| Author |
Message |
mike
Guest
|
 Posted:
Sun Mar 20, 2005 6:55 pm Post subject:
Re: Device to open a circuit when a voltage is released? |
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Active8 wrote:
| Quote: | On Sat, 19 Mar 2005 17:30:06 -0800, mike wrote:
Active8 wrote:
On Sat, 19 Mar 2005 08:34:05 -0800, mike wrote:
Active8 wrote:
On Fri, 18 Mar 2005 14:58:25 -0800, mike wrote:
Bruce W.1 wrote:
snip
It's much safer and less stressful on the cells if you use some kind of
slope termination with a secondary safety based on temperature.
mike
Slope termination... You mean throttle back the charging rate
(current)?
I don't understand what you mean.
I meant that I didn't understand what you meant :)
What I meant was...
Depending on the cell chemistry, terminate when dV/dt is appropriate
with safety termination on temperature limit.
Oh! *That* slope.
I'd think that when a cell of any chemistry is almost done charging,
that a continued constant current charge would stress it and isn't
that why it heats up? IIRC the internal resistance of the cell drops
as it's voltage increases. I think that is what leads to the temp
rise.
Suggest you do some more research.
I said "I'd think...".
I plan to whenever I get around to that deep cycle charger I'll
need. No need to worry about NiCds now.
I'm going to put each separate thought in it's own paragraph so you
don't misunderstand me. Read them as if the others weren't there.
|
Thanks, I need all the help I can get.
| Quote: |
In my experience, a properly charged cell with dV/dt termination does
NOT heat much.
Nothing I said was intended to suggest that it would. I opined that
a constant current all the way up to the end might heat it up.
I said that IIRC the internal resistance of a cell decreases as it
reached full charge.
I said I think that a current through a small valued resistor causes
more heat (heat = work = energy = P*T) than the same current through
a larger value resistor.
|
Mr. Ohm and those starting from his equations are all turning over in
their graves about now.
| Quote: |
I don't have any charge curves handy AFAIK but I'm guessing (hell, I
can almost picture curves I've seen in the past) that when it's
almost up to full charge, dV/dt is less than when it started
charging.
|
I'm not guessing, my battery charger plots the curves every time it charges.
For NiCds dV/dt peaks before it levels off then turns negative.
| Quote: |
I take it you mean it's always under constant current charge for the
dV/dt in question.
|
That's a reasonable assumption.
| Quote: |
The only reason for the temperature cutoff is SAFETY
when something goes wrong.
My cordless drill batts have a thermistor inside. I thought that was
the *only* thing that stops the charge.
|
It is. There are many cheap chargers that do just that. They charge
the thing until it gets hot enough to turn off. Great way to cook
batteries to death. I've rebuilt about a dozen drill packs. Usually,
all the cells have vented.
FWIW, this technique works reasonably if you
fully discharge the pack...but not too fully to reverse a cell...before
charging. During the full recharge time, there's enough heat built up
to bring the thermal mass up to cutoff without serious damage. Problem
is when you put a 70% charged pack on charge. The internal temperature
comes up too fast for the external sensor to trip. Cell vents.
A key phrase seems to have been snipped from my earlier post"
"In my experience, a properly charged cell..."
Temperature only cutoff is not a properly designed charger.
Today, you'd use that only if you cared about the lowest possible
product sales price, and you made a bundle off replacement batteries.
| Quote: |
With -dV/dt cutoff, the most likely
Now I have a minus sign to ponder.
|
Google 0-delta-v or -delta-v or some permutation of spaces and dashes.
| Quote: |
combination of factors is 1) low charge current, 2)mismatched initial
state of charge so they peak at different times and the total curve of
the sum of the series string never goes negative.
Is that the -dV/dt above? The cell or string starts uncharging?
|
Nope, still charging. There's lots of stuff on the web on proper
charging of NiCd cells.
Google 0-delta-v or -delta-v or some permutation of spaces and dashes.
Or start at the cadex website.
| Quote: |
THEN it gets hot.
Another problem situation is with a poorly designed charger that depends
on none of the cells being shorted.
I'd like to know how to detect and/or deal with that situation. I'd
guess that I'd have to determine that the string never reached it's
expected voltage at the expected time.
|
No problem. Make the charge current independent of pack voltage.
