# [EVDL] How to measure A123 resistance



## EVDL List (Jul 27, 2007)

I am not sure, but I think the way to do it would be to apply a
sinusoidal signal symmetric to the resting voltage of the battery to
it, and measure the current and voltage delivered. If you alternately
charge and discharge the battery like this, it would stay at the same
SOC, so voltage change should be be from internal resistance (either
mechanical or chemical). You'll need a good signal generator and a
oscilloscope to do this. I am not sure what frequency range you would
need to get a good result.

Z



> Osmo S. <[email protected]> wrote:
> > I=B4m going to do some tests with A123 to see how 10 of them behave in
> > parallel. But first I thought I would measure the resistance of the
> > cells to see if it is related to the behaviour of the cells. I haven
> ...


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## EVDL List (Jul 27, 2007)

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## EVDL List (Jul 27, 2007)

> Bill Dube <[email protected]> wrote:
> > The standard technique is to apply a 400 Hz
> > signal and measure the phase shift in the current versus the voltage.
> >
> ...


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## EVDL List (Jul 27, 2007)

Bill,

Do you know if A123 has correlation charts between impedance
at 400Hz and internal resistance measured "normal" way
(delta V divided by delta I). R_int on DC is what really relevant
really as you charge/discharge, the voltage drops on active
R_int, not reactive component of impedance.

In other words what portion of the impedance value reactive
component has? Do they know? It seems that 400Hz AC measurement
is in common use because this is standard equipment available
for such use, not because this gives you more accurate
(and relevant) value than U and I deltas (slope).

Phase shift between I and U at some frequency only tells you
that R_int has reactive component, either inductive or capacitive
depending on the shift direction. But this component is not
relevant when you load the battery to the traction system
which consumes DC current, though not pure DC. Reactive component
does not "rob" power. It just may have direct correlation
with real R_int and that's what is valuable to know.

Finally, I wonder if R_int on charge is the same as R_int
on discharge (of course measured at the same two current values
to get the slope, just in and out of the battery).

--
Victor
'91 ACRX - something different



> Bill Dube wrote:
> > The standard technique is to apply a 400 Hz
> > signal and measure the phase shift in the current versus the voltage.
> >
> ...


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## EVDL List (Jul 27, 2007)

Bill Dube kirjoitti 27.12.2007 kello 23.39:

> If you do a DC load test, you must do
> two different loads and then compute the slope of
> the line plotted on the I versus V graph.


Do you mean V versus TIME graph like here:
http://www.batteryuniversity.com/parttwo-41.htm

If not, can you show an example of a I versus V graph?

>
> Be sure to measure voltage on a separate
> contact on the terminals from the contact you use to draw the current.

What=B4s the reason for this? Maybe temperature rise on the current =

drawing contact?

Thanks,
Osmo

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## EVDL List (Jul 27, 2007)

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## EVDL List (Jul 27, 2007)

>
> Bill Dube kirjoitti 27.12.2007 kello 23.39:
>
>> If you do a DC load test, you must do
>> two different loads and then compute the slope of
>> the line plotted on the I versus V graph.
>
>
> Do you mean V versus TIME graph like here:
> http://www.batteryuniversity.com/parttwo-41.htm

That is a I versus V graph, the time component is just to insure the
voltage has stabilize. If you look at the top of the chart you will see
the formula for calculating resistance (Rdc) and it doesn't include time.

>
> If not, can you show an example of a I versus V graph?
>
>>
>> Be sure to measure voltage on a separate
>> contact on the terminals from the contact you use to draw the current.
>
> What=B4s the reason for this? Maybe temperature rise on the current
> drawing contact?

Because the contact will have resistance, ohms law tells us that this
resistance will incure a voltage drop relative to the current, this
voltage drop will effect your readings.
If you measure with the same contact then you will be measuring the
contact resistance as well as the batteries resistance and won't be able
to determine how much of the resistance comes from each point.

Because the meter draws so little current, the voltage drop across it's
contact will be negligible and won't effect your reading. So just make
sure the meter touches a different point on the terminals from the load,
i.e if the load is on clamps around the terminals, touch (firmly) the
meter leads to the top of the terminals.

>
> Thanks,
> Osmo
>
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## EVDL List (Jul 27, 2007)

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## EVDL List (Jul 27, 2007)

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## EVDL List (Jul 27, 2007)

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## EVDL List (Jul 27, 2007)

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## EVDL List (Jul 27, 2007)

> If you measure with the same contact then you will be measuring the
> contact resistance as well as the batteries resistance and won't be 
> able
> to determine how much of the resistance comes from each point.

Ok I understand this, I just thought maybe there are other (not so 
obvious) reasons for it.

I thought I would use a screw to connect both wires (with lugs on 
them) on the same tab hole of the A123 cell. I just want to compare 
the cells, not to get their absolute resistances. Of course the 
result will then include the tab and lug resistance also. But I 
suppose I can exclude the load contact resistance by putting the 
meter lug against the tab and the load connector on top of it?

I think screw is better than soldering if you want consistent 
results, at least with my soldering skills...

Osmo


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## EVDL List (Jul 27, 2007)

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## EVDL List (Jul 27, 2007)

Measuring deltas between two loads *close to your application
loads* yields not only most meaningful result, but is actually
correct way. If you plot the line that way you will see that it
does not cross U axis where E is. But if you do only one
load measurement, the other point to make plot is at zero amps
(and E instead of U), and the slope of it will be different.

Also you will find that this line is not a line but a curve,
so you get different R_int depending on the load,

You will find it highly dependent on the temp and SOC

You might also find it different for charging and discharging
even with the same current values, but I'm not certain
about this one - different manufacturers and chemistries
have no or conflicting data.

Victor



> Seppo Lindborg wrote:
> > Do you really need to measure two different load currents?
> >
> > Shouldn't this work: Consider the battery consisting of two series-
> ...


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## EVDL List (Jul 27, 2007)

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