Modifying a Cordless Screwdriver

[ms]

I have several cordless screwdrivers with dead batteries. I realize it
is possible that someoneo loaded the motor and burned it out, don't know
the history of these units.

As I also have a working cordless screwdriver, I want to modify one of
these items to run from a wall adapter. The unit is a Skil 2205,
normally has(IIRC), 2 nicad batteries, about 3 VDC total. I tried up to
a 6 VDC 300 MA adapter to the screwdriver switch, the motor did not
work. I measured across the motor terminals and there is continuity, so
I assume the motor is OK. I applied the voltage at the motor terminals
and still no luck. I tried the other identical Skil screwdriver and same
results.

The normal nicad batteries can supply a high peak current.

Does the motor in this unit typically need a high current to run even no
load, and the adapter I'm using is current limiting so the motor never
turns on?

If so, what kind of current does it draw?

TIA

Mike

 

[Dave D]

I have several cordless screwdrivers with dead batteries. I realize it
is possible that someoneo loaded the motor and burned it out, don't know
the history of these units.

As I also have a working cordless screwdriver, I want to modify one of
these items to run from a wall adapter. The unit is a Skil 2205,
normally has(IIRC), 2 nicad batteries, about 3 VDC total. I tried up to
a 6 VDC 300 MA adapter to the screwdriver switch, the motor did not
work. I measured across the motor terminals and there is continuity, so
I assume the motor is OK. I applied the voltage at the motor terminals
and still no luck. I tried the other identical Skil screwdriver and same
results.

The normal nicad batteries can supply a high peak current.

Yes

Does the motor in this unit typically need a high current to run even no
load, and the adapter I'm using is current limiting so the motor never
turns on?

Probably need several hundred mA just to overcome the drag of the
gears/bearings.

If so, what kind of current does it draw?

Likely several amps under load.

Dave

 

[Engineer]

I have several cordless screwdrivers with dead batteries. I realize it
is possible that someoneo loaded the motor and burned it out, don't know
the history of these units.

It's almost certainly the batteries, not the motors.

As I also have a working cordless screwdriver, I want to modify one of
these items to run from a wall adapter. The unit is a Skil 2205,
normally has(IIRC), 2 nicad batteries, about 3 VDC total. I tried up to
a 6 VDC 300 MA adapter to the screwdriver switch, the motor did not
work. I measured across the motor terminals and there is continuity, so
I assume the motor is OK. I applied the voltage at the motor terminals
and still no luck. I tried the other identical Skil screwdriver and same
results.

The normal nicad batteries can supply a high peak current.

Indeed they do - see below!

Does the motor in this unit typically need a high current to run even no
load, and the adapter I'm using is current limiting so the motor never
turns on?

If so, what kind of current does it draw?

Lots - see below!

TIA

Mike

Dead electric screwdrivers are a problem. I have found a
less than ideal solution - sort of.

They are 3.6 volts nominally. DC Wall warts don't work at
the screwdrivers need a lot of current, over 6 amps
stalled. They pull about an amp or so running free. After
failing to find replacement NiCads that fitted I built a
mains power supply.

This is a 12.6 VAC, 3A, center-tapped filament transformer,
two 5406 rectifiers and a 0.5 mH choke input to a 4,700 uF
capacitor. Open circuit voltage around 9 volts, but that
doesn't matter. Free running voltage is about 5 volts due to
choke input (does not seem to be a problem - nice fast
screwdriver on light load!) However, final torque under load
is limited. Near stall the current goes way up (over 6
amps) and the voltage goes down to about 2 1/2 to 3 VDC - so
torque suffers, it's less than with good NiCads.

As a system it's a bit of a pain - not portable - but I
leave it plugged in on the bench for light work.

I actually managed to salvage a good NiCad cell from another
dead screwdriver (a Skil flexicharge thing, or whatever it
is called) and used it to replace a dead cell in my Canadian
Tire screwdriver that died - but it does not keep a charge
long so I am still looking or a replacement (see
above.)

Cheers,

Roger

--
Roger Jones, P.Eng.
Thornhill, Ontario,
Canada.

"Friends don't let friends vote Liberal"

 

[Dave D]

Aren't these batteries just an assembly of ordinary "AA" size nicads? I seem
to recall that most rechargible battery packs are simply assemblies of the
common cell.

Not so?

They're most often sub-C cells or C cells.

Dave

 

[Engineer]

They're most often sub-C cells or C cells.

Dave

They are indeed Sub-C's (not C's) but just a fraction
shorter than the Radio Shack and several other industrial
replacements. That, and the fact that the originals are
welded together (and you need to wire replacements) means
they don't fit. Is this a conspiracy?!

Who sells 3-cell NiCad direct screwdriver replacements?
There must be a huge market for them!

Cheers,

Roger
--
Roger Jones, P.Eng.
Thornhill, Ontario,
Canada.

"Friends don't let friends vote Liberal"

 

[ms]

It's almost certainly the batteries, not the motors.


Indeed they do - see below!

