# Pack Configuration and Voltage Sag



## jeffcoat (Apr 16, 2012)

Voltage sag increases with increased current draw. Since a higher voltage draws less current for the same power output (P = VI), the higher voltage system would have less voltage sag for the same power output.


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## major (Apr 4, 2008)

jeffcoat said:


> Voltage sag increases with increased current draw. Since a higher voltage draws less current for the same power output (P = VI), the higher voltage system would have less voltage sag for the same power output.


Sag is the voltage drop due to the internal resistance of the cells. It is equal to the current times the internal resistance. Since winz is using the same 30 modules either in a 6P5S or 5P6S configuration, 40 kW draws the same current per module either way and therefore the sag per module is the same regardless of the P-S configuration.

Since sag is voltage drop, the higher voltage configuration (5P6S) actually has greater sag. However it would be equal as a percentage of pack voltage either way.

The factor needed to determine the answer to the question is the internal resistance number. If it is high enough to yield a sag greater than 2.2 volts per module, then you'll have to go with the 5P6S to stay above 100V.

You'll find that no matter how you arrange the modules, for the same total power, each module will see the same V, I and sag. And as long as the pack voltage including sag at load is higher than your desired motor voltage, it does not matter what P-S configuration you choose.


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## winzeracer (Apr 3, 2012)

major said:


> Sag is the voltage drop due to the internal resistance of the cells. It is equal to the current times the internal resistance.  Since winz is using the same 30 modules either in a 6P5S or 5P6S configuration, 40 kW draws the same current per module either way and therefore the sag per module is the same regardless of the P-S configuration.
> 
> Since sag is voltage drop, the higher voltage configuration (5P6S) actually has greater sag. However it would be equal as a percentage of pack voltage either way.
> 
> ...


Major,

Thank you, I suspected this was the case but did not know how to arrive at the answer. The 5P6S configuration also works better for building the pack as well, so based on these two factors I will build it this way.

The internal resistance as measured by my PL8 is between 1.5mOhms and 1.9mOhms, though the packaging says that they are ohm matched at 1.2mOhms. Since the internal resistance is non-ohmic, I would assume that the PL8 does a linear approximation. But this value is only calculated while charging and varies by rate of charge. Lets assume the highest I.R. and negligible VD over the wires. Then voltage drop across the traction pack would be 400*.019=7.9v drop. However I suspect that once heat generation begins the non-ohmic I.R. values raises based on temperature, because 7.9 volt drop is considerably less than we will see in actual conditions. Is there a better way to approximate Voltage Sag?

Thanks,
Brock


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## Ziggythewiz (May 16, 2010)

Can't the PL8 give you reistance and sag values while discharging? Unless you're interested in fast charging limits the apparent IR during a charge doesn't matter.


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## winzeracer (Apr 3, 2012)

Ziggythewiz said:


> Can't the PL8 give you reistance and sag values while discharging? Unless you're interested in fast charging limits the apparent IR during a charge doesn't matter.


No PL8 only does IR while charging, and even if it did give IR while discharging it will only discharge at 40amps, I would need to discharge at 80amps per 22.2v 5ah brick to simulate the 400amp traction pack load.

EDIT: Ziggy, also the pack will be routinely charged at 3C though not really relevant to this topic


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## major (Apr 4, 2008)

It is unclear what you mean by IR. Internal Resistance or I*R (voltage drop)? Anyway, the manufacturer's spec is rarely if ever spot-on and usually way optimistic. The only way I know to find the true R_int is to measure it by taking voltage at 2 different loads and using Ohm's Law. Doing this gives you a good number to use for DC internal resistance and will apply to discharging, charging and most of the SOC. It can vary with temperature and will vary with age or SOH.

When building a pack, my advice is to test each cell, or module in your case. Get a resistor to approximate your load, ammeter, voltmeter and stop watch at minimum. A CellLog8S is better as you can record individual V/c vs time.


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## Ziggythewiz (May 16, 2010)

If you use the pl8 to discharge at 20A and 40A you should be able get figure out pretty accurately what the apparent IR is and what sag would be at 80A.


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## Elithion (Oct 6, 2009)

Because both are the same energy, and both use the same cells, then, under the same generated power, both will have exactly the same sag (measured as a percentage of pack voltage).

Specifically, given the Short Discharge Time of Turnigy cells, the sag will be:

Sag [%] =100 [%] * Specific current [1/h] * Short Discharge Time  / 3600 [s/h] 

That will be true, regardless of arrangement of the cell.

Unfortunately, I don't have a number for Short Discharge Time for Turnigy cells, because they are of uncertain origins and there are no official specs out there.


