Capacitor to reduce DC motor start up current draw.

[~misfit~]

Hi folks,

I'm a noob when it comes to electronics but am trying to learn.

A while back I bought a cheap battery powered rotary tool / large engraver.
It didn't run long on a chanrge and didn't have much power so I opened it
up. It had three NiCad cells end-to-end as it's energy store, a simple
switch and charged through a jack to which you'd connect the 5v DC power
supply.

I decided to replace the NiCads with an 18650 Li-Ion cell and use the
following module to charge the cell and prevent discharging;
http://www.dx.com/p/219454

I've used those modules before with cells taken from old laptop battery
packs and LEDs and they work well. However after I modded the case, soldered
everything together and tried it (yeah I should have tested it outside the
case...) I found it wouldn't switch on. However if I momentarilly
shorted the DC out to the battery connection the motor would run just fine
except if put under very heavy loads, in which case it'd stop. (I killed a
couple of the modules messing around trying to fit a momentary contact
switch to short across to start the motor.)

Some guy in a forum on that site suggested it was because of the high
current draw on start-up of the motor and that I put a capacitor in parallel
(?) with the motor it might fix it. However I was unable to engage him any
further to get him to tell me more, what size and type of cap etc.

I'm not rich and don't have the funds to buy electronic components and try
whenever possible to use salvaged parts. I keep old PCBs and tend to use
them as my supply of parts whenever possible...

Can anyone tell me if doing this is likely to work and if so what type and
value of capacitor to use please? I seem to recall seeing something similar
somewhere, what appeared to be a tantalum capacitor wired across a DC motor.
I can see quite a few of those on the various PCBs that I have. If someone
can confirm that it might work them I'll get to unsoldering and try to work
out what values the caps are that I can find.

Cheers, and TIA.
--
Shaun.

"Humans will have advanced a long, long way when religious belief has a cozy
little classification in the DSM*."
David Melville (in r.a.s.f1)
(*Diagnostic and Statistical Manual of Mental Disorders)

 

[Jasen Betts]

On 2015-11-16, ~misfit~ <shaun.at.pukekohe@gmail.com> wrote:

I've used those modules before with cells taken from old laptop battery
packs and LEDs and they work well. However after I modded the case, soldered
everything together and tried it (yeah I should have tested it outside the
case...) I found it wouldn't switch on. However if I momentarilly
shorted the DC out to the battery connection the motor would run just fine
except if put under very heavy loads, in which case it'd stop. (I killed a
couple of the modules messing around trying to fit a momentary contact
switch to short across to start the motor.)

problem is your module isn't tough enough to start the motor.
adding capacitors will likely make matters worse.

Adding a resistor in series with the motor might help, say 0.47 ohm 5W
then put a switch parallel with the resistor to bypass it once the
motor starts.

--
\_(ツ)_

 

[Computer Nerd Kev]

~misfit~ <shaun.at.pukekohe@gmail.com> wrote:

Hi folks,

I'm a noob when it comes to electronics but am trying to learn.

A while back I bought a cheap battery powered rotary tool / large engraver.
It didn't run long on a chanrge and didn't have much power so I opened it
up. It had three NiCad cells end-to-end as it's energy store, a simple
switch and charged through a jack to which you'd connect the 5v DC power
supply.

I decided to replace the NiCads with an 18650 Li-Ion cell and use the
following module to charge the cell and prevent discharging;
http://www.dx.com/p/219454

I can't help but point out that even the sales picture shows a board
with an LED misaligned and almost shorting to a nearby resistor, which
looking at the board traces, would probably damage the rearby IC.
I'm not saying that the board is the problem, I just think it says
something about the sellers of this stuff that they use a board like
that as the demo.

I've used those modules before with cells taken from old laptop battery
packs and LEDs and they work well. However after I modded the case, soldered
everything together and tried it (yeah I should have tested it outside the
case...) I found it wouldn't switch on. However if I momentarilly
shorted the DC out to the battery connection the motor would run just fine
except if put under very heavy loads, in which case it'd stop. (I killed a
couple of the modules messing around trying to fit a momentary contact
switch to short across to start the motor.)

