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'[EE]: battery backup strategy'
2000\11\26@065146 by Graham

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This is my first attempt at a battery backup strategy. = no experience

The requirement is to continue powering a 16f84 + a counter chip + two
relative encoders, total around 3 to 50 ma at 5v, depending on activity, for
'a few seconds', then grab the count from the counter and move it to
internal eeprom, then power everything down until main supply
returns.....for a price+simplicity

My proposed solution is:

use ICL7673 with two external PNP transistors, as per their data sheet
circuit to maintain a supply to the whole system.

I added a switching FET at the backup battery supply. Use an input pin on
PIC to monitor main supply line, use another one as output to hold the FET
on so backup battery is available to the 7673.

If the main supply line goes lower than battery (by 50mv) the 7673 will
switch....next check (or interupt driven) the pic sees the main supply line
now low....it then goes into a 'preserve critical data in eeprom' routine(ie
demands a final count from the counter chip)....after which it powers down
the FET control line in the battery supply and cuts it's own throat. Next
power up it goes back to monitor main supply and holds FET on to make
available battery back up ....(loop)

will it fly ??.....gotchas ???....experiences ??...

I am concerned that the switch over time of the icl7673 is too long (50us)

better way ???...(I can see more complex and much more expensive ways)

(I added a simple resistor+diode to trickle the batteries)

Graham




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2000\11\26@101505 by Olin Lathrop

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> The requirement is to continue powering a 16f84 + a counter chip + two
> relative encoders, total around 3 to 50 ma at 5v, depending on activity,
for
> 'a few seconds', then grab the count from the counter and move it to
> internal eeprom, then power everything down until main supply
> returns.....for a price+simplicity

Your switched battery idea seems workable, but I would also investigate
using a large capacitor instead since you only need a relatively small
amount of energy once every time main power goes off.

For example, a 20,000uF cap will droop 5V after delivering 50mA for 2
seconds.  Therefore, if you kept a 20,000uF cap at 5V over your regulator
drop out voltage, it would power your circuit until it would shut down
anyway.  That probably means the cap needs to be kept at around 11 volts
normally.

I have seen "memory backup" capacitors which are physically small with very
high capacitance, from about .05F to 1F (yes, one Farad).  The ones I've
seen were only
a little over 5 volts, but higher voltages may be available.  Note that a 1F
cap droops only 100mV if drained at 50mA for 2 seconds.  This may be all you
need connected directly to your 5V supply if you can guarantee that the
regulator won't draw current when reversed.  Another approach would be to
use a chopper to produce the 5V from a range of DC voltages like 2.5 to 5V,
just like many battery circuits.  That would allow more droop from the
storage cap before the 5V went away.


*****************************************************************
Olin Lathrop, embedded systems consultant in Devens Massachusetts
(978) 772-3129, .....olinKILLspamspam@spam@embedinc.com, http://www.embedinc.com

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2000\11\26@103623 by staff

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Olin Lathrop wrote:
{Quote hidden}

Just a thought, since neither the PIC, the counter or encoders
will be that voltage sensitive, how about just using a large cap
and saving status before the volts drop from 5v to 3.5v (example)?
Exactly how long will it take to get the last data sample and
save status??

I too have seen the 1 Farad caps, called "Goldcaps" they are common
in VCRs about 12 years old, they were used for the channel memory.
I have a few types in my collection. Most of the newer VCRs have
0.047F and 0.1F 3.3v caps, these are not as good. The very newest
VCRs all seem to have flash memory.
:o)
-Roman

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2000\11\26@115009 by mike

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On Sun, 26 Nov 2000 06:51:46 -0500, you wrote:

>This is my first attempt at a battery backup strategy. = no experience
>
>The requirement is to continue powering a 16f84 + a counter chip + two
>relative encoders, total around 3 to 50 ma at 5v, depending on activity, for
>'a few seconds', then grab the count from the counter and move it to
>internal eeprom, then power everything down until main supply
>returns.....for a price+simplicity
If the PIC can switch off the encoders as soon as the supply
disappears, this will give you a lot more breathing space. Also you
could put the PIC in sleep mode whan you complete the eeprom write, so
the 1-10mS write time happens when the PIC is asleep, to save more
power (I think this works with '84 internal eeprom - it would
certainly work with an external one).

