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'[EE] Battery Capacity'
2008\07\04@201250 by

People have said to me already that a battery's "capacity" depends on
what current you're drawing from it, and that's why you can't put a
concrete figure on it as simple as "This battery has 760 milliwatt hours".

Is the reason that it depends on the current you're drawing from it to
do solely with the internal resistance of the battery? If so, would it
not be best to make a statement such as "This battery has 760 milliwatt
hours and has an internal resistance of 5 ohms", instead of giving a
list of different capacities for different currents?

If it's to do with more than the internal resistance though then please
let me know! :-D

Tomás Ó hÉilidhe wrote:
> People have said to me already that a battery's "capacity" depends on
> what current you're drawing from it, and that's why you can't put a
> concrete figure on it as simple as "This battery has 760 milliwatt hours".
>
> Is the reason that it depends on the current you're drawing from it to
> do solely with the internal resistance of the battery? If so, would it
> not be best to make a statement such as "This battery has 760 milliwatt
> hours and has an internal resistance of 5 ohms", instead of giving a
> list of different capacities for different currents?
>
> If it's to do with more than the internal resistance though then please
> let me know! :-D
>
>
The internal resistance changes with the current draw.

Rolf
> you can't put a concrete figure on it as simple as "This battery has
> 760 milliwatt hours"

Manufacturers specify a drain current that equates to the mAh
capacity of a cell. 1A might flatten a battery in an hour. 1uA would
probably take a lot longer than 1,000,000 hours, but most batteries
have a shelf-life less than 100,000 hours

> would it not be best to make a statement such as "This battery
> has 760 milliwatt hours and has an internal resistance of 5 ohms",
> instead of giving a list of different capacities for different currents?

Because internal resistance goes up as the battery gets older. mAh
is a pretty good average indicator of capacity

You build your circuit or estimate consumption, and choose the
battery you need based on discharge curves. If you want to use a
particular battery then you may have to alter the circuit or battery
life expectations. How energy content is expressed is irrelevant if
all batteries are rated in the same way, and they are - mAh

>> would it not be best to make a statement such as "This battery
>> has 760 milliwatt hours and has an internal resistance of 5 ohms",

No, it doesn't work that way.  if it was only an internal resistance,
it wouldn't affect capacity, only maximum current draw.  The
reduction in capacity is due to chemical effects (in ability to
recombine chemical byproducts of the electricity-producing reactions
at the full rate of production, for example) that aren't easily
modeled by purely electrical characteristics.

BillW

Tomás Ó hÉilidhe wrote:
> People have said to me already that a battery's "capacity" depends on
> what current you're drawing from it, and that's why you can't put a
> concrete figure on it as simple as "This battery has 760 milliwatt
> hours".
>
> Is the reason that it depends on the current you're drawing from it to
> do solely with the internal resistance of the battery?

Think about it.  If a perfect battery with 0 internal resistance was rated
for 1AH and you put 10ohms in series with it, how many AH can the battery
and resistor deliver to the load?

********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000.

William "Chops" Westfield wrote:
> No, it doesn't work that way.  if it was only an internal resistance,
> it wouldn't affect capacity, only maximum current draw.

Well if the internal resistance is high, and if the load is heavy, then
you'll get a non-negligible amount of power being expended across the
internal resistance. That will be energy that never makes it into the
load, so it will affect the capacity as far as the load's concerned.

> Well if the internal resistance is high, and if the load is heavy, then
> you'll get a non-negligible amount of power being expended across
> the internal resistance. That will be energy that never makes it into
> the load, so it will affect the capacity as far as the load's concerned

It's all relative. Short out a fully-charged NiCd D-cell sometime. Short
out a tractor battery with a garden fork until we got sworn at and chased
off by the man next door.....

Olin Lathrop wrote:

> Think about it.  If a perfect battery with 0 internal resistance was rated
> for 1AH and you put 10ohms in series with it, how many AH can the battery
> and resistor deliver to the load?
>
>
> ********************************************************************
> Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
> (978) 742-9014.  Gold level PIC consultants since 2000.

Could you calculate it for me? I'm too lazy to do it myself.

