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Subject: Re: Reducing contact resistance for low volt use?
Date: Wed, 11 Dec 2002 02:14:50 -0800
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John Muchow wrote:
> >>>Perhaps you could use an active discharge circuit or place a massive
> >>>power supply with current limiting in series with your cell. No need
> >>>to worry about contact and wire resistance. It's a great way to
> >>>reverse charge your cell, just like series connected cells. Beware of
> >>>exploding cells.
> I may be misunderstanding you but the circuit I have will be active (I
> think). It's a constant-current discharger using linearly-biased
> MOSFETs as the load.
> Hmm...I don't know enough to understand how to use a power supply to
> sink 100A from the cell. If it's a way to avoid having to worry about
> contact and wire resistance, I'm all ears regarding any sources of
> Wait a sec...are you saying that I can place the cell in series with
> the output of a power supply (and thus increasing it's open circuit
> voltage by 1.2V and then shorting the series connected sources
> together? The 1.2V from the cell will discharge through the output
> stage of the power supply (if it can handle the heat)? Or should the
> cell be put in "backwards" so the cell's POS terminal is connected to
> the power supply's POS terminal?
> If the cell will not start reverse charging until it's completely
> discharged, that's OK. It will get so hot above 40A discharge current
> that we'll never be able to use for anything else anyway....a
> destructive test this definitely is. We have a box-withn-a-box test
> jug to hold the cell while discharging. To date, no cell has done
> more than just "pop" a little, with zero projectile electrolyte. But,
> we're not taking any chances. Thanks for the warning.
> John Muchow
You can build a negative zero volts current
limited power supply that can source -100A.
Let me try that again. It's a negative power
supply, but the nominal voltage is zero.
It goes into current limit when the voltage is
You can probably get there with a simple current
sink if you use a good op-amp and a bunch
of parallel fets. You'll need multiple wires and
contact points anyway.
You could always test two cells a time. Hook them
up in reverse with a 24 milliohm resistor.
But let's back up a bit.
There are ways to counteract any amount of
resistance in the measurement...BUT
At 100A, each milliohm of contact resistance will
dissipate 10 Watts. Where's that heat
gonna go? Doesn't take very many milliohms to
smoke whatever you use.
Your whole effective load resistance is only 12
I didn't catch the physical geometry of the cells.
Any cell capable of 100A should have something
better than simple pressure on the ends.
I don't think you'll get there with any kind of
I've been experimenting with spot welding battery
tabs. Not sure I could get to 100
amps with a spot weld.
You'll need a Kelvin connection to get any
Wonder if there's another way to accomplish what
you really need?
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