You may want to include some precautions to deal with the eeprom write
being currupted if the supply disappears too quickly, e.g. use 2
alternate data areas with checksums so if a write fails you have the
previous data intact - obviously whether you can do this  depends on
the application.
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2000\11\26@125106 by Graham

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Olin, Roman...thanks for the rapid reactions....

I'm off to search for 'supercaps', I was aware of them from prior consumer
electronic projects, but am unaware of the math to calculate capacity....I
had only seen them used for simple memory retention in high volume consumer
products before.

based on your math Olin then a 1F supercap would get me there...I'll
research availability and try to find the equations I need , anyone know
useful sites ??
I think Panasonic was the supplier we used in the past....

experience with strategy for stopping the regulator sinking instead of
sourcing anyone ?? (simple diode at it's output perhaps ? and eat the
voltage drop in main supply, or jack up the regulator with another diode in
the gnd leg to get it back)

my last read time is hard to specify fully, as the system can be integrated
into other people mechanics that I have no control over, normally in the
100's of ms range, but if they have significant 'over run' in their
mechanics it could stretch out to say 2 seconds and in all cases I should
allow another 2 seconds to catch hysterisis/settlement in the encoder
coupling system...so  5 seconds to be safe, to my last read, and 10 seconds
power capacity to be safe again, but as roman points out I can drift down to
3.5v or so over that time.

Graham







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2000\11\26@211911 by Bill Westfield

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For what it's worth, I think a LARGE percentage of fancy charging,
switching, battery-back-up circuits have been replaced by much simpler
circuits that use a primary lithium coin cell.  It seems (amoung other
things) that a NiCd battery has a shorter lifespan when carefully charged
and maintained than a Li Coin cell has when simply ignored.  If your
expected requirement for backup involves relative few outages of relatively
brief duration, then something like the cr2032 used on many PC motherboards
is likely to suit things just fine (when was the last time you had a PC
motherboard battery go bad?)  At 225mAh, the cell can probably provide your
3 to 50mAh for at least several hours of use.  That's a LOT of outages if
the circuit only needs to operate for a few seconds at each outage.

(cisco's routers used to contain battery-backup up ram (32k worth or so) for
storing configuration.  The initial products used a multibus card that had
three * 3.6V NiCd setup, with a lot of associated and expensive electronics.
Rather shortly thereafter, we started using the Dallas NVRAM modules with
built-in Li batteries and supposed ~10y life.  I don't think we've gotten
any complaints about the batteries going bad.  (OTOH, a NVRam circuit has
highly predictable current requirements - things are more complicated when
you have a full microcontroller circuit you need to power.))

BillW

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2000\11\27@031713 by Vasile Surducan

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On Mon, 27 Nov 2000, Roman Black wrote:

> Olin Lathrop wrote:
> > I have seen "memory backup" capacitors which are physically small with very
> > high capacitance, from about .05F to 1F (yes, one Farad).  The ones I've
> > *****************************************************************
> > Olin Lathrop, embedded systems consultant in Devens Massachusetts
>
>
> I too have seen the 1 Farad caps, called "Goldcaps" they are common
> in VCRs about 12 years old, they were used for the channel memory.
> I have a few types in my collection. Most of the newer VCRs have
> 0.047F and 0.1F 3.3v caps, these are not as good. The very newest
> VCRs all seem to have flash memory.

> -Roman
>
 Olin, Roman, I don't want to be impolite but instead of just reading
1F or .047F and say: oh my God one farad capacitor! have you the
curiosity to measure the real value of these capacitors?
Because 1F is *not* one farad!
Best regards, Vasile

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2000\11\27@034902 by staff

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Vasile Surducan wrote:
{Quote hidden}

Hi Vasile, they are special parts and very cleary marked.
They are used instead of NiCd batteries in many appliances.
I have never measured the capacitance, but I did run a
hi-intensity led from a 1F capacitor for over a minute.
-Roman

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2000\11\27@040810 by Vasile Surducan

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On Mon, 27 Nov 2000, Roman Black wrote:

>
> Hi Vasile, they are special parts and very cleary marked.
> They are used instead of NiCd batteries in many appliances.
> I have never measured the capacitance, but I did run a
> hi-intensity led from a 1F capacitor for over a minute.
> -Roman

 OK then ! I've asked you because many times I've found
capacitors marked 1F or .1F and measured it was .1F = 0.1 micro farad
which show just another marking solution.
Vasile

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2000\11\27@120332 by Olin Lathrop

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>   Olin, Roman, I don't want to be impolite but instead of just reading
> 1F or .047F and say: oh my God one farad capacitor! have you the
> curiosity to measure the real value of these capacitors?

