From: paul dove via EV <ev@lists.evdl.org>
>Great then explain (96-72) x .08 = 300 amps.

That should be (96v-72v) / .08ohms = 300 amps. This is just Ohms law. 96v is 
the unloaded battery pack voltage. 72v is the pack voltage under load. 0.08 
ohms is the ESR. So with a load of 300a, the pack voltage drops from 96v to 
72v. Do you agree with that part?

> How will the controller going to see 24 volts?

It doesn't; it sees 72v. 24v is dropped across the internal resistance of the 
battery pack. This voltage drop, and the internal resistance is what makes the 
current 300a.

>> The PWM only controls the output voltage by switching it on and off.
>> The duty cycle then sets the average voltage the motor sees.

Not quite. The motor is an inductive load. It actually sees the full on/off 
voltage switching. The motor voltage is going from (in this case) about 1v to 
73v at the switching frequency of the controller.

The average motor voltage will surprise you. It is *not* the average of the 
PWM. The motor has back EMF; i.e. it *generates* a voltage proportional to its 
RPM and current. So the average motor voltage is a complex function of PWM, 
current, and RPM.

Note that there is a great big filter capacitor bank across the input of the 
controller. Even though the switches inside the controller are switching 
on/off, the capacitors filter this into a more-or-less constant battery voltage 
and current. A voltmeter at the input of the controller does indeed show 72v 
under these conditions. It is this low voltage that is causing the Zilla 
controller to say "whoops; back off the current; we're overloading the 
batteries".

>> It may have a low voltage cutoff circuit but that just shuts down the output 
>> in the event the battery falls below 72volts.... thus the controller would 
>> not be working if the voltage was 96-72 = 24volts.

(Again, 96-72=24v is the voltage drop across the ESR; not the controller input 
voltage).

But as Roger said, this just results in an on/off oscillation. As soon as the 
load is removed, the battery voltage pops right back up to 96v. The controller 
comes right back on. So there is a rapid on/off switching, exactly like PWM. 
Controller designers know this; and explicitly design their controllers to work 
predictably under these conditions. They limit current when you reach the 
low-voltage threshold so the battery simply *stays* at the low-voltage 
threshold.

>> And yes hooking batteries straight up you would have no control over the 
>> current but I still don't believe lead acid would put out 1200 amps.

I can only say from direct personal experience that they certainly will. Many 
EV drag racers who have won trophies will confirm it. Millions of hard-to-start 
diesel engines also pull 1000a from their lead-acid batteries every time they 
start.

Paul, at some point, I think you will have an "Aha!" moment, and realize that 
what we're saying is really how it works. Or, you will go out and do some 
actual testing and see it for yourself. It's easy to fool yourself; but very 
hard to fool the actual devices. :-)

--
Excellence does not require perfection. -- Henry James
--
Lee A. Hart http://www.sunrise-ev.com
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