Slow due to 96V pack?

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Slow due to 96V pack?

Electric Vehicle Discussion List mailing list
I'm helping a guy with a used conversion he just bought.
It's a S-10 with 96V of Golf Cart batteries, Zilla 1K, and a 8 or 9" motor.
Even fully charged - it's acceleration is poor.
I was never able to get it to draw more than 200 Amps in any gear at any speed.
(Zilla limit is set to 500)
About 18KW max.

Is this likely to be just because it's such a low voltage pack?

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Civicwithacord for sale

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Hi,
   The current owner of my conversion, a Gen 5 lead acid (92) Civic is asking 3k. I've said he needs to put it on the tradingpost, and not just the Medford, OR craigslist, but he's a busy pilot who "just hasn't gotten to it."
Lots of things built into it for safety. There is an EVAlbum entry for more info.

Bob Bath, from his iPod, so any misspellings are from autocorrect or fat fingers on a small device, not cluelessness...

> On Sep 8, 2017, at 8:51 PM, John Lussmyer via EV <[hidden email]> wrote:
>
> I'm helping a guy with a used conversion he just bought.
> It's a S-10 with 96V of Golf Cart batteries, Zilla 1K, and a 8 or 9" motor.
> Even fully charged - it's acceleration is poor.
> I was never able to get it to draw more than 200 Amps in any gear at any speed.
> (Zilla limit is set to 500)
> About 18KW max.
>
> Is this likely to be just because it's such a low voltage pack?
>
> --
> Try my Sensible Email package!  https://sourceforge.net/projects/sensibleemail/
> _______________________________________________
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>
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Re: Slow due to 96V pack?

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There is both a battery low voltage limit and a motor high voltage limit.
There is also a motor current limit and a battery current limit.

You need to set all four to the appropriate values.

You never said if you were reading battery current or motor current.
Makes a bit of difference. Also, it would be useful to know the battery
voltage under load.

Bill D.

On 9/8/2017 9:51 PM, John Lussmyer via EV wrote:

> I'm helping a guy with a used conversion he just bought.
> It's a S-10 with 96V of Golf Cart batteries, Zilla 1K, and a 8 or 9" motor.
> Even fully charged - it's acceleration is poor.
> I was never able to get it to draw more than 200 Amps in any gear at any speed.
> (Zilla limit is set to 500)
> About 18KW max.
>
> Is this likely to be just because it's such a low voltage pack?
>
> --
> Try my Sensible Email package!  https://sourceforge.net/projects/sensibleemail/
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>
>

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Re: Slow due to 96V pack?

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When I was driving my S-10 with 20 6v golf cart batteries (120v pack) I
was never able to draw more than 240-300 amps from the battery pack even
when the batteries were new. (and the acceleration wasn't great in
anything but 1st gear from 0-20mph...)

If your friend's pack is used, 200 amps seems reasonable, especially for
a 96v pack, which will probably have even more of a voltage sag than my
120 volt pack.

I definitely recommend upgrading to LiIon (perhaps from a wrecked Leaf
pack, which is what I did...)

Jay

On 09/09/2017 12:57 AM, Bill Dube via EV wrote:

> There is both a battery low voltage limit and a motor high voltage limit.
> There is also a motor current limit and a battery current limit.
>
> You need to set all four to the appropriate values.
>
> You never said if you were reading battery current or motor current.
> Makes a bit of difference. Also, it would be useful to know the battery
> voltage under load.
>
> Bill D.
>
> On 9/8/2017 9:51 PM, John Lussmyer via EV wrote:
>> I'm helping a guy with a used conversion he just bought.
>> It's a S-10 with 96V of Golf Cart batteries, Zilla 1K, and a 8 or 9"
>> motor.
>> Even fully charged - it's acceleration is poor.
>> I was never able to get it to draw more than 200 Amps in any gear at
>> any speed.
>> (Zilla limit is set to 500)
>> About 18KW max.
>>
>> Is this likely to be just because it's such a low voltage pack?
>>
>> --
>> Try my Sensible Email package!  
>> https://sourceforge.net/projects/sensibleemail/
>> _______________________________________________
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>> Please discuss EV drag racing at NEDRA
>> (http://groups.yahoo.com/group/NEDRA)
>>
>>
>
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Re: Slow due to 96V pack?

