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https://www.evwest.com/catalog/product_info.php?products_id=463

I am referring to the Tesla battery module listed in the above link.

It says a single cell has the following specs:

Nominal Voltage = 3.8V

Cell capacity = 3400 mAh

The cells are arranged in 74p6s config, therefore the voltage of the module is 3.8V * 6 = 22.8V The capacity should be 74 * 3400 mAh = 251,600 mAh or 251.6 Ah, 5.7 kWh

However, in the link, it says the module capacity is 232 Ah, 5.3 kWh

I am unable to figure out why it is mention 232 Ah, 5.3 kWh which doesn't work out in calculation.

Kindly advice

Update 1:

What is bothering me is the consistency of the inconsistency in calculation found in many websites. So I figure there must be something I am missing.

For example

http://store.evtv.me/proddetail.php?prod=TeslaBattModule

https://stealthev.com/product/tesla-module/

Update 2:

Thanks for the valuable inputs everybody. It makes sense to me to think that the manufacturers will limit the module / pack capacity using their Battery Management Systems to prolong lifetime of the cells.

I will just leave it at that. Because I am only working on developing some simulation models I can live with my calculation and set a buffer if needed.

user401393
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  • Honestly, I wouldn't get hung up on it. Could have been two different people doing the measurements on different batches or calculations from different sets of raw data. Battery mAh depend on discharge rate anyways and even if they didn't they are tough to measure accurately to begin with. – DKNguyen Sep 16 '20 at 03:29
  • Could also be byproduct of degradation from assembly into a pack. – DKNguyen Sep 16 '20 at 03:43
  • Looks strange. But how about voltage?Last time I looked at Li-Ion, they discharge down to 3.0V, then they are considered empty. (They can go lower, but they are not likely to be rechargeable anymore.) I would say (3.0+3.8) /2= 3.4 V gives a more true Wh number. Numbers still get wrong, but this might help someone else to explain better. – jmn Sep 16 '20 at 18:02

4 Answers4

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The batteries simply last longer, if they are not used up to the full capacity. Maybe the battery pack charge level is not allowed to go fully empty and they leave it 3% charged, and maybe the pack is also not fully charged but only up to 97%. The pack also has to provide a certain capacity over a temperature range, charge cycles, and battery age, so that is why the de-rating is needed.

Justme
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  • I think this is the nub of it. Tesla (and you can examine their powerwall datasheets to see this) doesn't allow their systems to completely discharge. For example, they may rate them as having a totla of 14 kWhr but will also rate them as only providing 13.5 kWhr under optimal circumstances. One might wonder why the difference. But it's about maintaining a minimal charge. The Toyota Prius Prime, for example, keeps them at or above 20% charge level at all times to maximize battery life. So +1 for catching that. – jonk Sep 16 '20 at 06:55
  • This raises the question what 100% and 0% charge level even mean. In the end it’s all about charging and discharging cut-off voltages and how much current/energy can be used between those two level. – Michael Sep 16 '20 at 19:43
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If you look up discussions about Porsche Tycan vs. Tesla people are discussing the fact that the Porsche only uses ~80% of the battery while the Tesla allows in the SW to set it to 100% (so I guess its not factory default)

Using only a part of the capacity has two effects:

  • more lifetime (wear of the electrodes)
  • faster charging (the curve gets flatter in the upper 20%)
schnedan
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The individual cell capacity of 3400 mAh is only a nominal value at the given discharge rate. The actual value of individual cells in the pack would vary within the tolerance limits. As the strength of a chain depends on its weakest link so also in a battery pack its total capacity will be less than that which is calculated on the basis of the nominal value of an individual cell. The packaging of cells in the module may also contribute to some reduction in capacity. I believe 232 Ah, 5.3 kWh capacity mentioned by the manufacturer for the module is the guaranteed capacity.

Ranbir Khanna
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1

Used batteries will have a bit less than original capacity (at least a few percent probably), also the voltage during discharge is not constant (the first used pack site shows average around 3.6V not 3.8V, and it's not completely linear).

Also some losses probably in the wiring of the pack.

And actual capacity will depend somewhat on usage.

So there is a fair bit of approximation going on. To test the actual capacity vs. Panasonic's numbers you'd have to duplicate the exact test conditions.

Spehro Pefhany
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