I don't have any here right now, so I can't do that.  I never actually
measured them, but I do remember getting the expected performance in the
circuit.

> Because 1F is *not* one farad!

Actually it is.  However a 1 farad cap may not be 1 farad I suppose if it
got damaged, was bad in the first place, used outside spec range, etc.  I'm
not sure what your point is.  Are you claiming the manufacturers are
stretching the truth?


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2000\11\27@120336 by Olin Lathrop

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>   OK then ! I've asked you because many times I've found
> capacitors marked 1F or .1F and measured it was .1F = 0.1 micro farad
> which show just another marking solution.

Don't you think a factor of 1 million in the capacitance would be obvious
just by looking at the thing!?  Imagine how big a 1F 50V capacitor would
have to be and what it would look like.  I think I'd notice when I was
instead expecting one of those mylar caps maybe 10 x 20 mm.


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Olin Lathrop, embedded systems consultant in Devens Massachusetts
(978) 772-3129, olinEraseMEspam.....embedinc.com, http://www.embedinc.com

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2000\11\27@123654 by Robert Rolf
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The 1F capacitor isn't really a capacitor as we know it.
It's a chemical capacitor which means that it has timing constraints.
I.E. you won't get 1Amp for 1 Second, but will likely get a milliamp for
1000seconds. Think of it as a nearly perfect rechargable battery.
That's why it can be made so small.


Olin Lathrop wrote:
{Quote hidden}

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2000\11\27@124901 by Bill Westfield

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   Don't you think a factor of 1 million in the capacitance would be obvious
   just by looking at the thing!?  Imagine how big a 1F 50V capacitor would
   have to be and what it would look like.

I would have expected such a thing, but the "real" 1F (2.5-5V) capacitors
seem to be about the same size as a 1000uF/35V "normal aluminum
electrolytic."  Or smaller Very depressing, from a physics education point
of view (how many of us heard "of course you could make a 1 Farad
capacitor, but it would be HUGE - perhaps the size of a car" or somesuch.
Sigh.  Now you've got to start talking about surface areas of fractal
solids and such.)  Digikey's 1F capacitors are about 20mm diameter by 5mm,
for instance.  The "0.047F" caps are the ones that really surprise you,
since that'd be a standard value in uF.  They should'a picked a different
"target" value (0.048F?) just to trick people into looking more closely...

BillW

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2000\11\27@131359 by Bill Westfield

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   The 1F capacitor isn't really a capacitor as we know it.  It's a chemical
   capacitor which means that it has timing constraints.  I.E. you won't get
   1Amp for 1 Second, but will likely get a milliamp for 1000seconds.

This was true briefly, but no longer (well, some of the smaller and cheaper
high-value caps are still like this, of course, but it's no longer a
technology limitation.)

See for instance http://www.powercache.com/ , where the smallest capacitor
(4g, 17x24x5mm) is 2F 2.5V and is designed to provide a peak current of 5
AMPS!!!  Apparently one of the major uses for such things is in parallel
with wimpy batteries, to provide the sort of peak current surges that are
not healthy for batteries...  Their largest capacitor is 2500 F - it's about
160mm long (7 inches) and designed to provide 400 A (normally.  Peak current
is 1800A.)  (Can you say "holy expletive deleted" kids?)

BillW

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2000\11\27@134109 by Don Hyde

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I've done one with similar requirements for saving information into EEROM.

If you are powered from an AC line, then there must already be enough C in
the power supply to carry you over from one half-cycle to the next, or 8.3
mS at 60Hz.  That is already probably long enough to grab a counter and
stuff it into an EEROM.  Increasing the C by a factor of 3 or 4 (or 10) to
give you some room to play will probably not cost very much at all.

The hard part is that you must recognize the loss of AC power as soon as
possible.  I did this by conditioning the AC half-cycles from a rectifier
into near-square-wave pulses clipped to GND and VCC, and sampling this
signal (connected to a digital I/O line) every couple of milliseconds.  If I
see no pulses for 16mS (one cycle at 60Hz), then I'm pretty certain the
power has failed (real world AC power can go away for half a cycle due to
someone turning on a big motor sometimes), but that I still have at least
20mS of usable power.