Some guy in a forum on that site suggested it was because of the high
current draw on start-up of the motor and that I put a capacitor in parallel
(?) with the motor it might fix it. However I was unable to engage him any
further to get him to tell me more, what size and type of cap etc.

That wouldn't work unless the capacitor's connection to the battery voltage
bypassed the power switch, so that it was charging all the time. Even then
it may trigger the current limiting circuitry, which may or may not cause
a problem depending on design. If it uses foldback current limiting, then
it would drop the voltage until the point that less that the maximum
current is flowing. As the capacitor charged, the current flow at that
voltage would be reduced, and as such the current limiting circuit would
progressively raise the voltage until both charge circuit and capacitor
were effectively at their full voltage. with a normal electrolytic
capacitor, this would always take less than a second to occour.

If however, the current limiting actually cuts off the power completely
when the maximum current is exceeded (for protection in the case of a
short perhaps), you would need to limit the power going to the capacitor
when it charges. This could be achieved by placing a current limiting
resistor (47 ohms should allow a quick enough charge time with most
large value electrolytics) in series with the capacitors connection
to the battery voltage (which is still before the power switch), and
a power diode (eg. 1N4004) in parallel with its Anode towards the
capacitor. The diode would supply current from the capacitor when the
power switch is turned on.

The capacitor value required would depend on the peak load exerted by
the motor (Storage Oscilloscope measurement), but I'd recommend just
trying the largest capacitance value electrolytic you have which is a
suitable size for the application. You might need to try some in
parallel if that isn't enough. You'd want the total capacitance well
above 1,000uF in any case. Quite likely above 10,000uF.

Note that in both cases, the connection of the capacitor bypassing the
power switch will cause a constant (though not relatively large) drain
on the battery. The exact current would depend on the leakage of the
capacitor, and therefore its capacitance, quality and age. The other
suggestion of a switched resistor in series with the motor might be
preferred if this is undesired. From the battery/module's point of
view, that is simply a manual equivalent to the capacitor solution.

Indeed a circuit could be constructed to automate the switching of
the motor series resistor if so desired.

I'm not rich and don't have the funds to buy electronic components and try
whenever possible to use salvaged parts. I keep old PCBs and tend to use
them as my supply of parts whenever possible...

Can anyone tell me if doing this is likely to work and if so what type and
value of capacitor to use please? I seem to recall seeing something similar
somewhere, what appeared to be a tantalum capacitor wired across a DC motor.
I can see quite a few of those on the various PCBs that I have.

Perhaps they were actually for reducing electrical noise from the motor,
ceramic capacitors are sometimes used for that purpose.


Sorry for the long post. And I just realised that I can't really speak
because I've got a cheap rotary tool I was given with a dead NiCad
battery, and it's still sitting disassembled somewhere.

--
__ __
#_ < |\| |< _#

 

[~misfit~]

Once upon a time on usenet Jasen Betts wrote:

On 2015-11-16, ~misfit~ <shaun.at.pukekohe@gmail.com> wrote:

I've used those modules before with cells taken from old laptop
battery packs and LEDs and they work well. However after I modded
the case, soldered everything together and tried it (yeah I should
have tested it outside the case...) I found it wouldn't switch on.
However if I momentarilly shorted the DC out to the battery
connection the motor would run just fine except if put under very
heavy loads, in which case it'd stop. (I killed a couple of the
modules messing around trying to fit a momentary contact switch to
short across to start the motor.)

problem is your module isn't tough enough to start the motor.
adding capacitors will likely make matters worse.

Ok, thanks. Like I said I didn't understand what the guy meant and I'm quite
new to electronics (and an 'old dog' - finding new tricks not as easy to
learn as they used to be).

Adding a resistor in series with the motor might help, say 0.47 ohm 5W
then put a switch parallel with the resistor to bypass it once the
motor starts.

Thanks. There's very little room in the case. I've already crwoded it with
the longer cell and charging module. The momentary swith I fitted was a thin
thing, a recessed reset switch scavenged from a dead ADSL modem and
hot-glued into the rotary tool case.

Cheers.
--
Shaun.

