Battery Powered Drills.
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,
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
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.
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
they are not all that easy to find second hand.
chuck that you hand tighten is the most convenient. However,
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
in Tek screws - and you won't have to open the chuck all that often.
the first thing to stop working with a battery powered drill, is the
battery. I have found that buying a replacement battery pack
the same or more than buying a new drill, so that is not a good
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.
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
NiCad's and starting from scratch, but it could be an option worth
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 -
you can also pick them up second hand. They are the type of
battery used in a lot of UPS's (Uninterruptible
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.
second option is to run a lead from
the drill to a larger 12 volt gell cell - 30 to 40 amp hour.
these are used in larger UPS applications, and can be picked up second
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
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.
will need some way of charging 12 volt gell cells. A battery
charger for wheel-chair batteries is ideal. A normal car
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.
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.
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.
a square battery does not have any way of being clipped onto the drill
The way I overcome this is by using large hose clamps.
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
handle will depend on the type of drill. The easiest way is
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
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.
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.
make the electrical
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
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
have to drill a hole in the handle to get the wires out that come from the motor, to connect to
the battery. I
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.
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.
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.
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.
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.
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.
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
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.
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.
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.
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.
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
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.
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.
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.
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.
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.