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Hi there, I have found some batteries for my conversion and have penned some calculations, that I need sanity checking.

The cells are Li-ion 3.7v Nom and 20Ah. They have a 5C continuous and a 10C peak rating.

I’m looking to have a 97S3P set up to give me circa 25KWh.

Am I right in calculating a continuous current delivery of circa 300A and peak of 600A?

Or do I need to go do some more reading in batteries?

I am looking to drive a Tesla SDU you see.

Thanks for your help in advance.

Tom
 

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Am I right in calculating a continuous current delivery of circa 300A and peak of 600A?
20Ah per cell, 5c continuous means 100amps, 10c peak means 200a.

You'll have 3 strings in parallel, so 300-600a is correct.

If you want someone to doublecheck your capacity...

You have 97 x 3 cells = 291 cells. Each are 3.7v and 20Ah.

291 * 3.7v * 20ah = 21,534 watt-hours. So, close to 25kwh, sure.

Keep in mind that'll decay as they age, and, you won't want to slam them to full depth shove them to full recharge every time, so, you'll probably get 10% less than 21kwh.

Looks like you've got a handle on this.
 

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Discussion Starter #3
Thanks, that’s brilliant. At least I know I’m not heading down the total wrong path.

To be honest The 25KWh, was worked out on the max voltage (4.15v) not the nominal, so my mistake there. I only realised after I had posted.

I’ve been working on a conservative 80% usage, so if I go on 90% that should give me some more miles.

Next thing is trying to work out the best way to BMS a 97S3P system. Back to the research!

Thanks again!
 

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If the cells are 3.7 v nom..... then most likely they are 4.2 v fully charged.

4.2x 97 x 60Ah = 24,444 kWh
Typo, you meant watt-hours, not kWh.

...

But why would you calculate capacity based on max voltage? The cells do not stay at that voltage throughout their discharge cycle.

For example, why not use 2.75v (empty) x 97 * 60Ah = 16,000 watthours?

So is the real number 24kWh, 16kWh? It's obviously in between.

That's the whole point of using the nominal voltage isn't it?
 

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Typo, you meant watt-hours, not kWh.

...

But why would you calculate capacity based on max voltage? The cells do not stay at that voltage throughout their discharge cycle.

For example, why not use 2.75v (empty) x 97 * 60Ah = 16,000 watthours?

So is the real number 24kWh, 16kWh? It's obviously in between.

That's the whole point of using the nominal voltage isn't it?
Yes sorry it would be 24kWh.

You have to use max voltage because the cell won’t contain 20Ah until it is full and a full cell is 4.2 volts.
 

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You have to use max voltage because the cell won’t contain 20Ah until it is full and a full cell is 4.2 volts.
...

Umm, no?

Or rather, you made two statements and they're not connected.

Correct that you could not get 20Ah of discharge from a cell unless it was full. Just like you can't get 5 gallons from a 5 gallon bucket if you don't fill it up all the way.

But, how is that related to "you have to use the max voltage"?

Have to use it for what?

"You have to charge the battery until it's full, to get the full Amp-hours out of it" is obvious, but unrelated to the amount of energy you can store in the battery.

When calculating the kWh of a battery, you use the nominal voltage, as a mid-point between charged and discharged.

I'm not sure how else to explain this, other than, if you were using a constant power (like, driving a steady speed), you would not get 25kWh out of this battery. You would get only get 21kWh, even from a fully charged battery.

I'm pretty sure you don't understand this correctly by the way your reasoning is working.
 

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yes, I see what you are saying.

These are just estimates or averages anyway since it depends on the rate of discharge. However, using the nominal voltage is probably a better estimate.
 

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...

But why would you calculate capacity based on max voltage?
Because that is what all the OEMs do to rate their packs. It is a known fixed maximum value that can be used for comparison of other car pack sizes.

Also when you are building the instrumentation and measurement system, it gives upper limit bounds for determining the dynamic range of the sensors and signal conditioning.
 

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Capacity is not calculated by OEMs based on max voltage, because that's physically nonsensical. If you want something better than using the nominal voltage, integrate the product of current and voltage, as voltage drops from the fully charged state to the completely discharged state, during which the rated capacity (integral of current over time) is exhausted. Divide this energy (watt-hours) by that charge (amp-hours) to get a voltage... which will likely turn out to be the nominal voltage, so you could have just done one multiplication. ;)

Obviously everything needs to be rated to operate at the maximum voltage, which has nothing to to with energy capacity.
 

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Tesla, Nissan, Mitsubishi all do this using the maximum allowed pack voltage multiplied by the maximum allowed discharge capacity, and i have no reason to assume that the other OEMs wouldn't follow this practice.

All this gives you is an estimate of the size of your "fuel tank" for sales and marketing comparison purposes, and that's why it is advertised as some round integer number of kWh.

For actual capacity measurements they do indeed integrate the current over time and it is reported in floating point precision over the CAN buss.
 

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Because that is what all the OEMs do to rate their packs.
This doesn't make any sense, but, regardless, doesn't make any sense for a DIYer either.

Why would you want to call a pack where you can only extract 20kWh, to be called a 25kWh pack?

Why if you measured the capacity of your own cells to discover the amp-hours, would you not want to know the energy storage of the battery?

I.E. If you have a 25kWh pack, and you are measuring energy removed as you drive, why would you want to run out of energy when you still appear to have 5kWh left?

Other than perhaps marketing (which does not apply to a DIYer), I see no advantage in lying to yourself and overestimating the capacity of your pack.
 

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Tesla, Nissan, Mitsubishi all do this using the maximum allowed pack voltage multiplied by the maximum allowed discharge capacity, and i have no reason to assume that the other OEMs wouldn't follow this practice.
The Audi E-Tron only allows about 90% of the battery capacity to be used.
 

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The Audi E-Tron only allows about 90% of the battery capacity to be used.
At the heart of the Audi e-tron® is its powertrain and battery, a powerful muscle that keeps the vehicle moving and whose performance determines how far the vehicle can go. A large 95 kWh lithium-ion battery is integrated into the floor of the passenger compartment, which creates a low center of gravity for dynamic handling and helps achieve the vehicle’s long-range capability. Innovative materials and design help optimize the battery’s capacity, durability and safety, making this one battery that’s built to perform.

In all, there are 36 battery modules in each pack, each about the size of a shoe box. In each module, Audi nestles 12 pouch-style 60-ah (3.5-volt) cells, for a total of 432 cells. Each of those cells is the size of a bag of coffee.

432 x 60ah x 3.67 v = 95kwh…… Looks like they use nominal voltage and advertise the whole capacity.

https://www.audiusa.com/models/audi-e-tron/battery
 

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432 cells at 4.1 max Vpc, times 90% of 60Ahr = 95.6 kWh for the Audi.

What is the pack voltage of an Audi? googly says 396 V, so that would be 108s4p cell configuration

Those modules are about 12V each at 3s4p, they might make useful starter batteries. Interesting that they use 12V blocks for building packs, seems a carry-over from lead acid.
 
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