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On Fri Sep 08 21:57:43 PDT 2017 [hidden email] said:
>There is both a battery low voltage limit and a motor high voltage limit.
>There is also a motor current limit and a battery current limit.
>
>You need to set all four to the appropriate values.

yes, I know.  Just forgot to list those Motor values as they are set to be irrelevant.
1000A and 150V.
And I was reporting Battery Amps of course.

I'm pretty sure now that it's just too low a voltage pack.  200 Batt amps was the max I could pull.
The GC batts are basically unused - but are a couple years old, and have had battery maintainers connected for that time.

I'm betting he has a Z1K LV - so that will limit how hi a voltage we can switch to when we switch to Lithium.
(also dealing with it having 2, 10A 48V chargers - one of which is dead, so you have to switch the other charger back and forth between pack halves.  sigh.)
This guy can't afford a bunch of upgrades immediately though.


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Re: Slow due to 96V pack?

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John,

As someone mentioned, even at 120v you won't get great acceleration with GC
batts.  That's why we often call them "lead sleds".

Peter Flipsen Jr

On Sep 9, 2017 7:14 PM, "John Lussmyer via EV" <[hidden email]> wrote:

> On Fri Sep 08 21:57:43 PDT 2017 [hidden email] said:
> >There is both a battery low voltage limit and a motor high voltage limit.
> >There is also a motor current limit and a battery current limit.
> >
> >You need to set all four to the appropriate values.
>
> yes, I know.  Just forgot to list those Motor values as they are set to be
> irrelevant.
> 1000A and 150V.
> And I was reporting Battery Amps of course.
>
> I'm pretty sure now that it's just too low a voltage pack.  200 Batt amps
> was the max I could pull.
> The GC batts are basically unused - but are a couple years old, and have
> had battery maintainers connected for that time.
>
> I'm betting he has a Z1K LV - so that will limit how hi a voltage we can
> switch to when we switch to Lithium.
> (also dealing with it having 2, 10A 48V chargers - one of which is dead,
> so you have to switch the other charger back and forth between pack
> halves.  sigh.)
> This guy can't afford a bunch of upgrades immediately though.
>
>
> --
>
> Try my Sensible Email package!  https://sourceforge.net/
> projects/sensibleemail/
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Re: Slow due to 96V pack?

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I seem to recall that the minimum input voltage for a Zilla is 72 volts.
Might be sagging the pack to 72 volts and that is that is simply amps it
puts out at that voltage.

On 9/9/2017 8:14 PM, John Lussmyer via EV wrote:

> On Fri Sep 08 21:57:43 PDT 2017 [hidden email] said:
>> There is both a battery low voltage limit and a motor high voltage limit.
>> There is also a motor current limit and a battery current limit.
>>
>> You need to set all four to the appropriate values.
> yes, I know.  Just forgot to list those Motor values as they are set to be irrelevant.
> 1000A and 150V.
> And I was reporting Battery Amps of course.
>
> I'm pretty sure now that it's just too low a voltage pack.  200 Batt amps was the max I could pull.
> The GC batts are basically unused - but are a couple years old, and have had battery maintainers connected for that time.
>
> I'm betting he has a Z1K LV - so that will limit how hi a voltage we can switch to when we switch to Lithium.
> (also dealing with it having 2, 10A 48V chargers - one of which is dead, so you have to switch the other charger back and forth between pack halves.  sigh.)
> This guy can't afford a bunch of upgrades immediately though.
>
>
> --
>
> Try my Sensible Email package!  https://sourceforge.net/projects/sensibleemail/
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>
>

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Re: Slow due to 96V pack?

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On Sat Sep 09 20:58:50 PDT 2017 [hidden email] said:
>I seem to recall that the minimum input voltage for a Zilla is 72 volts.
>Might be sagging the pack to 72 volts and that is that is simply amps it
>puts out at that voltage.

