# Re: [EVDL] Slow due to 96V pack?

```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
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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