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## Elithion (Oct 6, 2009)

*Caveat Emptor*



winzeracer said:


> Batteries are Turnigy Nano-Tech 35C Constant


I truly doubt that. As far as I know, no Li-ion cell can put out 35 C continuous (that's < 2 minute discharge), and do so unscathed. I am afraid that that's just a salesman saying that just to sell you cells .

"Unfortunately with other big brands; numbers, ratings and graphs can be fudged. Rest assured, TURNIGY nano-techs are the real deal, delivering unparalleled performance!:

...right!

(Yes, the GEB cells can do that, but they also have hardly any capacity in them.)


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## Ziggythewiz (May 16, 2010)

*Re: Caveat Emptor*

Many RC applications use a .5-5 min discharge.


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## Elithion (Oct 6, 2009)

*Re: Caveat Emptor*



Ziggythewiz said:


> Many RC applications use a .5-5 min discharge.


Wow! I had no idea. Thanks.

I just ordered one of those cells, so I can actually measure its Short Discharge Time. I'll let you know what I find.


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## winzeracer (Apr 3, 2012)

Davide,

What exactly is Short Discharge Time, I noticed it is indicated on your Enerdel module. 

Thanks, 
Brock


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## Elithion (Oct 6, 2009)

Hello Brock,



winzeracer said:


> What exactly is Short Discharge Time


In brief:
"Short Discharge Time" is constant characteristic of each battery cell technology, regardless of capacity or voltage, which enables to quickly compare various cell technologies, and to easily calculate the efficiency, resistance and voltage sag or a battery using those cells.

In detail:
Magazine article, most recent web page.

For example: list of many cell, battery and ultra-capacitor families, by Short Discharge Time (smaller = more efficient):


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## sirwattsalot (Aug 27, 2012)

I recently resumed work on one of my experimental circuits to reduce voltage sag, to increase performance and to increase range. I finished assembly of the new circuit last Sunday, 02/01/2015 This circuit passed it's bench test.


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## sirwattsalot (Aug 27, 2012)

I recently resumed work on one of my experimental circuits to reduce voltage sag, to increase performance and to increase range. Last week I received my new circuit boards from the board house that I use to produce my prototype boards. It was just like Christmas when I tore open the package. I started to assemble the circuit on Saturday which should be ready to test in another week. I will test, retest and test again. Results were good with my first prototype which was wired on perf board. I got a good boost of acceleration and I drove miles farther than ever before. First ask this question- If I hold the accelerator down to the floor and draw 400 amps, how many seconds will it take for the battery voltage to drop from peak (120v) to a low (96v)? Maybe 3 to 4 seconds? This is what I mean by voltage sag; low voltage and high current that causes the motor to lag and the battery to die quickly. It is even worse when the battery is weak. It is like I just shorted out the battery and I expected it to go like a wild mustang and then the hamster dies. What I am working on might be called a kind of voltage compensator that will give the car some snap. If you want to hit a homerun, you need to give the ball a sudden sharp surge of energy. Pushing the ball slowly would require more energy and would make for a dull game. Quick acceleration is not a gentle push but rather a sudden sharp surge, a jolt, a boost, a thrust, and whoosh zoom your off. I will report the results of the next test when I finish building my new circuit. I hope it will be a homerun.


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## grip911 (Dec 14, 2011)

this is fascinating stuff. I have pondered over whether to try a mechanical type of switching in more voltage, when the sag goes under a certain limit, but never really attempted it . Are you using this on an AC system perchance?
I would really like to know more about it, and would happily volunteer in any verification or application of this project.


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## sirwattsalot (Aug 27, 2012)

"Are you using this on an AC system perchance?
I would really like to know more about it, and would happily volunteer in any verification or application of this project. "

Yes, I have the AC-50 motor and Curtis controller with regenerative feature.
I was introduced to the concept of using Supercapacitors to help store energy during the regenerative breaking phase by a magazine article that I read. NASA developed a flywheel generator storage device that has potential to do the same which is being used in test cars now. My scheme however, does not require components as expensive as those. It is not a one shot deal. I tested my new circuit on Sunday and all is well. I need to install it in the car and make certain that there are no negative effects. I also need to determine that it truly is beneficial and that the results I got before last Summer are a product of the circuit and not some fluke. 
One challenge was to perform switching without welding contacts shut and without creating voltage spikes that could cause harm to sensitive electronics. A big box full of 200v capacitors protected with a DC circuit breaker will do. I will want someone else to test the circuit. I will keep in touch. I am getting supporting documents together for the complete network of components required to make this work; parts lists, schematic, diagrams, etc.


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