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From: jmuchow@SPAMMENOTcamlight.com (John Muchow)
Subject: Re: Reducing contact resistance for low volt use?
Date: Thu, 12 Dec 2002 05:13:42 GMT
Organization: MindSpring Enterprises
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>>>Let's wait for the real engineers to wake up and think about it, they
>>>might find out how to do it. :-)
>>>Maybe if we put our head together we can solve this.
>>>If we see it as three devices in series, battery, a shunt, a power
>>>supply. The voltage over the shunt is amplified and used to control
>>>the power supply.
>>>It is set so that the power supply is giving a voltage which is enough
>>>to push the current through the power supply plus the shunt.
>>>Assume the current is X to begin with (or zero).
>>>This gives a zero voltage over the shunt.
>>>The battery raises this to another value when it is connected and the
>>>series circuit is completed.
>>>The power supply compensates that increase, so the outer resistance is
>>>This allows the battery to raise the current a little more.
>>>That is compensated too.
>>>Until the current has become what the battery is capable of pushing
>>>through itself, with an outer circuit which is effectively a zero
>>>resistance, or some other low value of our choice.
>>>Hmmm.. sorry, my brain is out of steam here, but maybe someone else
>>>can build further on the line of thought, or something similar.
>>>As you said, others have touched the idea already.
OK, I think I understand this. :-)
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 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.
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
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.
As Mike asked about...
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
correctly) that any interconnect resistance between the extra cells
doesn't matter because I should still have at least enough voltage
left over to ensure 100A flows when the MOSFET's are biased fully on
(so I'm only dealing with the Rds-on of the MOSFET's and any
connection and contact resistances).
Seems like I need to balance the cost and time involved in getting a
mondo power supply in the circuit vs. using 2 or 3 cells every time I
want to test a new cell type/size. Big up front expense vs. $3-$10
extra every time I do a test.
Time to look for a power supply (probably 5V, 100A+) and do a little
math to figure out costs.
Thanks for your time (and any help in pointing out any remaining
glaring errors of mine)!
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