generous amount of silicone.

These days, my preferred option is a small push-on switch (easier to fit) that is used to activate the coil of a small 12 volt relay. The full power to the motor goes through the points of the relay and not through the switch. The switch only has to handle a few milliamp. Most small relays have two contacts, so it is best to connect these in parallel to spread the load. These small switches and relays (smaller than a match box, and hence fit in the handle) are available from electronic hobby places like Dick Smith or Tandy.

It is rare - but it does happen - for a motor to stop. If it does, then the first thing to look for (after you have checked all the connections) are the brushes. The cheaper drills will have these "built in" at manufacture, and there is not much that can be done. The more expensive ones usually have brushes that can be replaced, and if you like the drill, this is certainly an option. A shop that specializes in power tools will usually carry spares. Take the old ones and as many details about the drill as you can find, when you go to buy them.

If the motor has stopped, then I am assuming that a full investigation of the wiring, switches and relay (if used) is done before you start pulling the brushes out.  It is usually easy to get the brush end of the motor off, but much harder to get it back on. The brushes have to be held back so they will go over the commutator, and this is not easy as there is very little room. One method I have used is to tie the brushes back with a strand of very fine wire, and then cut and remove the wire when the brushes are in place. Another way is to drill a small hole for a fine nail at the end of the brush when it is fully back. When the back plate is home, you just pull the nails out and the brushes slide into position. Obviously the nail holes have to be just outside the diameter of the commutator.

Usually, when the chuck jams I give up.  No doubt it is possible to swap chucks if the thread is the same, but I haven't done it.

If the drill has come to the end of the line as far as a drill is concerned, all is still not lost. The motor (if it wasn't the major cause of failure) can still be used for hobbies or a small fan. If the motor has failed, then it is still worth pulling it apart for the permanent  magnets - they make good fridge magnets for holding paper notes and so on.

Sorry about the hand drawn circuit, but I haven't mastered a computer program to do it for me as yet.  I am not good at electronics, so there would be better solutions around no doubt. I put this together with bits I already had, and some ideas off the Internet, and it works, so I am happy.

Battery Charging Circuit.


1.    If you are not on solar, or don't have 24 volts handy, then you might have to resort to using a transformer/rectifier to produce somewhere between 17 and 25 volts to run the battery charger. This is shown at the bottom of the of the above image. If you are not experienced in mains AC, then get someone who is, to set it up for you.

2.    The LM317 needs to be bolted to a small heat sink - at least 50 mm square.

3.    The two 2N3055's also need to be bolted to a larger heat sink. They can either be isolated from the heat sink by mica washers, or you can just bolt them on to the heat sink, and then isolate the heat sink. If you bolt them direct to the heat sink, then that becomes the Collector (the imput from the power supply). When using mica washers, it is usual to use a special heat transfer white paste. If you don't have any handy - as I often don't - then you can  use white zinc suncream ointment.

4.     The 0.1 ohms resistor on the output of each 2N3055 is required - I am told - to stop one transistor going "lazy" and letting the other one do all the work. Since they have to handle all the power going out, they need to be "beefy". My solution is to get some metal "tie wire" and then using an ohm meter, measure out the length that gives 0.1 ohm. Cut two pieces the same length, and then curl them around a round object (such as a pencil) to reduce their length.

5.    In my charger I added another feature. I added a three way double pole switch (you can actually do it with a singe pole three way switch, as earth is common) to be able to swap the voltmeter between the out put (battery voltage) and the out put from the LM317. If you set the LM317 to 0.6 volts higher then your required final voltage (the 0.6 volts is lost through the 2N3055) then you can walk away and leave it. The battery will not be charge higher than your setting. If you don't have the switch, then you have to monitor your first run so that by adjusting the 1K variable resistor you end up with 14 volts and minimum amps. That is, the battery is fully charged, but is not being over charged - which is bad for gell cell batteries.

6.    The 75 ohm resistor between the output and the adjustment on the LM317 is necessary, but the value - I have found by trial and error - is not all that critical. If you have to solder several lessor value resistors together to get close to 75 ohms then it will still work.

7.    It is not shown in the drawing, but I have a fuse in the leads going to the battery you are charging.

8.    I haven't shown it in the circuit, but I have an adjustable resistor in series for one of the output leads. It needs to be able to take at least an amp without getting too hot. Wire wound ones with a sliding contact should work, and there are solid carbon ones that will carry the load too. The reason I did this is so that I can set the voltage and leave it.  Batteries should be initially charged between 10% and 20% of their capacity - but no more than 20%.  For the 7 amp/hour gell cells I usually start at 1.0 amp, and then as the voltage comes up they start to drop back.  I use the variable resistor to hold the current at or under 1.0 amp, and with the voltage already set at 14 volts maximum, the charger is safe even if I forget to check up on it. As the current falls below - say 0.5 amps - the variable resistor can be moved towards zero to make the charger more efficient.  

How it works.
A technical person could describe it more accurately, but as I understand it, the LM317 and the variable resistor can be set to a selected voltage - say 14.6 volts. Once set, it remains reasonably constant as long as you don't draw more than 1.0 amp from the LM317.

This set voltage applied to the base of the 2N3055 will allow it to pass current until the voltage in the battery being charged rises to 0.6 volts below the voltage set in the LM317.

The greater the difference between the voltage set by the LM317 and the battery being charged, the greater the flow of current. As the voltages get closer, the current drops off. This is good, because if you forget to turn it off, no harm is done to the battery. It also means that you can control the current being drawn through the 2N3055's. If the current is too high, drop the voltage set on the LM317 (see point 8 above for an alternate method to reduce the current). After the battery has absorbed some current its voltage will come up, and then you can push the voltage up on the LM317 in stages until you get to the final voltage.

The final voltage that you charge a battery to will depend on the battery type, and if you are charging for a duty cycle or stand-by. Sometimes this is given on the battery, or the specifications can be found on the Net. If it is not known, then I use 14 volts. Wet cells can be taken higher, but for gell cells that is usually their upper limit.

Picture of a modified drill.

A Modified Drill

The original switch has been replaced with a small push-on switch, which operates a small relay in the handle.  Notice the cut-out in the drill handle, and the clamp.  The packing tape over the wires and terminals has been pulled back.

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