--
Martin
On Fri, 4 Jul 2008 22:47:30 -0700
"William \"Chops\" Westfield" <westfwmac.com> wrote:

>
> >> would it not be best to make a statement such as "This battery
> >> has 760 milliwatt hours and has an internal resistance of 5 ohms",
>
> No, it doesn't work that way.  if it was only an internal resistance,
> it wouldn't affect capacity, only maximum current draw.  The
> reduction in capacity is due to chemical effects (in ability to
> recombine chemical byproducts of the electricity-producing reactions
> at the full rate of production, for example) that aren't easily
> modeled by purely electrical characteristics.
>
> BillW

Exactly... Check, just as an example (no endorsement):

http://www.pilasduracell.com.ar/oem/Pdf/new/MN2400_US_CT.pdf

On the first page, it shows the discharge with several different loads,
but the graph you're interested in, is on the second page, "Typical
discharge performance". At 50 mA, the mA capacity doesn't even reach
1.2Ah (1200 mAh).

DC internal resistance increases with discharge, as with all batteries,
and just determines terminal voltage.

John

Olin Lathrop wrote:
> Tomás Ó hÉilidhe wrote:
>
>> Is the reason that it depends on the current you're drawing from it to
>> do solely with the internal resistance of the battery?
>>
>
> Think about it.  If a perfect battery with 0 internal resistance was rated
> for 1AH and you put 10ohms in series with it, how many AH can the battery
> and resistor deliver to the load?
>

Depends on the resistance/impedance of the load. But if I have my device
and I know it has an "average resistance" of 100 ohms, then I can do the
maths to work it out.

Jinx wrote:
>> Well if the internal resistance is high, and if the load is heavy, then
>> you'll get a non-negligible amount of power being expended across
>> the internal resistance. That will be energy that never makes it into
>> the load, so it will affect the capacity as far as the load's concerned
>>
>
> It's all relative. Short out a fully-charged NiCd D-cell sometime. Short
> out a tractor battery with a garden fork until we got sworn at and chased
> off by the man next door.....

I'm putting a clutch in my car at the moment so I'm keeping the battery
stored in the boot. There the other day, just for the craic, I
short-circuited it using a metal rod. Sadly thought the battery was dead
so I didn't get fireworks :-(

Anyone know what the internal resistance of a car battery is?

Now we know why you are "Jinx"     :) :) :)

Jinx wrote:
> ... Short out a tractor battery with a garden fork until we got sworn at and chased off by the man next door.....
>
>
Olin's point is that the current flow through the internal "ideal
battery" is the same as that coming out of the physical battery,
regardless of whatever series resistance you place inside the battery.

Batteries are very complex devices to model. What's going on inside
are chemical reactions which depend on the ability of ions in solution
to move around to parts of the electrodes which have not yet reacted
with the electrolyte or other electrode. In addition, if the reaction
proceeds faster than the diffusion rates allow, then it will grind to
a halt quickly until you allow it time to recover.

So, in other words, no, the reduction in capacity is not just because
of internal resistance. The term "capacity" usually refers to the
current integrated over time, NOT the energy.

The effect of discharge rate on capacity is called the Peukert Effect
in Lead Acid batteries (I don't know to what extent other batteries
have this issue or what the effect is called in those cases). It is
due to what I was talking about above - the formation of a layer of
insulating material around grains of electrode material during fast
discharges.

With a Lead Acid battery rated for, say, 10AH at a 1 amp discharge
rate, if you try to discharge it at 10 amps, you will probably get
something like 63% of the 1amp rate capacity (so the battery will run
for only 0.63 hours instead of 1 hour. However, in order to get there,
you will also have to allow the battery voltage to drop a lot more
than usual because of the high current and high internal resistance,
which means that you get even LESS than 63% of the energy out.

HOWEVER, if you then allow this "dead" battery to sit open-circuit, it
will recover some of its capacity as electrolyte slowly seeps into the
electrode grains and then you can draw more AH out of it.

If you Google for Peukert Effect you will find some really interesting stuff.

Sean

On Sat, Jul 5, 2008 at 12:25 PM, Tomás Ó hÉilidhe <toelavabit.com> wrote:
{Quote hidden}

>
The internal resistance is really a misnomer in such a situation as
various mechanisms within the chemical reactions limit the maximum
current. However, for a car battery it would easily be something like
2000 amps if fully charged.

I would recommend that you NOT try this again. If you do, at least
wear safety glasses. It is possibly (although unlikely) to kill
yourself this way as the battery produces hydrogen and oxygen gasses
while in operation and the sparks from shorting it out can easily set
of a rather large explosion, sending bits of battery and liquid
battery acid at you. This can easily blind you permanently.

A better way to try such an (admittedly fun) experiment would be to
rig some way to short out the battery while you are, say, 10 or 20
meters away. This could be some kind of contraption for moving the
metal rod by a string or you could also attach heavy-duty cable to the
battery terminals and short it, say, 10 meters away from the battery.
Still beware of the bright flash you will get - you don't want to be
looking right at it.

