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

```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.```
```
Sent from my iPhone

> On Sep 13, 2017, at 6:09 PM, Roger Stockton via EV <ev@lists.evdl.org> wrote:
>
> paul dove wrote:
>
>> You said:Here's my guess: A 6v golf cart battery has an internal
>> resistance of about 5 milliohms (0.005 ohms). A 96v pack has 16 of them;
>> so the resistance is 0.005 x 16 = 0.08 ohms. Let's say the Zilla limits
>> voltage to 72v (i.e. it won't pull the pack below 72v). Then the most
>> current you can get is I = V/R = (96v-72v) / 0.08 ohms) = 300 amps.
>> **It is either going to be 96v/0.08, 72v/0.08, or somewhere in between.
>> The voltage won't be 24/0.08 at any time.So, somewhere between 1200 and
>> 900 amps.
>
> You are (almost) correct only if you *short* the battery terminals, so that
> the voltage on the load side of the internal resistance is 0V.
>
> This is *not* the case with a controller that enforces a low voltage cutoff,
> such as the 72V value in the example:
>
>
>             0.08 ohm
>            Internal R
>     +-------/\/\/\/\------oo------+
>     |                             |
>   --+-- ideal                     / 72V
>    ---  96V                       \ minimum load
>   ----- battery                   / voltage
>    -+-                            \
>     |                             |
>     +---------------------oo------+
>
> The controller effectively regulates the voltage on the load side of the
> battery internal resistance such that the voltage difference across the
> internal resistance will indeed be approximately as Lee states.  And the
> maximum battery current that *this* load can draw will be on the order of
> (96-72)/0.08=300A.
>
> Cheers,
>
> Roger.
>
>
>
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