Charge at the proper current relative to cell capacity. (there's
argument over exactly what this is. I usually use C"
Terminate on voltage slope rather than voltage.
Have three safety cutoffs
1)over temperature
2)over voltage
3)over charge I x T > 1.2 x pack capacity
And apply a trickle charge if the pack voltage is below some minimum
voltage. Only start fast charge if the pack voltage is within some
min/max range and the temperature is within some min/max range.
The only reason my charger needs to know the voltage is so it can
properly normalize the graph so every pack looks the same on screen
and I can immediately recognize a problem.
mike
--
Return address is VALID but some sites block emails
with links. Delete this sig when replying.
..
Wanted, PCMCIA SCSI Card for HP m820 CDRW.
FS 500MHz Tek DSOscilloscope TDS540 Make Offer
Wanted, 12.1" LCD for Gateway Solo 5300. Samsung LT121SU-121
Bunch of stuff For Sale and Wanted at the link below.
MAKE THE OBVIOUS CHANGES TO THE LINK
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Active8
Guest
|
| Posted:
Sun Mar 20, 2005 8:49 pm Post subject:
Re: Device to open a circuit when a voltage is released? |
|
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On Sun, 20 Mar 2005 04:55:43 -0800, mike wrote:
| Quote: | Active8 wrote:
snip
I'm going to put each separate thought in it's own paragraph so you
don't misunderstand me. Read them as if the others weren't there.
Thanks, I need all the help I can get.
|
I respect your modesty. Commo's a bitch sometimes.
| Quote: |
In my experience, a properly charged cell with dV/dt termination does
NOT heat much.
Nothing I said was intended to suggest that it would. I opined that
a constant current all the way up to the end might heat it up.
I said that IIRC the internal resistance of a cell decreases as it
reached full charge.
I said I think that a current through a small valued resistor causes
more heat (heat = work = energy = P*T) than the same current through
a larger value resistor.
Mr. Ohm and those starting from his equations are all turning over in
their graves about now.
|
And yet again my heads up my ass[-]backwards! :( Caffeine, sugar and
a long day or two? Day/night - whatever it was. "A V across..." That
kills that hypothesis.
| Quote: |
I don't have any charge curves handy AFAIK but I'm guessing (hell, I
can almost picture curves I've seen in the past) that when it's
almost up to full charge, dV/dt is less than when it started
charging.
I'm not guessing, my battery charger plots the curves every time it charges.
For NiCds dV/dt peaks before it levels off then turns negative.
|
Got some returns on dV/dt and dT/dt. It sounds similar to a slight
peak at the cutoff of a LP filter. From the recombination. I
probably saw that in the curves from the enerhell (sic) quidbook,
wherever that is at the moment. Last I checked, the online thing
sucked.
"With the NiMH battery the voltage depression is smaller, and harder
to detect than with the NiCad battery"
| Quote: |
I take it you mean it's always under constant current charge for the
dV/dt in question.
That's a reasonable assumption.
The only reason for the temperature cutoff is SAFETY
when something goes wrong.
My cordless drill batts have a thermistor inside. I thought that was
the *only* thing that stops the charge.
It is. There are many cheap chargers that do just that. They charge
the thing until it gets hot enough to turn off. Great way to cook
batteries to death. I've rebuilt about a dozen drill packs. Usually,
all the cells have vented.
FWIW, this technique works reasonably if you
fully discharge the pack...but not too fully to reverse a cell...before
charging. During the full recharge time, there's enough heat built up
to bring the thermal mass up to cutoff without serious damage. Problem
is when you put a 70% charged pack on charge. The internal temperature
comes up too fast for the external sensor to trip. Cell vents.
A key phrase seems to have been snipped from my earlier post"
"In my experience, a properly charged cell..."
|
"with dV/dt termination does
NOT heat much."
It's up there below my first comment. We'd have lost nothing if it
were snipped, I got it the first time. Didn't mean to suggest
otherwise.
| Quote: | Temperature only cutoff is not a properly designed charger.
Today, you'd use that only if you cared about the lowest possible
product sales price, and you made a bundle off replacement batteries.