"Humans will have advanced a long, long way when religious belief has a cozy
little classification in the DSM*."
David Melville (in r.a.s.f1)
(*Diagnostic and Statistical Manual of Mental Disorders)

 

[~misfit~]

Once upon a time on usenet Computer Nerd Kev wrote:

~misfit~ <shaun.at.pukekohe@gmail.com> wrote:
Hi folks,

I'm a noob when it comes to electronics but am trying to learn.

A while back I bought a cheap battery powered rotary tool / large
engraver. It didn't run long on a chanrge and didn't have much power
so I opened it up. It had three NiCad cells end-to-end as it's
energy store, a simple switch and charged through a jack to which
you'd connect the 5v DC power supply.

I decided to replace the NiCads with an 18650 Li-Ion cell and use the
following module to charge the cell and prevent discharging;
http://www.dx.com/p/219454

I can't help but point out that even the sales picture shows a board
with an LED misaligned and almost shorting to a nearby resistor, which
looking at the board traces, would probably damage the rearby IC.
I'm not saying that the board is the problem, I just think it says
something about the sellers of this stuff that they use a board like
that as the demo.

Actually I've noticed that with a lot of their products and so far nearly
all of the things that I've bought have been better made than the pictures.
I considered that maybe they get a prototype for the pic?

I've used those modules before with cells taken from old laptop
battery packs and LEDs and they work well. However after I modded
the case, soldered everything together and tried it (yeah I should
have tested it outside the case...) I found it wouldn't switch on.
However if I momentarilly shorted the DC out to the battery
connection the motor would run just fine except if put under very
heavy loads, in which case it'd stop. (I killed a couple of the
modules messing around trying to fit a momentary contact switch to
short across to start the motor.)

Some guy in a forum on that site suggested it was because of the high
current draw on start-up of the motor and that I put a capacitor in
parallel (?) with the motor it might fix it. However I was unable to
engage him any further to get him to tell me more, what size and
type of cap etc.

That wouldn't work unless the capacitor's connection to the battery
voltage bypassed the power switch, so that it was charging all the
time. Even then it may trigger the current limiting circuitry, which
may or may not cause a problem depending on design. If it uses
foldback current limiting, then it would drop the voltage until the
point that less that the maximum current is flowing. As the capacitor
charged, the current flow at that voltage would be reduced, and as
such the current limiting circuit would progressively raise the
voltage until both charge circuit and capacitor were effectively at
their full voltage. with a normal electrolytic capacitor, this would
always take less than a second to occour.

Yeah. That's a bit too complicated and fiddly for my liking.

If however, the current limiting actually cuts off the power
completely when the maximum current is exceeded (for protection in
the case of a short perhaps), you would need to limit the power going
to the capacitor when it charges. This could be achieved by placing a
current limiting resistor (47 ohms should allow a quick enough charge
time with most
large value electrolytics) in series with the capacitors connection
to the battery voltage (which is still before the power switch), and
a power diode (eg. 1N4004) in parallel with its Anode towards the
capacitor. The diode would supply current from the capacitor when the
power switch is turned on.

The capacitor value required would depend on the peak load exerted by
the motor (Storage Oscilloscope measurement), but I'd recommend just
trying the largest capacitance value electrolytic you have which is a
suitable size for the application. You might need to try some in
parallel if that isn't enough. You'd want the total capacitance well
above 1,000uF in any case. Quite likely above 10,000uF.

So about as big as the 18650 cell than?

Note that in both cases, the connection of the capacitor bypassing the
power switch will cause a constant (though not relatively large) drain
on the battery. The exact current would depend on the leakage of the
capacitor, and therefore its capacitance, quality and age. The other
suggestion of a switched resistor in series with the motor might be
preferred if this is undesired. From the battery/module's point of
view, that is simply a manual equivalent to the capacitor solution.

Ok. However the case is quite tight...

Indeed a circuit could be constructed to automate the switching of
the motor series resistor if so desired.

Heh, yep if I was up to the task and there was room.

I'm not rich and don't have the funds to buy electronic components
and try whenever possible to use salvaged parts. I keep old PCBs and
tend to use them as my supply of parts whenever possible...

Can anyone tell me if doing this is likely to work and if so what
type and value of capacitor to use please? I seem to recall seeing
something similar somewhere, what appeared to be a tantalum
capacitor wired across a DC motor. I can see quite a few of those on
the various PCBs that I have.

