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From: jmuchow@SPAMMENOTcamlight.com (John Muchow)
Subject: Re: Reducing contact resistance for low volt use?
Date: Thu, 12 Dec 2002 11:53:22 GMT
Organization: MindSpring Enterprises
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X-Server-Date: 12 Dec 2002 11:52:53 GMT
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>>>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
Ahh...I'm realizing that I didn't nderstand nearly as much as I
thought. I don't understand the distinction...unless it means that
the power supply's voltage would never exceed the level of the voltage
which is dropped across the contact/connection resistances.
>>>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.
Wow, I think I'll need a circuit example to understand this. It's
just whooshing straight over my head. My ignorance at work
>>>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.
Oops, I meant electronically, via op-amp.
>>>It would be better to use a minimal load and change the voltage to
>>>compensate the load, it needs less power, to begin with.
So I would keep the MOSFET's and adjust their Rds-on "manually" (by
setting a gate voltage) before discharging to set a desired fixed
"ideal" current flow? The power supply would compensate for any
additional resistance by varying its voltage?
Or, when you say "compensate the load", do you mean vary the
resistance of the load (by varying gate voltage) to compensate for
reduced current flow?
I wish I could do some research *anywhere* to get a better idea of
what I need to do. I've smoked half my neurons doing web searches but
I think I don't know what I'm really looking for.
>>>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.
Darn good point. Unless the cells were matched very closely, this
wouldn't work. Most of the cells track very closely but I couldn't
tell you if it was +/-1% or +/-10% without doing some serious
testing....something the new discharger would be great for. :-)
>>>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.
I think I need to understand how to do this manually in order to
understand how to get it all going in real-time electronically.
1) Get or build a programmable 0V-5V (maybe only 2V?), 100+ amp power
2) Place the battery and power supply in series.
3) Place the MOSFET load somewhere in that battery/power supply loop.
4) Adjust the MOSFET gate voltages for an ideal desired current flow.
5) Turn on the power supply to its lowest voltage and measure the
voltage and current.
6) Adjust the power supply voltage higher if not enough current is
flowing due to contact/connection resistances. Only compensate for
these other resistances, not the resistance of the MOSFET load (that
would essentially create a short-circuit, even if it was possible).
7) Continually monitor the current level and adjust the power supply
voltage to compensate for the dropping cell voltage. Since the cell
will only drop 1.2V max., the power supply only needs to be rated for
1.2V plus the total contact/connection/cable resistances....plus some
8) Keep doing this until the cell voltage has dropped to the desired
9) Design and build a circuit that does all this in real-time.
It's #6 and #7 that I'm not at all sure about. And that's only if I'm
right about using the fixed (but adjustable before discharging) MOSFET
load and my power supply voltage and current specs are reasonable.
>>>Well, my brain ran out of steam again, I hope somebody else can take
>>>over from there maybe.
I sincerely appreciate the time you've taken, thanks again. I wish I
knew enough to make better use of it...very frustrating! :-)
>>>Using a microcontroller feels like cheating, there should be some
>>>analog way to do it.
I totally agree, especially since I don't want to start from scratch
with a micro....haven't used one in years and I'm not looking forward
to re-familiarizing myself with assembler again!
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