Making a part you already have (the capacitor) biggger will have a much
smaller effect on your reliability than will adding several components
including an electrochemical one (battery) that has a higher intrinsic
failure rate.  Besides it will be smaller and cheaper.

> {Original Message removed}

2000\11\27@153228 by Peter L. Peres

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>supercaps

The supercaps are not rated for that kind of current. They show the
nominal capacity at drains in the uA range (a few of those). On account of
the high internal resistance you will not likely be able to draw 50mA from
them for any length of time (not at the nominal voltage at least).

A large electrolytic may work however, but its price and bulk may make it
loose against two lithium coin cells in series imho.

Special capacitors for this kind of use used to exist, but I haven't seen
any for a while now. They were some sort of mixture between electrolytic
capacitor and battery, in an elco case (usually with screw terminals). I
think that they were sealed lead acid or such technology (not sure). Used
to charge quickly drawing a lot of amps and then act like a battery
keeping the output at 6V I think, until they ran down.

Peter

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2000\11\27@153232 by Peter L. Peres

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Hi Graham, imho you are overdoing it a little. All you need is a single
switch if you already have a backup battery. A shottky diode network
(2 of them) should power your circuit both from the battery and from the
regulated power. A pic pin will sense the incoming power going down. You
can use a R divider to sense the voltage before the regulator. The switch
will be a MOSFET (logic level) and will cut off the supply to the
peripherals (leaving the PIC connected to the battery through the backup
system). When the PIC senses power down it will do what it does and then
turn the MOSFET off, then go to sleep. You can arrange for it to come back
when the power comes back, by implementing a wakeup on portb change or
such. You can also arrange for the watchdog to run while sleeping and the
PIC to briefly wake up every 2 seconds or so, check conditions, and sleep
again. This is the way I'd do it anyway.

Peter

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2000\11\28@115405 by Bill Westfield

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   The supercaps seem to be more like a lead acid battery than
   a normal electrolytic. They are normally sealed with black
   epoxy, not a rubber plug. The electrolyte is very corrosive
   and after about 10 years it corrodes past one or both legs
   and leaves crusty yellow salts on the bottom of the cap.
       :
   So expect 10 years full time use maximum from these parts.

Perhaps.  What was the expected lifetime of most electrolytics we were
talking about not too long ago?  Not that great, either :-(

There's a reasonable summary covering some of the high-discharge rate
capacitors at:

 http://www.techweb.com/se/directlink.cgi?EET20000508S0079

(aside from screwing up the specs on AVX's entries.  God, I hope no one
starts using mF to actually mean milliFarads!!!)

One of the interesting things that shows up in the article is that there
seem to be quite a few different technologies in use.  The AVX "BestCaps"
apparently have a solid electrolyte, for instance.  The Powerstore cells are
based on carbon aerogels (while most of the others are based on "activated"
carbon?)  Tadarian already offers a lithium battery with integrated supercap
for 'pulsed' power applications.

I guess it's been about 20 years since the very-high capacitance capacitors
started to appear, but it looks like significant progress is still being
made.  I'll bet the capabilities continue to improve for some time, and the
line between batteries and capacitors continues to blur...

BillW

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2000\11\28@125126 by Simon Nield

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>God, I hope no one starts using mF to actually mean milliFarads!!!

p = pico  = x10^-12
n = nano  = x10^-9
u = micro = x10^-6
m = milli = x10^-3
k = kilo  = x10^3
M = mega  = x10^6
G = giga  = x10^9

what else would you want to suggest mF means ?

Regards,
Simon

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2000\11\28@130414 by Bill Westfield

face picon face
> >God, I hope no one starts using mF to actually mean milliFarads!!!

> what else would you want to suggest mF means ?

You got the emphasis wrong.  Of cours mF *means* milliFarads.  However,
english typerwriters and ascii both lack a greek mu character, and older
publications (and part markings, IIRC) have used "mF" as micro-farads (after
all, milliFarads was impossibly large and useless!)  *using* mF to mean
millifarads is a bad idea.

You can see the result in the article I mentioned.  In one sentence, mF
is used "correctly" for millifarads, and later in the next paragraph
someone has incorrectly "expanded" the abreviation to microfarads...