Perhaps they were actually for reducing electrical noise from the
motor, ceramic capacitors are sometimes used for that purpose.

Thanks.

Sorry for the long post. And I just realised that I can't really speak
because I've got a cheap rotary tool I was given with a dead NiCad
battery, and it's still sitting disassembled somewhere.

LOL, sounds like me. It was well worth it re-powering with the Li-Ion cell.
It's more powerful (and doesn't seem to overheat or anything) and of course,
even with a salvaged cell last longer between charges and can sit in the
drawer for a month between uses and still have charge.

I originally modded this with a simple Li-Ion charge module and it worked
great. But then I got worried about over-discharging so changed to this
thing. I'm starting to think that the easiest answer might be to run the
motor from the battery terminals and connect an LED to the 'load' terminals.
When the LED goes out then it's time to stop using the tool.

There is continuity between the load out + and battery + so I could switch
positive to both the LED and the motor. Then have the LED - going to load
and motor - going to the battery teminals of the module. When the battery
drops below 2.5v the LED should go out. That should work don't you think?

I dunno, I'm winging it here.
--
Shaun.

"Humans will have advanced a long, long way when religious belief has a cozy
little classification in the DSM*."
David Melville (in r.a.s.f1)
(*Diagnostic and Statistical Manual of Mental Disorders)

 

[Computer Nerd Kev]

~misfit~ <shaun.at.pukekohe@gmail.com> wrote:

Once upon a time on usenet Computer Nerd Kev wrote:
~misfit~ <shaun.at.pukekohe@gmail.com> wrote:

I decided to replace the NiCads with an 18650 Li-Ion cell and use the
following module to charge the cell and prevent discharging;
http://www.dx.com/p/219454

I can't help but point out that even the sales picture shows a board
with an LED misaligned and almost shorting to a nearby resistor, which
looking at the board traces, would probably damage the rearby IC.
I'm not saying that the board is the problem, I just think it says
something about the sellers of this stuff that they use a board like
that as the demo.

Actually I've noticed that with a lot of their products and so far nearly
all of the things that I've bought have been better made than the pictures.
I considered that maybe they get a prototype for the pic?

Huh, you never can tell with this Chinese stuff.

I've used those modules before with cells taken from old laptop
battery packs and LEDs and they work well. However after I modded
the case, soldered everything together and tried it (yeah I should
have tested it outside the case...) I found it wouldn't switch on.
However if I momentarilly shorted the DC out to the battery
connection the motor would run just fine except if put under very
heavy loads, in which case it'd stop. (I killed a couple of the
modules messing around trying to fit a momentary contact switch to
short across to start the motor.)

Some guy in a forum on that site suggested it was because of the high
current draw on start-up of the motor and that I put a capacitor in
parallel (?) with the motor it might fix it. However I was unable to
engage him any further to get him to tell me more, what size and
type of cap etc.

That wouldn't work unless the capacitor's connection to the battery
voltage bypassed the power switch, so that it was charging all the
time. Even then it may trigger the current limiting circuitry, which
may or may not cause a problem depending on design. If it uses
foldback current limiting, then it would drop the voltage until the
point that less that the maximum current is flowing. As the capacitor
charged, the current flow at that voltage would be reduced, and as
such the current limiting circuit would progressively raise the
voltage until both charge circuit and capacitor were effectively at
their full voltage. with a normal electrolytic capacitor, this would
always take less than a second to occour.

Yeah. That's a bit too complicated and fiddly for my liking.

You know most of that was describing the technicalities of how it
worked? The actual circuit is just the capacitor with the existing
power switch and control module:

Argh, I couldn't be bothered to do this last time... _._
CAP | |
_____| |_________ + _________ | |
-------- + | | | *__|_____| | | C |
| |______|__________*\ ON | |---| E |
| MOTOR | POWER SWITCH- \ | CONTROL | | L |
| | *OFF | MODULE |---| L |
| |------------------------------|_________| | |
-------- - - |___|

The only added component is the capacitor.

