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'[EE:] Using two 9V in parallel for greater capacit'
2004\02\16@004206 by Jesse Lackey

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Hello all,

I have a project that involves blinking LEDs that the client would like
to have run for 2 months per battery or battery set.

2 9V wired in parallel would do it and is preferable to 4 AA.

Is this a reasonable thing to do?  Is it dangerous at all if both are
fresh of the same kind?  How about if not?  Would a 5ohm resistor in
series with each before being wired in parallel alleviate any dangers
from users who don't follow directions?  (its a consumer product).  The
current is pretty low so the loss due to a small resistor is not a problem.

Any advice?
Thanks ...
Jesse



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2004\02\16@004206 by Jesse Lackey

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face
Hello all,

I have a project that involves blinking LEDs that the client would like
to have run for 2 months per battery or battery set.

2 9V wired in parallel would do it and is preferable to 4 AA.

Is this a reasonable thing to do?  Is it dangerous at all if both are
fresh of the same kind?  How about if not?  Would a 5ohm resistor in
series with each before being wired in parallel alleviate any dangers
from users who don't follow directions?  (its a consumer product).  The
current is pretty low so the loss due to a small resistor is not a problem.

Any advice?
Thanks ...
Jesse



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2004\02\16@023643 by ahid Sheikh

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I've seen that done before and have wondered why they didn't just use 4
AAs instead. Specially when in that case they were using a linear
regulator after the 9V source.

I would recommend using ORing diodes instead of a resistor though.

Shahid

{Original Message removed}

2004\02\16@023643 by ahid Sheikh

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I've seen that done before and have wondered why they didn't just use 4
AAs instead. Specially when in that case they were using a linear
regulator after the 9V source.

I would recommend using ORing diodes instead of a resistor though.

Shahid

{Original Message removed}

2004\02\16@025758 by Jinx

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> 2 9V wired in parallel would do it and is preferable to 4 AA

Why is it preferable ? You get more bang for your buck from
4xAAs than you do from a 9V. The only place I ever use a 9V
(rechargeable) is in my DMM, because that's all that will fit.
Anything else has either a 2, 4, or 6-pack of AAs

> Is this a reasonable thing to do?  Is it dangerous at all if both are
> fresh of the same kind?  How about if not?

I'm having a guess at this, but I'd have thought the higher of two
unequal batteries would be wasting power driving the internal
resistance of the other. If the lesser of the two is flat it's dead
weight basically and just adds to the load of the survivor. And
I'm not too sure about the chemical effects of trying to recharge
one battery with another. Like June Allyson, you could get some
serious leakage problems

> Would a 5ohm resistor in series with each before being wired
> in parallel alleviate any dangers

There shouldn't be any dangers with small batteries, but any series
component wastes power as heat. Even the voltage drop of a diode.
If you can make it run on 4xAA you'll not be wasting power dropping
that higher voltage of the 9V

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2004\02\16@025758 by Jinx

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> 2 9V wired in parallel would do it and is preferable to 4 AA

Why is it preferable ? You get more bang for your buck from
4xAAs than you do from a 9V. The only place I ever use a 9V
(rechargeable) is in my DMM, because that's all that will fit.
Anything else has either a 2, 4, or 6-pack of AAs

> Is this a reasonable thing to do?  Is it dangerous at all if both are
> fresh of the same kind?  How about if not?

I'm having a guess at this, but I'd have thought the higher of two
unequal batteries would be wasting power driving the internal
resistance of the other. If the lesser of the two is flat it's dead
weight basically and just adds to the load of the survivor. And
I'm not too sure about the chemical effects of trying to recharge
one battery with another. Like June Allyson, you could get some
serious leakage problems

> Would a 5ohm resistor in series with each before being wired
> in parallel alleviate any dangers

There shouldn't be any dangers with small batteries, but any series
component wastes power as heat. Even the voltage drop of a diode.
If you can make it run on 4xAA you'll not be wasting power dropping
that higher voltage of the 9V

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2004\02\16@034248 by Russell McMahon

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> I have a project that involves blinking LEDs that the client would like
> to have run for 2 months per battery or battery set.
>
> 2 9V wired in parallel would do it and is preferable to 4 AA.
>
> Is this a reasonable thing to do?  Is it dangerous at all if both are
> fresh of the same kind?  How about if not?  Would a 5ohm resistor in
> series with each before being wired in parallel alleviate any dangers
> from users who don't follow directions?  (its a consumer product).  The
> current is pretty low so the loss due to a small resistor is not a
problem.


Not "by the book".
Entirely doable. I'd say it was probably always safe *IF* both batteries
were always new.

HOWEVER Changing one battery and not the other - eg replacing one battery at
a time when flat while the other was still in place - WOULD lead to
discharge between batteries. This could produce significant energy losses in
the good battery and heating in both. Probably up to an amp or so worst case
and several watts dissipation.

