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

```There is no voltage adjustment on input voltage in any motor controller I ever
used.
The PWM only controls the output voltage by switching it on and off. The duty
cycle then sets the average voltage the motor sees.
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.
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.```
```
From: Roger Stockton <rstock...@delta-q.com>
To: Electric Vehicle Discussion List <ev@lists.evdl.org>
Cc: paul dove <dov...@bellsouth.net>
Sent: Thursday, September 14, 2017 12:19 PM
Subject: RE: [EVDL] Slow due to 96V pack?

paul dove wrote:

> That made no sense to me but a DC motor controller takes the input voltage
> to power mosfets or igbts which switch the power on and off with a pwm
> signal to get the desired output to drive a motor. It has no knowledge of
> the battery impedance.

The controller does not have, or need, knowledge the battery impedance; this is
simply Ohms Law at work.

The controller PWMs the power from the battery, and the duty cycle of the PWM
can be varied to limit whatever parameter the controller logic cares about at
any given time: input voltage, input current, output voltage, or output current.

Since the battery has a finite, non-zero internal impedance, its terminal
voltage will sag as current is drawn from it.  If the motor controller enforces
a minimum input (battery) voltage (as does the controller in question), then
its logic will vary/limit the PWM duty cycle to prevent the voltage at the
input of the controller from falling below the target level.

Since the controller logic will limit/vary the input current to prevent the
input voltage from falling below the threshold, then the maximum voltage
difference across the battery internal resistance is the open circuit battery
voltage minus the minimum input voltage limit of the controller: 96V - 72V =
24V, in this example.

The controller does not know what the internal impedance of the battery is, but
Ohms Law still applies to limit the maximum current from the battery to
*whatever* value results in 24V drop across the internal resistance: 24V / 0.08
ohms = 300A in this example.

*IF* the sophisticated controller were replaced with a simple contactor
controller, then if the contactor controller allowed the entire 96V battery to
be connected directly to a stalled motor (~0 ohms), the voltage drop across the
battery internal resistance would ~approach~ 96V - 0V = 96V, and the peak
current into the motor would approach 96V / 0.08 ohms = 1200A.

Hope this helps,

Roger.

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