If however, the current limiting actually cuts off the power
completely when the maximum current is exceeded (for protection in
the case of a short perhaps), you would need to limit the power going
to the capacitor when it charges. This could be achieved by placing a
current limiting resistor (47 ohms should allow a quick enough charge
time with most
large value electrolytics) in series with the capacitor's connection
to the battery voltage (which is still before the power switch), and
a power diode (eg. 1N4004) in parallel with its Anode towards the
capacitor. The diode would supply current from the capacitor when the
power switch is turned on.

CAP RESISTOR(47R)
| | ______
-| |---|______|-- _._
| | | | | | |
| | |\ | | | |
| |-| >|------| + _________ | |
-------- + |DIODE> |/ | *__|_____| | | C |
| |______|__________*\ ON | |---| E |
| MOTOR | POWER SWITCH> \ | CONTROL | | L |
| | *OFF | MODULE |---| L |
| |------------------------------|_________| | |
-------- - - |___|

The capacitor value required would depend on the peak load exerted by
the motor (Storage Oscilloscope measurement), but I'd recommend just
trying the largest capacitance value electrolytic you have which is a
suitable size for the application. You might need to try some in
parallel if that isn't enough. You'd want the total capacitance well
above 1,000uF in any case. Quite likely above 10,000uF.

So about as big as the 18650 cell than?

Quite likely (having just looked up the size of one of these mystical
Lithium beasts). Possibly smaller if you can find 6V or 10V rated caps
in the right range. Much smaller if you use a couple of Supercaps.

Sorry for the long post. And I just realised that I can't really speak
because I've got a cheap rotary tool I was given with a dead NiCad
battery, and it's still sitting disassembled somewhere.

LOL, sounds like me. It was well worth it re-powering with the Li-Ion cell.
It's more powerful (and doesn't seem to overheat or anything) and of course,
even with a salvaged cell last longer between charges and can sit in the
drawer for a month between uses and still have charge.

I originally modded this with a simple Li-Ion charge module and it worked
great. But then I got worried about over-discharging so changed to this
thing. I'm starting to think that the easiest answer might be to run the
motor from the battery terminals and connect an LED to the 'load' terminals.
When the LED goes out then it's time to stop using the tool.

There is continuity between the load out + and battery + so I could switch
positive to both the LED and the motor. Then have the LED - going to load
and motor - going to the battery teminals of the module. When the battery
drops below 2.5v the LED should go out. That should work don't you think?

I dunno, I'm winging it here.

Oh boy, here we go again...
LED ~120R
| /| ______
----|< |--|______|- _._
| | \| ________| + _________ | |
-------- + | | *_______| | | C |
| |______| *\ ON | |---| E |
| MOTOR | POWER SWITCH- \ | CONTROL | | L |
| | *OFF | MODULE |---| L |
| |------------------------------|_________| | |
-------- - - |___|

Hmm, if that's what you meant then we are in trouble...

I think this is really what you want:
LED ~120R _This is a bridge,
|\ | ______ / not a join
----| >|--|______|- / _._
| |/ | | / + _________ | |
-------- + | *__|____| | | C |
| |______|__________*\ ON | | |---| E |
| MOTOR | POWER SWITCH- \ | | CONTROL | | L |
| | *OFF | | MODULE |---| L |
| |------------------------------|_________| | |
-------- - / - |___|
/
That's the join (Resistor to -)

It's drawn a bit confusingly because I'm getting lazy(er), but
the LED should be on until the voltage gets below ~2V, or the
power switch is turned off.

Now looking at the notes from the a Lithium battery controller I
recently designed (but also haven't got around to doing yet...),
I had the cut-off voltage at 3V. Perhaps against best advise, I
trust myself in my past reasoning and would suggest that 2V is
too low a cut-off point. However I remember I got all my
information from the website "Battery University" (I'm out of
time now, so can't look it up myself to check).

In any case, a regular diode in series with the LED would make
the cut-off point ~2.7V, and that might be acceptable. In
practice it will grow dimmer towards the cut-off point instead
of suddenly going out.


Sorry for the whining, don't feel you put me under any
obligation to do all this.