A sure fire safe way (ie no fires :-) ) , at the loss of some capacity,
would be to place a diode in series with each battery. This would take about
0.6v off the available voltage. For an eg 6 volts endpoint for your
equipment, instead of 9 to 6 volts you'd get 8.4 to 6 volts at the terminals
or a useable battery voltage of 9 to 5.6v. I'd guess maybe 10 capacity loss
depending on acceptable endpoint voltage and current drain.

Using small Schottky diodes would reduce the energy losses. By how much
depends on current characteristics.

Knowing the usage pattern would help greatly in predicting what minimum
series R would make things safe.
eg is this run 24/7 for 2 months or intermittently. What is the average
drain and what is the peak drain (max instantaneous when LEDs on).
Capacitors may make the mean battery current approximate the peak etc.

5 ohms in series with both would give a MAXIMUM current of 9/5 = 2A and far
far less inter-battery in practice.

Adding a resistor in series with ONE battery only would reduce losses and
give similar protection (think about it)

I assume you are specifying Alkaline batteries to achieve this life time. If
not, you should, as you could then use one battery.

If 2 batteries in parallel suit your need the acceptable minimum terminal
voltage must be about 6 volts. If an inverter is not used it could be
considered - quite possibly cost precludes.

AA cells have over FOUR  times the energy capacity of a 9v cell or over
twice the capacity of 2 x 9v cell (PP3 type). Maybe the cost of 2 x battery
snaps versus an AA cell holder etc makes a difference. IF you can get by on
4v endpoint then 4 x AA absolutely walk all over 2 x PP3. At 6v endpoint = 6
AA cells The AA holder becomes larger and annoying but is even more superior
to 2 x 9V. If 2 x 9V giver 2 months operation then 6 x AA give 4 months plus
!!!

Even the dreaded 4 or 6 AAA have equivalent capacity to 2 x 9v PP3 !!!!
( 4 x AAA have less volume than 2 x PP3 and 6 x AAA are perhaps slightly
larger)

4 x AA have slightly more volume than 2 x PP3. If you can live with the 4v
end-point (3.6v to absolute last gasp) of 4 x AA I'd really really try to
use them. Battery snapped holders are available. The 4 month+ life is
presumably attractive.



       Russell McMahon

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2004\02\16@034248 by Russell McMahon
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face
> I have a project that involves blinking LEDs that the client would like
> to have run for 2 months per battery or battery set.
>
> 2 9V wired in parallel would do it and is preferable to 4 AA.
>
> Is this a reasonable thing to do?  Is it dangerous at all if both are
> fresh of the same kind?  How about if not?  Would a 5ohm resistor in
> series with each before being wired in parallel alleviate any dangers
> from users who don't follow directions?  (its a consumer product).  The
> current is pretty low so the loss due to a small resistor is not a
problem.


Not "by the book".
Entirely doable. I'd say it was probably always safe *IF* both batteries
were always new.

HOWEVER Changing one battery and not the other - eg replacing one battery at
a time when flat while the other was still in place - WOULD lead to
discharge between batteries. This could produce significant energy losses in
the good battery and heating in both. Probably up to an amp or so worst case
and several watts dissipation.

A sure fire safe way (ie no fires :-) ) , at the loss of some capacity,
would be to place a diode in series with each battery. This would take about
0.6v off the available voltage. For an eg 6 volts endpoint for your
equipment, instead of 9 to 6 volts you'd get 8.4 to 6 volts at the terminals
or a useable battery voltage of 9 to 5.6v. I'd guess maybe 10 capacity loss
depending on acceptable endpoint voltage and current drain.

Using small Schottky diodes would reduce the energy losses. By how much
depends on current characteristics.

Knowing the usage pattern would help greatly in predicting what minimum
series R would make things safe.
eg is this run 24/7 for 2 months or intermittently. What is the average
drain and what is the peak drain (max instantaneous when LEDs on).
Capacitors may make the mean battery current approximate the peak etc.

5 ohms in series with both would give a MAXIMUM current of 9/5 = 2A and far
far less inter-battery in practice.

Adding a resistor in series with ONE battery only would reduce losses and
give similar protection (think about it)

I assume you are specifying Alkaline batteries to achieve this life time. If
not, you should, as you could then use one battery.

If 2 batteries in parallel suit your need the acceptable minimum terminal
voltage must be about 6 volts. If an inverter is not used it could be
considered - quite possibly cost precludes.

AA cells have over FOUR  times the energy capacity of a 9v cell or over
twice the capacity of 2 x 9v cell (PP3 type). Maybe the cost of 2 x battery
snaps versus an AA cell holder etc makes a difference. IF you can get by on
4v endpoint then 4 x AA absolutely walk all over 2 x PP3. At 6v endpoint = 6
AA cells The AA holder becomes larger and annoying but is even more superior
to 2 x 9V. If 2 x 9V giver 2 months operation then 6 x AA give 4 months plus
!!!

