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How do I calculate total resistance of a battery pack? Plus nickel plus wires etc

Posted: Dec 18 2020 6:13pm
by mbgjt1
Sony 21s12p 36 Ah vtc6 battery pack. So let's take the internal battery resistance we can add all the series resistance together and then do a parallel resistance calculation, which is pretty easy to do.

But then, how do I factor in resistance of the pure nickel strips? Resistance of wires? BMS? Etc.

Will this resistance increase or decrease with resistance.

I've taken it as 0.1 ohms for the entire battery pack. Is this accurate?

I'd like to use this in the grin motor simulations to get a more accurate idea of what my overall performance will be

Re: How do I calculate total resistance of a battery pack? Plus nickel plus wires etc

Posted: Dec 18 2020 9:55pm
by MadRhino
Your battery internal resistance is not a constant value. The number on the spec sheath are sometimes average, sometimes measured in specific conditions of load and temperature.

Of course, the calculations are easy to do for the whole pack after you know the specs of one single cell, but it is not a precise and constant value. The simulator is not a precise tool for that purpose, because the true resistance between the battery chemical reaction and the motor windings does imply too many variables. The result is an approximation, even if you could enter a precise value for battery pack internal resistance. The calculations that are made from single cell spec are enough to give an idea of the total power that your battery can feed. More precise power limit values can only be obtained via temp and Amp monitoring in real life usage.

Re: How do I calculate total resistance of a battery pack? Plus nickel plus wires etc

Posted: Dec 20 2020 10:36pm
by mbgjt1
MadRhino wrote:
Dec 18 2020 9:55pm
Your battery internal resistance is not a constant value. The number on the spec sheath are sometimes average, sometimes measured in specific conditions of load and temperature.

Of course, the calculations are easy to do for the whole pack after you know the specs of one single cell, but it is not a precise and constant value. The simulator is not a precise tool for that purpose, because the true resistance between the battery chemical reaction and the motor windings does imply too many variables. The result is an approximation, even if you could enter a precise value for battery pack internal resistance. The calculations that are made from single cell spec are enough to give an idea of the total power that your battery can feed. More precise power limit values can only be obtained via temp and Amp monitoring in real life usage.
Hey mate

Yes I've heard it changes based on a few factors but I was hoping to get an average ball park figure. Some threads have suggested that a ballpark figure can be obtained by taking the total resistance of all the series and parallel cells and multiplying it by 2.

In my case if I do that, I will get an overall resistance of 0.1 ohms.

Not sure how close this would be to a real life value though.

Have you measured the resistance on your battery pack?

Re: How do I calculate total resistance of a battery pack? Plus nickel plus wires etc

Posted: Dec 21 2020 2:48am
by MadRhino
Ohm law says that resistance adds in a series and divides in parallel.

The logic is simple: the current in a series will have to go through the resistance of each cell, thus they add. Now add another series and wire it in parallel with the first one so the current does flow through both series at once, then the resistance is cut by half.

Understanding the basic laws of electricity can be made simpler by comparing with water flowing through a pipe.

Re: How do I calculate total resistance of a battery pack? Plus nickel plus wires etc

Posted: Dec 21 2020 3:59am
by serious_sam
mbgjt1 wrote:
Dec 20 2020 10:36pm
Some threads have suggested that a ballpark figure can be obtained by taking the total resistance of all the series and parallel cells and multiplying it by 2.
In my case if I do that, I will get an overall resistance of 0.1 ohms.
Not sure how close this would be to a real life value though.
Have you measured the resistance on your battery pack?
It really depends on the cell interconnects, your wire gauge and length, and the type and qty of MOSFETs in the BMS, and any additional switches, connectors, or fuses you have in the circuit. None of which you have provided any information on.

All of these things can be accurately calculated. The biggest variable (as stated) is the cell IR, but even that can be estimated as a fixed value (it sits pretty constant through the middle section of SoC of a cell). You'd be fairly safe using 20mOhm for VTC6.

Having said that, 0.1 ohms is a good conservative estimate for your pack.

Courtesy of docware:
viewtopic.php?f=14&t=103092&start=400#p1553365
SONY VTC DCIR after 700 cycles.jpg
SONY VTC DCIR after 700 cycles.jpg (174.37 KiB) Viewed 143 times