--
__ __
#_ < |\| |< _#

 

[~misfit~]

Once upon a time on usenet Computer Nerd Kev wrote:

And before you redefine your understanding of capacitor theory, the
capacitor in those diagrams should be connected with its negative to
the battery negative, not to the motor positive. eg.
_._
CAP | |
_____| |_________ + _________ | |
-------- + | | |+ *__|_____| | | C |
|______|__________*\ ON | |---| E |
MOTOR | | POWER- \ | CONTROL | | L |
| | SWITCH *OFF | MODULE |---| L |
|------------------------------|_________| | |
-------- - - |___|

Sorry about that.

Thanks for taking the time to help and sorry it's been so long since I
visited here.

I was thinking of something along the lines of:


LED ~120R _This is a bridge,
|\ | ______ / not a join
----| >|--|______|- / _._
| |/ | | / + _________ | |
-------- + | *__|____| | | C |
| |______|__________*\ ON | | |---| E |
| MOTOR | POWER SWITCH- \ | | CONTROL | | L |
| | *OFF | | MODULE |---| L |
| | ----|_________| | | |
-------- ------------------------------------------ |___|

Where the motor positive and LED positive are switched but motor negative
goes straight to the battery bypassing the protection circuit (which seems
to operate on the negative - the battery positive and load out positive are
common).

That way only the LED is on the protection circuit (so motor shouldn't trip
overcurrent protection) and would turn off when the voltage drops below the
predefined low for the 18650 (18mm diameter, 650 long, the most ubiquitous
Li-Ion cell there is - it's in flashlights, laptop batteries, powertool
batteries and even powers Prius and Tesla cars).

Cheers,
--
Shaun.

"Humans will have advanced a long, long way when religious belief has a cozy
little classification in the DSM*."
David Melville (in r.a.s.f1)
(*Diagnostic and Statistical Manual of Mental Disorders)

 

[~misfit~]

Once upon a time on usenet ~misfit~ wrote:

Once upon a time on usenet Computer Nerd Kev wrote:
And before you redefine your understanding of capacitor theory, the
capacitor in those diagrams should be connected with its negative to
the battery negative, not to the motor positive. eg.
_._
CAP | |
_____| |_________ + _________ | |
-------- + | | |+ *__|_____| | | C |
|______|__________*\ ON | |---| E |
MOTOR | | POWER- \ | CONTROL | | L |
| | SWITCH *OFF | MODULE |---| L |
|------------------------------|_________| | |
-------- - - |___|

Sorry about that.

Thanks for taking the time to help and sorry it's been so long since I
visited here.

I was thinking of something along the lines of:


LED ~120R _This is a bridge,
|\ | ______ / not a join
----| >|--|______|- / _._
| |/ | | / + _________ | |
-------- + | *__|____| | | C |
|______|__________*\ ON | | |---| E |
MOTOR | POWER SWITCH- \ | | CONTROL | | L |
| *OFF | | MODULE |---| L |
| ----|_________| | | |
-------- ------------------------------------------ |___|

Where the motor positive and LED positive are switched but motor
negative goes straight to the battery bypassing the protection
circuit (which seems to operate on the negative - the battery
positive and load out positive are common).

That way only the LED is on the protection circuit (so motor
shouldn't trip overcurrent protection) and would turn off when the
voltage drops below the predefined low for the 18650 (18mm diameter,
650 long, the most ubiquitous Li-Ion cell there is - it's in
flashlights, laptop batteries, powertool batteries and even powers
Prius and Tesla cars).

D'oh! Of course 18650s aren't 650mm long they are 65mm long. Sorry about
that. :-/
--
Shaun.

"Humans will have advanced a long, long way when religious belief has a cozy
little classification in the DSM*."
David Melville (in r.a.s.f1)
(*Diagnostic and Statistical Manual of Mental Disorders)

 

[~misfit~]

Once upon a time on usenet Computer Nerd Kev wrote:

~misfit~ <shaun.at.pukekohe@gmail.com> wrote:
Once upon a time on usenet Computer Nerd Kev wrote:
And before you redefine your understanding of capacitor theory, the
capacitor in those diagrams should be connected with its negative to
the battery negative, not to the motor positive. eg.
_._
CAP | |
_____| |_________ + _________ | |
-------- + | | |+ *__|_____| | | C |
|______|__________*\ ON | |---| E |
MOTOR | | POWER- \ | CONTROL | | L |
| | SWITCH *OFF | MODULE |---| L |
|------------------------------|_________| | |
-------- - - |___|

Sorry about that.