Well, as far as I could tell by the Volt meter, the pack wasn't sagging below 90V.



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Re: Slow due to 96V pack?

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On Sep 9, 2017 9:20 PM, "John Lussmyer via EV" <[hidden email]> wrote:

On Sat Sep 09 20:58:50 PDT 2017 [hidden email] said:
>I seem to recall that the minimum input voltage for a Zilla is 72 volts.
>Might be sagging the pack to 72 volts and that is that is simply amps it
>puts out at that voltage.

Well, as far as I could tell by the Volt meter, the pack wasn't sagging
below 90V.


Is that measured at the battery terminals or at the input to the Zilla?
Voltage might be really sagging through the cabling or a high resistance
connection.
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Re: Slow due to 96V pack?

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In reply to this post by Electric Vehicle Discussion List mailing list
                                    Hi John and All,
                                            Since they haven't been worked they are going to need at least 5 cycles to waken up and likely 10 cycles to get to their real
potential.
                                                 Jerry Dycus


--------------------------------------------
On Sat, 9/9/17, John Lussmyer via EV <[hidden email]> wrote:

 Subject: Re: [EVDL] Slow due to 96V pack?
 To: "Electric Vehicle Discussion List" <[hidden email]>
 Cc: "John Lussmyer" <[hidden email]>
 Date: Saturday, September 9, 2017, 10:14 PM
 
 On Fri Sep 08 21:57:43 PDT 2017
 [hidden email]
 said:
 >There is both a battery low
 voltage limit and a motor high voltage limit.
 >There is also a motor current limit and a
 battery current limit.
 >
 >You need to set all four to the appropriate
 values.
 
 yes, I know.  Just
 forgot to list those Motor values as they are set to be
 irrelevant.
 1000A and 150V.
 And I was reporting Battery Amps of course.
 
 I'm pretty sure now that
 it's just too low a voltage pack.  200 Batt amps was
 the max I could pull.
 The GC batts are
 basically unused - but are a couple years old, and have had
 battery maintainers connected for that time.
 
 I'm betting he has a Z1K
 LV - so that will limit how hi a voltage we can switch to
 when we switch to Lithium.
 (also dealing
 with it having 2, 10A 48V chargers - one of which is dead,
 so you have to switch the other charger back and forth
 between pack halves.  sigh.)
 This guy
 can't afford a bunch of upgrades immediately though.
 
 
 --
 
 Try my Sensible Email package!  https://sourceforge.net/projects/sensibleemail/
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Re: Slow due to 96V pack?

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In reply to this post by Electric Vehicle Discussion List mailing list


On 09/09/2017 10:14 PM, John Lussmyer via EV wrote:

> I'm betting he has a Z1K LV - so that will limit how hi a voltage we can switch to when we switch to Lithium.
> (also dealing with it having 2, 10A 48V chargers - one of which is dead, so you have to switch the other charger back and forth between pack halves.  sigh.)
> This guy can't afford a bunch of upgrades immediately though.
>

I have a Curtis 1231c, so I had to keep my pack voltage at 120-130 volts
when upgrading to lithium. I had to re-arrange my Leaf pack to put every
3 modules in parallel reducing the voltage to 131.2 fully charged.

If you can fit an extra 4 GC batteries into the truck somewhere (mine
had them under the hood up front) to raise your pack voltage to 120v
nominal I can sell you my old Lead Acid chargers (Zivan for 240v  &
Quick Charge for 110v) very cheaply....

Jay
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Re: Slow due to 96V pack?

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In reply to this post by Electric Vehicle Discussion List mailing list
96V  = Slug


On Friday, September 8, 2017, John Lussmyer via EV <[hidden email]>
wrote:

> I'm helping a guy with a used conversion he just bought.
> It's a S-10 with 96V of Golf Cart batteries, Zilla 1K, and a 8 or 9" motor.
> Even fully charged - it's acceleration is poor.
> I was never able to get it to draw more than 200 Amps in any gear at any
> speed.
> (Zilla limit is set to 500)
> About 18KW max.
>
> Is this likely to be just because it's such a low voltage pack?
>
> --
> Try my Sensible Email package!  https://sourceforge.net/
> projects/sensibleemail/
> _______________________________________________
> UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub
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> group/NEDRA)
>
>
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Re: Slow due to 96V pack?

