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From: Roger Johansson
Subject: Re: Reducing contact resistance for low volt use?
Date: Thu, 12 Dec 2002 09:47:16 +0100
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jmuchow@SPAMMENOTcamlight.com (John Muchow) wrote:
>OK, I think I understand this. :-)
The zero resistance idea is similar to the idea of using a power
supply to draw current out of the battery, but I see no reason to
force current through the battery, that does not give any useful
What you want to know is the ability of the battery to deliver current
by its own power, at very low resistance values as load.
So you can take Mike's idea, but use some regulator to make sure it is
only the force of the battery itself which is pushing current, so the
power supply should only compensate for outer resistance you do not
want to influence the battery.
>I would take the voltage across the shunt and compare it to another
>voltage I have set up that equals the voltage that should be across
>the shunt for the current level I want. If the shunt voltage is too
>low, I would raise the power supply voltage. If it's too high, I'd
>lower the supply voltage.
I did not consider manual regulation of the power supply, it is
doubtful if you can regulate it fast enough to remove all influence,
negative or positive, on the decharching process.
It would be much better to make that electronically.
>I have a MOSFET variable load. I guess I could set the gate voltage
>to give a resistance that would be right for the current level I
>wanted (assuming no connection resistances) and just let the power
>supply provide the extra voltage needed to push that current through
>the MOSFET load, any extraneous resistances, and on through the shunt
>so I can measure the shunt voltage and keep adjusting the supply
>voltage. As the battery voltage lowers, I would keep raising the
>power supply voltage to compensate.
We should try to find a way to do that regulation automatically, to
get accurate values of the capacity of only the battery to push
current through the extremely small, or zero load.
But I think you have understood the method in general.
>Some random thoughts:
>- I need to make sure that the power supply can provide enough voltage
>to make sure that 100A can flow when the battery is at its lowest
>voltage (I'll assume zero volts).
>- Do I need to have the load the MOSFET's provide become variable
It would be better to use a minimal load and change the voltage to
compensate the load, it needs less power, to begin with.
>based on anything? It seems that I don't since the power supply
>varies to compensate for any resistances in the circuit. The MOSFET's
>become a fixed (but easily adjustable) load.
>There would be some calibration involved though.
We need electronic regulation to make it really useful.
The outer resistance needs to be zero during the whole process, and
that means tracking any unbalance and compensating for it fast.
>I could not use the power supply and keep the constant-current
>MOSFET's that vary the current through them based on the voltage
>across a shunt (using an amplifier and reference voltage) and just
>have 2 or 3 cells discharging in series. I realized (hopefully
This would not work as I see it. because the batteries would influence
The discharge curves for the individual batteries would need to be
identical to make this method useful.
Think of a simple example:
Battery A gives 100A to begin with, and 20 A after time T, when
Battery B gives 90A to begin with, and 10A after the same time T, when
Now what do you think would be the resulting curve of them working
together in series?
A completely new situation. The combined measurement would say very
little about each battery's performance alone.
Putting batteries in series is completely useless and would give very
misleading results as far as I can understand.
The individual discharge curves are different in Amps, Voltage, and
Time. The series method assumes incorrectly that only the voltage
factor would be different between the two discharge curves.
But all three factors will be different for each individual battery.
Back to the zero resistance circuit:
Let's see now, we can measure voltage and current in the outer circuit
in every moment, and from these values compute the resistance in that
circuit in every moment, now we only need to keep it constant by
varying the power supply voltage, and then we can also keep it zero.
Well, my brain ran out of steam again, I hope somebody else can take
over from there maybe.
The computations I mention above should be done by analog or digital
circuitry in real time, of course.
Using a microcontroller feels like cheating, there should be some
analog way to do it.
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