BillW

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2000\11\28@150104 by Chris Carr

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Snip, Snip

> One of the interesting things that shows up in the article is that there
> seem to be quite a few different technologies in use.  The AVX "BestCaps"
> apparently have a solid electrolyte, for instance.  The Powerstore cells
are
> based on carbon aerogels (while most of the others are based on
"activated"
> carbon?)  Tadarian already offers a lithium battery with integrated
supercap
> for 'pulsed' power applications.
>
> I guess it's been about 20 years since the very-high capacitance
capacitors
> started to appear, but it looks like significant progress is still being
> made.  I'll bet the capabilities continue to improve for some time, and
the
> line between batteries and capacitors continues to blur...
>
> BillW
>
Since this string started I have been looking for a recent article in one of
the
tech mags....with no success. I believe it was "Electronics Weekly",
however, in an over-enthusiastic cleanout I find they have been consigned to
File 13.

Anyway the thing that stuck in my braincell was the remark that supercaps
were now being developed for use in alternative car technologies instead of
conventional batteries due to the superior characteristics of the new
devices.

Previously I had considered these devices as only capable of providing their
stored energy at low currents. This delusion was recently (and rapidly)
revised when I was recovering useful components from a terminally defective
VCR. I accidentally placed the soldering iron across the leads of a 1.2 volt
1F capacitor, the BANG and spark was sufficient to cause a very robust
reaction.

After that painful experience (I threw myself off the stool I was sitting
on - backwards) I now treat these devices with respect. And these can be
over 10 years old.

Chris

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2000\11\29@003451 by Peter L. Peres

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>I would have expected such a thing, but the "real" 1F (2.5-5V) capacitors
>seem to be about the same size as a 1000uF/35V "normal aluminum
>electrolytic."  Or smaller Very depressing, from a physics education point
>of view (how many of us heard "of course you could make a 1 Farad
>capacitor, but it would be HUGE - perhaps the size of a car" or somesuch.

Robert Rolf is correct, you need to analyze them from the point of view of
batteries, not capacitors. So compare a 1F supercap with a coin cell or
two. The coin cells will win by a factor of at least 10:1 in all respects
except perhaps self discharge.

Peter

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2000\11\29@003459 by Peter L. Peres

picon face
>Olin, Roman, I don't want to be impolite but instead of just reading
>1F or .047F and say: oh my God one farad capacitor! have you the
>curiosity to measure the real value of these capacitors?
>Because 1F is *not* one farad!

I have had the curiosity and it is as it says on the box, i.e. 1F. FYI you
need to measure with a constant current sink (breadboarded opamp circuit
in my case, set to draw 25 uA) as per the manufacturer's indications. See
my previous posting wrt. use of supercaps. At the current level where this
is done you need to disconnect the 20M/V DVM most of the time to avoid
false readings during testing ;-).

Peter

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2000\11\29@003509 by Peter L. Peres

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William, 1800A is not a very high peak current for capacitors. Simple
calculations show that you basically need to short it into a wire barely
longer than the distance between the pins to obtain that current, unless
it is charged to kilovolt range voltages. For example doorknob caps rated
0.01 uF 20kV can supply 5000A peak, easily, into about 0.5 ohms of circuit
resistance (total). This is more credible than 1800A at 5V (which requires
a total load circuit resistance of 5/1800 = 0.0027 ohms - not your average
wire imho - plain screw terminals are more than that). Also plain NiCd and
lead acid batteries can supply that kind of current for several seconds
without too many problems (not that it helps them live longer). Even cheap
D NiCd cells are rated 80 to 120A by the manufacturer.

Peter

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2000\11\29@090344 by Olin Lathrop

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> Robert Rolf is correct, you need to analyze them from the point of view of
> batteries, not capacitors. So compare a 1F supercap with a coin cell or
> two. The coin cells will win by a factor of at least 10:1 in all respects
> except perhaps self discharge.

And surge current!


*****************************************************************
Olin Lathrop, embedded systems consultant in Devens Massachusetts
(978) 772-3129, KILLspamolinspamBeGonespamembedinc.com, http://www.embedinc.com

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'[EE]: battery backup strategy'
2000\12\01@042058 by Peter L. Peres
picon face
>And surge current!

Olin, only the special (i.e. $pecial) supercaps are capable of high
current. The normal ones normally found in consumer equipment and on the
shelves of retailers don't. Please trust me on this, I did some study on
the matter and the conclusion was that NiCd or Lithium cells beat them in
price, volume, and availability. This was not that long ago.

A 1F supercap will appear as a good quality (but inductive) 0.1uF
capacitor when examined with 'normal' C-meter instruments. It reaches 1F
when discharged at under 100uA (5-10uA is better).

Peter

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