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In reply to this post by Electric Vehicle Discussion List mailing list
From: lektwik via EV <[hidden email]>
>96V  = Slug

Not necessarily; but in this case, probably correct.

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.

In practice, the resistance of the pack is a bit higher due to the wire and connectors. So a 200a max current is probably the most you can draw from the golf cart batteries before their voltage under load falls to 72v.

Now, if you had a different kind of batteries with less internal resistance, or a controller that would cheerfully pull the battery voltage even lower, you'd have a lot more current, and peppier performance.

For example, my old ComutaVan had a 72v pack of golf cart batteries, and a contactor controller. It would cheerfully pull 1000a from the batteries, and spin the tires if you "floored it" from a dead stop. I tried a Curtis 1221 400a controller; and it was *worse* than the contactor controller for accelleration, because it never came anywhere near 400a due to voltage sag.

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Re: Slow due to 96V pack?

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I don't think that math is right but you won't get 100% efficiency out a controller.
I think Curtis advertises 88%.
Current out of a battery is not calculated based on internal resistance. Internal resistance or Impedance of a battery can be modeled and measured but it doesn't act like a normal load. It's a chemical reaction and different chemistries will give you different currents.
If it is not due to limits on the controller and the efficiency drop then I would say the batteries aren't at full capacity any more.
The battery industry uses a 100-hour rate as an index to compare batteries of different types and sizes. The 100-hour rate is the amount of Ahs the battery will deliver during a 100-hour discharge. The capacity of a battery, in Ahs, is a dynamic number that is dependent on the discharge current.

For example, a battery that is discharged at 10A will give you more capacity than a battery that is discharged at 100A. With the 100-hr rate, the battery is able to deliver more Ahs than with the 20-hr rate because the 100-hr rate uses a much lower discharge current than the 20-hr rate. Both rates are used as baselines. Either rate, however, will give you the same view of a battery. A higher capacity battery will have higher 5 and 20 hour rates than a battery with lower capacity.
They publish tables that you can look up to see these values.


      From: Lee Hart via EV <[hidden email]>
 To: Electric Vehicle Discussion List <[hidden email]>
Cc: Lee Hart <[hidden email]>
 Sent: Monday, September 11, 2017 11:40 PM
 Subject: Re: [EVDL] Slow due to 96V pack?
   
From: lektwik via EV <[hidden email]>
>96V  = Slug

Not necessarily; but in this case, probably correct.

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.

In practice, the resistance of the pack is a bit higher due to the wire and connectors. So a 200a max current is probably the most you can draw from the golf cart batteries before their voltage under load falls to 72v.

Now, if you had a different kind of batteries with less internal resistance, or a controller that would cheerfully pull the battery voltage even lower, you'd have a lot more current, and peppier performance.

For example, my old ComutaVan had a 72v pack of golf cart batteries, and a contactor controller. It would cheerfully pull 1000a from the batteries, and spin the tires if you "floored it" from a dead stop. I tried a Curtis 1221 400a controller; and it was *worse* than the contactor controller for accelleration, because it never came anywhere near 400a due to voltage sag.

--
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Re: Slow due to 96V pack?

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From: paul dove <[hidden email]>
> Current out of a battery is not calculated based on internal resistance.

Actually, it is. The Equivalent Series Resistance (ESR) of a battery is not a theoretical number. It is found empirically by measuring the voltage drop caused by a high-current load. The voltage of a 6v golf cart battery really does drop by about 2.5 volts with a 500 amp load. In other words, it acts like an ideal 6v battery with a 0.005 ohm resistor in series. Thus, it's a reasonable way to estimate what you are likely to get.

> you won't get 100% efficiency out a controller.

No, of course not. But the efficiency is pretty high, and can usually be ignored for back-of-the-envelope estimates. That's all I was trying to do with my estimate.