With -dV/dt cutoff, the most likely
Now I have a minus sign to ponder.
Google 0-delta-v or -delta-v or some permutation of spaces and dashes.
combination of factors is 1) low charge current, 2)mismatched initial
state of charge so they peak at different times and the total curve of
the sum of the series string never goes negative.
Is that the -dV/dt above? The cell or string starts uncharging?
Nope, still charging. There's lots of stuff on the web on proper
charging of NiCd cells.
|
I read only a bit of what I've stumbled across years ago re: R/C.
| Quote: | Google 0-delta-v or -delta-v or some permutation of spaces and dashes.
Or start at the cadex website.
THEN it gets hot.
Another problem situation is with a poorly designed charger that depends
on none of the cells being shorted.
I'd like to know how to detect and/or deal with that situation. I'd
guess that I'd have to determine that the string never reached it's
expected voltage at the expected time.
No problem. Make the charge current independent of pack voltage.
Charge at the proper current relative to cell capacity. (there's
argument over exactly what this is. I usually use C"
|
The quote is not a typo?
| Quote: | Terminate on voltage slope rather than voltage.
Have three safety cutoffs
1)over temperature
2)over voltage
3)over charge I x T > 1.2 x pack capacity
And apply a trickle charge if the pack voltage is below some minimum
voltage. Only start fast charge if the pack voltage is within some
min/max range and the temperature is within some min/max range.
The only reason my charger needs to know the voltage is so it can
properly normalize the graph so every pack looks the same on screen
and I can immediately recognize a problem.
If the day comes that I need to really get into it, I can add that |
logging into one of my progs. How many uses for a DSP/FIR/IIR with
time domain/freq domain and plotting prog?
What is this? :
http://www.repairfaq.org/ELE/F_Battery_info.html
seach find: "Type ANSI" (no quotes)
What's this C5maH ? 5*C in maH ?
Hey! I think we did well on commo this time. And with me wired on
caffeine.
--
Best Regards,
Mike |
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mike
Guest
|
| Posted:
Mon Mar 21, 2005 6:10 am Post subject:
Re: Device to open a circuit when a voltage is released? |
|
|
Active8 wrote:
snip
It's right under the chart:
Note that this is the capacity when the battery is discharged over 5
hours time period.
Remember that cell capacities are 3-4X what they were when that was written.
mike
| Quote: |
Hey! I think we did well on commo this time. And with me wired on
caffeine.
|
--
Return address is VALID but some sites block emails
with links. Delete this sig when replying.
..
Wanted, PCMCIA SCSI Card for HP m820 CDRW.
FS 500MHz Tek DSOscilloscope TDS540 Make Offer
Wanted, 12.1" LCD for Gateway Solo 5300. Samsung LT121SU-121
Bunch of stuff For Sale and Wanted at the link below.
MAKE THE OBVIOUS CHANGES TO THE LINK
ht<removethis>tp://www.geocities.com/SiliconValley/Monitor/4710/
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Active8
Guest
|
| Posted:
Mon Mar 21, 2005 9:51 pm Post subject:
Re: Device to open a circuit when a voltage is released? |
|
|
On Sun, 20 Mar 2005 17:49:45 -0800, mike wrote:
| Quote: | Active8 wrote:
snip
What is this? :
http://www.repairfaq.org/ELE/F_Battery_info.html
seach find: "Type ANSI" (no quotes)
What's this C5maH ? 5*C in maH ?
It's right under the chart:
Note that this is the capacity when the battery is discharged over 5
hours time period.
|
Oh. I read the lines below, duh. At any discharge rate? Is it a
permanent loss of capacity?
| Quote: | Remember that cell capacities are 3-4X what they were when that was written.
|
About that dV/dt thing:
"Another reason memory effect is a myth since all the consumer
charger's I've seen actually overcharge until there is a slight
voltage drop (due to an increase in resistance from the formation of
larger cadmium hydroxide particules that cause contact loss). It's
because consumer chargers actually overcharge that you have to give
the battery a deep discharge from time to time. It has nothing to do
with memory."
Does this mean that they go further into voltage depression than you
would allow them using the technique you recommend?
--
Best Regards,
Mike |
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