Sean

On Sat, Jul 5, 2008 at 12:32 PM, Tomás Ó hÉilidhe <toelavabit.com> wrote:
>
>

>
> Anyone know what the internal resistance of a car battery is?
>
>
Tomás Ó hÉilidhe wrote:
>> Think about it.  If a perfect battery with 0 internal resistance was
>> rated for 1AH and you put 10ohms in series with it, how many AH can
>> the battery and resistor deliver to the load?
>
> Depends on the resistance/impedance of the load.

No, it doesn't.  That was the point of the mental exercise.  You were
supposed to realize that the internal resistance of the battery has no
bearing at all on the amp-hours it can deliver.

> But if I have my
> device and I know it has an "average resistance" of 100 ohms, then I
> can do the maths to work it out.

OK, go ahead.

********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000.

Tomás Ó hÉilidhe wrote:
> Olin Lathrop wrote:
>
>>> Think about it.  If a perfect battery with 0 internal resistance was rated
>>> for 1AH and you put 10ohms in series with it, how many AH can the battery
>>> and resistor deliver to the load?
>>>
>>>
> Depends on the resistance/impedance of the load. But if I have my device
> and I know it has an "average resistance" of 100 ohms, then I can do the
> maths to work it out.

OK so let's say our battery is as follows:
Voltage = 3 V
Capacity = 1 Ahr = 3 Whr
Internal resistance = none (it's the perfect battery)

And our circuit is as follows:
Resistor in series = 10 ohms
Resistance of load = 100 ohms

The total energy in the battery is 3 Whr   or   10.8 kJ.

Looking at the ratio of load resistance to "series resistance", we can
see that ten elevenths of the battery's energy will be expended in the
load, while one eleventh will be expended in the "series resistor".

Therefore, we've got a total of (10.8 / 11 * 10) for the actual load,
which is 9.818 kJ (or  2.727 Whr).

The voltage across the load will be (3 / 11 * 10), which is equal to
2.727 V.

Therefore the Ahr for the battery, (i.e the amount of coulombs it has
available), is 1 Ahr (but this is figure is misleading if you don't know
what voltage you have across the load).

So scrap my original idea of Ahr and internal resistance. I'd prefer Whr
and internal resistance. But anyway this is irrelevant now because I've
been told that a battery's internal resistance rises as more current is
drawn from it.

Tomás Ó hÉilidhe wrote:
>     Capacity = 1 Ahr = 3 Whr

There is one of your misconceptions.  Amp-hours do not equal Watt-hours.
The first is a measure of charge, the second a measure of energy.  You can
use the AH rating and the battery voltage to get a rough idea of energy, but
there are even more factors you have to take into account to get energy
actually delivered to a load than the charge delivered to the load.

> Therefore the Ahr for the battery, (i.e the amount of coulombs it has
> available), is 1 Ahr

Exactly.  Note that the internal or external resistance is irrelevant to
this calculation.

> (but this is figure is misleading if you don't
> know
> what voltage you have across the load).

It's not misleading if you know what AH are and how to use them.  You have
to understand that there is more to battery ratings than AH and that it only
gives you a rough guide to the total energy the battery can deliver.  AH is
still a useful rating because it is the closest simple value that describes
the battery independent of the external circuit, although it's not totally
independent and you have to understand how.

********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000.
I found out at an early age that high currents can weld the wire you're casually drawing sparks with onto the car battery terminal ... then I found out that wire gets red hot very quickly when lots of amps are passed through it. Further discoveries included (in rapid succession): the smell of burning thumb, just what a huge quantity of smoke you can make from a few yards of bell wire insulation, that having a pair of pliers to hand in such a situation is very useful, how long it takes for the stink of burned thumb and insulation to clear, that my neighbours are alert and knock on the door when they see lots of smoke coming from a bedroom window ... and that mothers take a very dim view of such activities :)

Anyway to drag this back on topic, I more recently discovered that the capacities you see marked on Lithium Ion rechargeable packs or cells are usually about 20% higher than you can realistically expect to achieve. The "industry standard" is to rate them as if they were to be discharged down to the sort of low voltage that actually destroys the cell ( likely to lead to "venting with flame"). Any sensible safety circuitry will cut off the discharge at a point that avoids this mischief. So yeah, you can get the rated capacity ... only the once though!

Cheers,
Robin.