Lots - see below!


Dead electric screwdrivers are a problem. I have found a
less than ideal solution - sort of.

They are 3.6 volts nominally. DC Wall warts don't work at
the screwdrivers need a lot of current, over 6 amps
stalled. They pull about an amp or so running free. After
failing to find replacement NiCads that fitted I built a
mains power supply.

This is a 12.6 VAC, 3A, center-tapped filament transformer,
two 5406 rectifiers and a 0.5 mH choke input to a 4,700 uF
capacitor. Open circuit voltage around 9 volts, but that
doesn't matter. Free running voltage is about 5 volts due to
choke input (does not seem to be a problem - nice fast
screwdriver on light load!) However, final torque under load
is limited. Near stall the current goes way up (over 6
amps) and the voltage goes down to about 2 1/2 to 3 VDC - so
torque suffers, it's less than with good NiCads.

As a system it's a bit of a pain - not portable - but I
leave it plugged in on the bench for light work.

I actually managed to salvage a good NiCad cell from another
dead screwdriver (a Skil flexicharge thing, or whatever it
is called) and used it to replace a dead cell in my Canadian
Tire screwdriver that died - but it does not keep a charge
long so I am still looking or a replacement (see
above.)

Cheers,

Roger

--
Roger Jones, P.Eng.
Thornhill, Ontario,
Canada.

"Friends don't let friends vote Liberal"

Thanks, that really explains my results.

The only reason I want to have a AC supply for a cordless screwdriver is
the chuck fits many useful bits and has, of course, a slow RPM. Your
details give me a way to go.

I will reevaluate my need as building that kind of high current supply
is more than I planned on. I had hoped to use a wall wart.

Mike

 

[ms]

They are indeed Sub-C's (not C's) but just a fraction
shorter than the Radio Shack and several other industrial
replacements. That, and the fact that the originals are
welded together (and you need to wire replacements) means
they don't fit. Is this a conspiracy?!

Who sells 3-cell NiCad direct screwdriver replacements?
There must be a huge market for them!

Cheers,

Roger
--
Roger Jones, P.Eng.
Thornhill, Ontario,
Canada.

"Friends don't let friends vote Liberal"

I have tried to replace a bad cell in a string with a better cell from
another unit. But the welded tabs are very difficult to solder to, and
clip leads have too much resistance for the high current in use.

Advice?

Mike

 

[nick hull]

They are indeed Sub-C's (not C's) but just a fraction
shorter than the Radio Shack and several other industrial
replacements. That, and the fact that the originals are
welded together (and you need to wire replacements) means
they don't fit. Is this a conspiracy?!

Who sells 3-cell NiCad direct screwdriver replacements?
There must be a huge market for them!

Try Josh Sponenberg <[email protected]>, he has replacement batteries
for almost anything.

 

[Engineer]

Engineer wrote:
(snip)

(snip)

Thanks, that really explains my results.

The only reason I want to have a AC supply for a cordless screwdriver is
the chuck fits many useful bits and has, of course, a slow RPM. Your
details give me a way to go.

I will reevaluate my need as building that kind of high current supply
is more than I planned on. I had hoped to use a wall wart.

Mike

Mike, here are the actual numbers for the above P/S on my
"battery dead" Skil Supertwist (I was too lazy in the above
post to go to the basement to get them!)

At stall: 2.15 VDC (too low, of course) but still 6.4 amps
(amazing!) This is why the torque is a bit disappointing.
Free running: 5 volts (a bit high but OK), 1.2 to 1.25 amps

If I did this again (I won't!) I would design a P/S with
better regulation - the choke input in important to keep the
reservoir ripple current reasonable low. Ideally, I would
use a solid state regulator, say 4 VDC, 10 amps minimum, but
I have not looked for one.

Yes, it's much more than a wall wart! When the NiCads work
they deliver scads of current. When they fail they leave us
with a drawer full of dead screwdrivers!

Cheers,

Roger
--
Roger Jones, P.Eng.
Thornhill, Ontario,
Canada.

"Friends don't let friends vote Liberal"

 

[Engineer]

(big snip)

I had said...

Mike said:

I have tried to replace a bad cell in a string with a better cell from
another unit. But the welded tabs are very difficult to solder to, and
clip leads have too much resistance for the high current in use.

Advice?

Mike

I've done this (see other post.) I identified the dead cell
by measuring the voltage both on and off charge - it should
be obvious which one. Then I literally ripped the stack
apart and cut the thin welded strips (you can use scissors)
leaving enough on the good cell to solder onto (it can be
done, use a good flux cored solder and hot iron, tin
everything first, don't cook the cell!) Then I ripped up
the target stack the same way and soldered the replacement
cell in - you need to be careful with the space the wire
takes up. Then install charge the refurbished battery. To
date it still works... but I'm still looking to buy a
complete new battery.

How we waste our time salvaging stuff!

Cheers,

Roger

--
Roger Jones, P.Eng.
Thornhill, Ontario,
Canada.

"Friends don't let friends vote Liberal"