Thanks for taking the time to help and sorry it's been so long since
I visited here.

I was thinking of something along the lines of:


LED ~120R _This is a bridge,
|\ | ______ / not a join
----| >|--|______|- / _._
| |/ | | / + _________ | |
-------- + | *__|____| | | C |
|______|__________*\ ON | | |---| E |
MOTOR | POWER SWITCH- \ | | CONTROL | | L |
| *OFF | | MODULE |---| L |
| ----|_________| | | |
-------- ------------------------------------------ |___|

Where the motor positive and LED positive are switched but motor
negative goes straight to the battery bypassing the protection
circuit (which seems to operate on the negative - the battery
positive and load out positive are common).

OK, that's alright if you actually don't mind pulling over 3A from
the Lithium cell. For the breif moment at start-up it probably
wouldn't be too much of a problem (but you might want to check the
technical specs, because I'm no battery expert). The issue might be
if you put too much load on the tool and cause it to stall, then the
full stall current will flow and may damage the cell.

The cell can handle it. Also unlikely to have full stall current for more
than a second if that as it's hand-held. Even 'low draw' 18650s can handle
5A continuous draw (although it will shorten their lives somewhat) and the
higher ones as much as 20A.

You've also left out the second diode in series with the LED, that I
recommended to get the low voltage warning to trigger closer to 3V.

I know there's a page about the correct discharge cut-off voltages for
Lithium-Ion bateries at this website:
http://batteryuniversity.com/
But the bookmark is on another computer and I can't find it in the
index.

OK, just tried looking on other computer, written notes, forehead.
Can't
find that bookmark anywhere, perhaps it doesn't exist, I'm sure it
does though. Argh, try another website.

I wouldn't mind a slightly higher trigger voltage to extend cell life but
it's a trade-off with storage capacity. As it is I've got so many usable
salvaged cells that they're likely to die of old age (even stored at ~40%
charged in the fridge) before I find uses for them all.

Cheers, and thanks again for the input.
--
Shaun.

"Humans will have advanced a long, long way when religious belief has a cozy
little classification in the DSM*."
David Melville (in r.a.s.f1)
(*Diagnostic and Statistical Manual of Mental Disorders)

 

[Computer Nerd Kev]

~misfit~ <shaun.at.pukekohe@gmail.com> wrote:

I wouldn't mind a slightly higher trigger voltage to extend cell life but
it's a trade-off with storage capacity. As it is I've got so many usable
salvaged cells that they're likely to die of old age (even stored at ~40%
charged in the fridge) before I find uses for them all.

One of the things it said on the page was that 95% of the energy is exhausted
from the cell when it reaches 3V. I actually did finally find the page
yesterday (they'd changed it on me so I didn't realise when I found it), but
of course I bookmarked it on another computer and now I don't have time to
find it again... It's in the index under some fairly obvious name.

--
__ __
#_ < |\| |< _#

 

[~misfit~]

Once upon a time on usenet Computer Nerd Kev wrote:

~misfit~ <shaun.at.pukekohe@gmail.com> wrote:

I wouldn't mind a slightly higher trigger voltage to extend cell
life but it's a trade-off with storage capacity. As it is I've got
so many usable salvaged cells that they're likely to die of old age
(even stored at ~40% charged in the fridge) before I find uses for
them all.

One of the things it said on the page was that 95% of the energy is
exhausted from the cell when it reaches 3V. I actually did finally
find the page yesterday (they'd changed it on me so I didn't realise
when I found it), but of course I bookmarked it on another computer
and now I don't have time to find it again... It's in the index under
some fairly obvious name.

Yes, you're right - it was the other end that I was thinking of (~4.2v). I'd
be quite happy to stop draining my cells at 3v. Trouble is almost all
control modules I can find pull them down to around 2.6v.

Cheers,
--
Shaun.

"Humans will have advanced a long, long way when religious belief has a cozy
little classification in the DSM*."
David Melville (in r.a.s.f1)
(*Diagnostic and Statistical Manual of Mental Disorders)