{Original Message removed}

Sean Breheny wrote:
> However, for a car battery it would easily be something like
> 2000 amps if fully charged.
>

Deadly! :-D

> A better way to try such an (admittedly fun) experiment would be to
> rig some way to short out the battery while you are, say, 10 or 20
> meters away. This could be some kind of contraption for moving the
> metal rod by a string or you could also attach heavy-duty cable to the
> battery terminals and short it, say, 10 meters away from the battery.
> Still beware of the bright flash you will get - you don't want to be
> looking right at it.

If I'm standing 10 metres away with heavy-duty cable, will I not get
sparks at the point where I touch the crocodile clips together?

Something I'm curious about: What causes the sparks?

In the case of spark plugs, I realise that the voltage is so high that
it makes the air conduct, but since a car battery is only 12 V, I don't
see what causes the sparks.

seriously.....

ever heard of positive and negative electrons?

ever seen lightning?

I think my delete key is going to be worn out....

--- On Mon, 7/7/08, Tomás Ó hÉilidhe <toelavabit.com> wrote:

{Quote hidden}

> -
Lightning is plasma. These sparks are bits of molten metal and such...

Ever see a wrench dropped across the battery posts? ;) Most older mechanics
will remember the first time they did that. They won't have done it twice most
likely. ;)

alan smith wrote:
{Quote hidden}

>
Positive electrons, lol. :-D

Although Tomas's question is valid, and Dr. Skip is correct in saying that
when you see sparks at 12V, they're bits of metal melted/ignited by the high
current.

{Original Message removed}
Tomás Ó hÉilidhe wrote:
> If I'm standing 10 metres away with heavy-duty cable, will I not get
> sparks at the point where I touch the crocodile clips together?
>
> Something I'm curious about: What causes the sparks?

Actually they are not sparks, although they look similar.  A spark is
glowing plasma caused by electricity flowing thru the air itself.  When you
touch two pieces of metal together at 12V, the metal at the point of contact
gets vaporized due to the high currents the battery can deliver.  This can
also weld the two pieces of metal together.

********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000.
Hi Tomas,

Yes, you will get sparks, but they will not be close to the explosive
gasses which may be leaking out of the battery.

The sparks can still be bad to look at (just from sheer brightness and
also from UV light they produce, just like an arc welder).

Actually, the question on what causes the sparks is a valid one. You
are correct that 12V is not nearly enough to break down any reasonable
amount of air.

What's happening is something called "inductive kick". The length of
wire (even if it is only the distance between the battery terminals)
has some inductance. When current is flowing, energy gets stored in
this inductance in the form of a magnetic field. When you try to
interrupt the current flow, the current cannot stop instantaneously
because of the energy stored in the magnetic field. As the field
"collapses", it forces some current to continue flowing for a short
time (perhaps microseconds) by creating a large voltage.

When you touch two wires together (or pull them apart), you get switch
"bounce" - that is, the wires actually connect and disconnect many
times within the first few milliseconds of contact. During this time
the inductance causes voltage spikes which are enough to cause arcing
across the tiny air gap. The voltage doesn't really need to get very
high because the air gap at that point might only be 0.1 millimeter
(it takes about 3kV per millimeter so 300 volts is enough in this
case).

Making the inductance greater (longer wire, thinner wire, more loop
area of the entire loop of wire, or wrapping the wire into multiple
turns) or increasing the current both make the arc bigger.

This effect is actually the same thing that happens in a boost
converting switching power supply. Current is drawn from the low
voltage side for a large fraction of the converter's cycle, put
through an inductor, and then that inductor is connected to the output
for a short time. Since the output draws less current than the input,
the voltage rises as the inductor tries to keep the current constant
(the extra current goes into the output capacitors). This is a more
controlled case because at no point is the inductor ever totally
disconnected - just being switched between a low impedance and a high
impedance.

Sean

On Mon, Jul 7, 2008 at 5:13 PM, Tomás Ó hÉilidhe <toelavabit.com> wrote:
>
> If I'm standing 10 metres away with heavy-duty cable, will I not get
> sparks at the point where I touch the crocodile clips together?
>
> Something I'm curious about: What causes the sparks?
>
> In the case of spark plugs, I realise that the voltage is so high that
> it makes the air conduct, but since a car battery is only 12 V, I don't
> see what causes the sparks.
>
>
On Mon, Jul 7, 2008 at 6:07 PM, Sean Breheny <shb7cornell.edu> wrote:

> This effect is actually the same thing that happens in a boost
> converting switching power supply. Current is drawn from the low
> voltage side for a large fraction of the converter's cycle, put
> through an inductor, and then that inductor is connected to the output
> for a short time. Since the output draws less current than the input,
> the voltage rises as the inductor tries to keep the current constant
> (the extra current goes into the output capacitors). This is a more
> controlled case because at no point is the inductor ever totally
> disconnected - just being switched between a low impedance and a high
> impedance.