Even the dreaded 4 or 6 AAA have equivalent capacity to 2 x 9v PP3 !!!!
( 4 x AAA have less volume than 2 x PP3 and 6 x AAA are perhaps slightly
larger)

4 x AA have slightly more volume than 2 x PP3. If you can live with the 4v
end-point (3.6v to absolute last gasp) of 4 x AA I'd really really try to
use them. Battery snapped holders are available. The 4 month+ life is
presumably attractive.



       Russell McMahon

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2004\02\16@192644 by Jesse Lackey

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Wow thanks so much everyone for the comments and suggestions.

I need to run @ 5V (well ... 4.75V minimum), and a DC/DC to go from 9V
to 5V is simpler and cheaper than doing something from 4x AAA (no space
for 4x AA) which will be > 5V initially.  2x AA is doable space-wise,
but I will have to do some calculations / cost analysis to see if this
is better than 2x 9V.  The attraction of the 2x 9V is that my client can
decide yes/no on the extra space later, or can tell users they can run
with 1 or 2 9V, their choice.  At the least I want to present two
options and he can decide.

The power consumption is very peak-y, pulses of up to 50mA for <2msec to
run an IR LED, and 10msec PWM of 20mA for a red LED, and some other
stuff that is <5msec and <2mA.

Its a real balancing act all this, power vs. cost vs. physical size vs.
complexity.

Thanks to everyone for for the diode tip.  Duh of course...!  and the
small voltage drop is no problem for the DC/DC.

Best!
Jesse



Russell McMahon wrote:
{Quote hidden}

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and those who don't."

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2004\02\16@192644 by Jesse Lackey

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face
Wow thanks so much everyone for the comments and suggestions.

I need to run @ 5V (well ... 4.75V minimum), and a DC/DC to go from 9V
to 5V is simpler and cheaper than doing something from 4x AAA (no space
for 4x AA) which will be > 5V initially.  2x AA is doable space-wise,
but I will have to do some calculations / cost analysis to see if this
is better than 2x 9V.  The attraction of the 2x 9V is that my client can
decide yes/no on the extra space later, or can tell users they can run
with 1 or 2 9V, their choice.  At the least I want to present two
options and he can decide.

The power consumption is very peak-y, pulses of up to 50mA for <2msec to
run an IR LED, and 10msec PWM of 20mA for a red LED, and some other
stuff that is <5msec and <2mA.

Its a real balancing act all this, power vs. cost vs. physical size vs.
complexity.

Thanks to everyone for for the diode tip.  Duh of course...!  and the
small voltage drop is no problem for the DC/DC.

Best!
Jesse



Russell McMahon wrote:
{Quote hidden}

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and those who don't."

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2004\02\17@003527 by William Jacobs

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Jesse,
I did something similar to this some time ago.  I used a PIC 12C509 with
the LED across an output pin and no resistor.  The program turned the
led on and went to sleep for a 22mS(1 sleep cycle).  Then when it came
out of sleep it would clear the port so the led would be off and I would
enter sleep again for about 100 mS(4 sleep cycles). Then on again for 22
mS and then off for 2.3 S.  The power source was 2 AA batteries and the
life was just about a year.  This produced a definite double blink of
the LED.
bill


Jesse Lackey wrote:
{Quote hidden}

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2004\02\17@003527 by William Jacobs

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Jesse,
I did something similar to this some time ago.  I used a PIC 12C509 with
the LED across an output pin and no resistor.  The program turned the
led on and went to sleep for a 22mS(1 sleep cycle).  Then when it came
out of sleep it would clear the port so the led would be off and I would
enter sleep again for about 100 mS(4 sleep cycles). Then on again for 22
mS and then off for 2.3 S.  The power source was 2 AA batteries and the
life was just about a year.  This produced a definite double blink of
the LED.
bill


Jesse Lackey wrote:
{Quote hidden}

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2004\02\17@172432 by Peter L. Peres

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Why not use a 6V photo lithium battery ? Or 3V with upconverter ?

Peter

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2004\02\17@185116 by Russell McMahon

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> Why not use a 6V photo lithium battery ? Or 3V with upconverter ?

Lithium typically has very little extra amp-hour capacity over an alkaline
for a given size. Main gains are temperature range, discharge rate and shelf
life, at the expense of price. There are of course several lithium
chemistries available and some have rather higher capacities than others. .

Useful comparison table (although it's intended as a catalog page)

       http://www.eurobatteries.com/sitepages/lithcambat.asp



       RM

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2004\02\18@025747 by William Chops Westfield

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On Tuesday, Feb 17, 2004, at 13:23 US/Pacific, Peter L. Peres wrote:

> Why not use a 6V photo lithium battery ? Or 3V with upconverter ?

  Note that there have been recent drastic reductions in prices
on photo lithium batteries, due to their increasingly common use in
things like flashlights.  You can get a CR123A sized lithium battery
for less than $2 from a number of online sources.  For instance, $1.25
from http://www.batterystation.com

Of course, they're still about $9 each from the local drugstore,
which might be a problem for a nominally "consumer" product.

BillW

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