>The battery industry uses a 100-hour rate as an index to compare batteries of different types and sizes.

They do when they are trying to exaggerate the capacity of their battery. For EV use, the 100-hour rate is useless. Even the more common 20-hour rate is nowhere near the rate at which the battery will be used. What you want to look at is the 1-hour or Reserve Capacity rates, which are what you will get when discharged at high currents. For a golf cart battery, the reserve capacity is measured at 75 amps, and the 1-hour rate at about 100 amps.

But, I don't think these capacity numbers are what is limiting the maximum current in this case. My guess is that these golf cart batteries can only deliver 200a before their voltage sags to 72v and the controller starts limiting current.


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Re: Slow due to 96V pack?

Electric Vehicle Discussion List mailing list
Lee,
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.

In practice, the resistance of the pack is a bit higher due to the wire and connectors. So a 200a max current is probably the most you can draw from the golf cart batteries before their voltage under load falls to 72v.
**obviously something else is going on since you don't see 1200 Amps.

Now, if you had a different kind of batteries with less internal resistance, or a controller that would cheerfully pull the battery voltage even lower, you'd have a lot more current, and peppier performance.

For example, my old ComutaVan had a 72v pack of golf cart batteries, and a contactor controller. It would cheerfully pull 1000a from the batteries, and spin the tires if you "floored it" from a dead stop. I tried a Curtis 1221 400a controller; and it was *worse* than the contactor controller for accelleration, because it never came anywhere near 400a due to voltage sag.
**Exactly, the controller is influencing the current which is what I said. I ould be surprised if you could get a sustained 900 amps out of a lead battery but I will have to take your word since I've never tried it.


      From: Lee Hart via EV <[hidden email]>
 To: Electric Vehicle Discussion List <[hidden email]>
Cc: Lee Hart <[hidden email]>
 Sent: Tuesday, September 12, 2017 7:36 PM
 Subject: Re: [EVDL] Slow due to 96V pack?
   
From: paul dove <[hidden email]>
> Current out of a battery is not calculated based on internal resistance.

Actually, it is. The Equivalent Series Resistance (ESR) of a battery is not a theoretical number. It is found empirically by measuring the voltage drop caused by a high-current load. The voltage of a 6v golf cart battery really does drop by about 2.5 volts with a 500 amp load. In other words, it acts like an ideal 6v battery with a 0.005 ohm resistor in series. Thus, it's a reasonable way to estimate what you are likely to get.

> you won't get 100% efficiency out a controller.

No, of course not. But the efficiency is pretty high, and can usually be ignored for back-of-the-envelope estimates. That's all I was trying to do with my estimate.

>The battery industry uses a 100-hour rate as an index to compare batteries of different types and sizes.

They do when they are trying to exaggerate the capacity of their battery. For EV use, the 100-hour rate is useless. Even the more common 20-hour rate is nowhere near the rate at which the battery will be used. What you want to look at is the 1-hour or Reserve Capacity rates, which are what you will get when discharged at high currents. For a golf cart battery, the reserve capacity is measured at 75 amps, and the 1-hour rate at about 100 amps.

But, I don't think these capacity numbers are what is limiting the maximum current in this case. My guess is that these golf cart batteries can only deliver 200a before their voltage sags to 72v and the controller starts limiting current.


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Lee A. Hart http://www.sunrise-ev.com
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Re: Slow due to 96V pack?

Electric Vehicle Discussion List mailing list
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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Re: Slow due to 96V pack?

Electric Vehicle Discussion List mailing list
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 <[hidden email]> 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.
>
>
>
> _______________________________________________
> UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub
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> Please discuss EV drag racing at NEDRA (http://groups.yahoo.com/group/NEDRA)
>


.
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Re: Slow due to 96V pack?

Electric Vehicle Discussion List mailing list
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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Re: Slow due to 96V pack?

Electric Vehicle Discussion List mailing list
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 <[hidden email]>
 To: Electric Vehicle Discussion List <[hidden email]>
Cc: paul dove <[hidden email]>
 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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