One small addendum - the fact that the current is drawn from the input
for a longer time than it is supplied to the output side is what makes
the circuit obey energy conservation. Capacitors are added on the
input and output to allow the source and load to see a fairly constant
current but the switching inside the converter does spend more time
"building" the magnetic field in the inductor than it does dumping it
into the output.

>
> Sean
>
>
alan smith wrote:
> ever heard of positive and negative electrons?

Um, you might want to check your physics.  I'd like to see a jar of them
positive electrons.

********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000.
I'm embarrassed that I didn't think of the metal melting - that is
probably happening. However, I still think that inductive effects come
into play as well.

Sean

On Mon, Jul 7, 2008 at 6:09 PM, Olin Lathrop <olin_piclistembedinc.com> wrote:
{Quote hidden}

>
They're the ones that are a lot happier than the negative ones... :)

Olin Lathrop wrote:
> alan smith wrote:
>> ever heard of positive and negative electrons?
>
> Um, you might want to check your physics.  I'd like to see a jar of them
> positive electrons.
>
>
> ********************************************************************
> Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
> (978) 742-9014.  Gold level PIC consultants since 2000.
I believe that you will find that the 'spark' from an inductive load will
produce a blue arc. This is similar to lightening in that the potential
increases to the point of breaking down the gap. It's high voltage built across
a gap.

However, the orange bits are heated metal. This is from resistive heating and
motion. Usually, when you draw a wire on a battery post, since the wire is
cylindrical, you're probably hitting it with a 'corner', which is much less
contact area than even the wire size (although at these amperages it probably
doesn't matter). It melts quickly and blows off, as you press more wire into it.

One can make the orange sparks by brushing a wire across a 1.5v cell too...
It's current driven.

Sean Breheny wrote:
> I'm embarrassed that I didn't think of the metal melting - that is
> probably happening. However, I still think that inductive effects come
> into play as well.
>
> Sean
>
> On Mon, Jul 7, 2008 at 6:09 PM, Olin Lathrop <olin_piclistembedinc.com> wrote:
>
On Mon, 2008-07-07 at 18:32 -0400, Olin Lathrop wrote:
> alan smith wrote:
> > ever heard of positive and negative electrons?
>
> Um, you might want to check your physics.

Technically they are called positrons, but "positive electrons" are a
common name for them (or at least a common description of them). They
are identical to electrons except for having the exact opposite charge.

> I'd like to see a jar of them
> positive electrons.

Since positrons are anti-matter, they would quickly annihilate the
closest piece of "normal" matter.

In order to have them in a jar (I'm assuming the jar is made out of
"normal" matter) you'd have to ensure the positrons stay away from the
jar, and obviously the inside of the jar would have to be a total
vaccuum.

TTYL

> -----Original Message-----
> From: piclist-bouncesmit.edu On Behalf Of Olin Lathrop
> Sent: Monday, July 07, 2008 6:09 PM
>
> Actually they are not sparks, although they look similar.  A spark is
> glowing plasma caused by electricity flowing thru the air itself.

Actually the way others are using the word spark agrees with its primary
definition. Your usage is a secondary definition added much later.

Paul Hutch

>  When you
> touch two pieces of metal together at 12V, the metal at the point
> of contact
> gets vaporized due to the high currents the battery can deliver.  This can
> also weld the two pieces of metal together.

>> ever heard of positive and negative electrons?
>
>Um, you might want to check your physics.  I'd like to see a
>jar of them positive electrons.

Ain't they what they call 'positrons' ???
Alan B. Pearce wrote:
>>> ever heard of positive and negative electrons?
>>
>> Um, you might want to check your physics.  I'd like to see a
>> jar of them positive electrons.
>
> Ain't they what they call 'positrons' ???

The original question was in relation to jumper cables and a ordinary car
battery.

********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000.
> Now we know why you are "Jinx"     :) :) :)

Oh, you don't know the half of it (undecided on emoticon)

Sometimes I think a down-town roof-top and a .303 would
be a pleasant way to spend the afternoon......

Dr Skip wrote:
> Lightning is plasma. These sparks are bits of molten metal and such...
>
> Ever see a wrench dropped across the battery posts? ;) Most older mechanics
> will remember the first time they did that. They won't have done it twice most
> likely. ;)
>
>
>

As they reminisce through their cloudy eyes.
-
Martin

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