Battery Health question when buying a used i3 as first time electric car buyer

[John_S]

Hi All,

Thanks for reading this and my apologies if a similar question has been asked before.

I’ve never owned an electric car before but I’m looking at getting my first one and it’s a 2019 i3 with the 120Ah battery.

I followed YouTube videos which showed me how to get the Batta Kappa battery health estimate and it showed a figure of Batt.Kapa.max 36.4

But I don’t really have any idea of whether this number is good, bad or about normal for what you should expect on a 5 year old i3?

If anybody else has any advice it will be greatly appreciated!

Many thanks,

John

 

[EvanstonI3]

BMW's advertised "useable capacity" for that battery is 37.9, so as far as warranty goes you are at 96%

 

[John_S]

Hi Evanstoni3,

Thanks for your quick reply which is much appreciated!

That’s really helpful because I’ve never had an electric car before let alone an i3.

Because I’m completely new to the world if electric cars I’m trying to get up to speed fast with all of the terminology and things to look out for.

So just to make sure I’ve got my head around this correctly can I just ask this.

Following your message so if assuming back when a brand new 120Ah 42.2kWh i3 was first delivered if everything were delivered in order as it’s meant to be if you’d of checked on the cars display doing the Battery Kappa test instead of showing 42.2kWh it would of instead read 37.9kWh because that’s all that BMW wanted you to be able to use up to as the useable capacity of the battery versus the total capacity of the battery?

So if now this 2019 used car at 5 years old with the battery kappa test reading at 36.4kWh (so 96% of the original capacity) sounds pretty okay to me in terms of battery degradation so far.

My apologies for this racers question fir clarification but just because I’m completely new to electric cars if you were able to let me know if my understanding is correct (or potentially completely wrong) that would be fantastic!

Thank you again.

Best regards,

John

 

[EvanstonI3]

BMW's "advertised" useable capacity is slightly de-rated from what it would have been when new.
I am guessing it was actually around 39 when new but don't know for certain.

For example: My car has the 94aH battery which has a Gross Rating of 33kWh.
BMW's "advertised useable capacity" was 27.2, yet when I got the car it was 3 years old with 19K miles and the BATT.KAPPA was 29.5 So higher than 100%. I'm pretty sure that when new, my battery would have read 30kWh.

No one was really sharing these numbers when they got their cars new so everything is just a guess right now.

 

[Gonville]

The BKM (Kappa) in the hidden menu is not a measure of the battery condition. It is a mathematical summation of the current last put into the batteries - so is an estimate. For all intents and purposes it is useless. The battery will degrade by 1-2% per annum no matter what. No matter how many cycles, voltage, charges AC/DC, or how it is driven. Yes it could be the pack is unbalanced which is why charging it to full indicated (which isn’t 100% actual) is a good idea to allow cell balancing. It does one cell each time the vehicle is charged. That has far more of an impact than does calendar age. A miss balanced pack will show much lower range. BimmerFlow might give some data but doesn’t really know the battery condition. More modern BMW models do have more data.

 

[Gonville]

BMW Max Kappa
Max Kappa (short for “Kapazität”) is the BMS estimated snapshot capacity at that point in time, its based on external factors (temperature, current load), coulomb count, these factors are then fed into a statistical model to provide an educated guess of what the current capacity of the pack. This is what is shown in the service menu. It is not like a fuel tank you can't just measure it as there is chemistry involved.

Max Kappa as such is not a measure of anything, it is a guess of what the BMS thinks the pack is doing at that moment in time and therefore how many kWhs are available. It is not a true reflection of capacity.

Consider the following scenario, it has been cold overnight you haven't preconditioned so the battery is at 4c well below its preferred operating temperature of 24-28c, Max Kappa will show a corresponding low number as due to the cold the pack is less chemically active, as you drive the load starts to warm the pack up, so you then lose range but Max Kappa goes up due to battery reaching operating temperature.

The only way to test true capacity and health of a battery is to charge and discharge the pack to known points and record the input and output rates. It is perfectly possible for Max Kappa to read 19.1kWh (on a 60aH) on a full charge but the battery actual be degraded, or vice versa for it to read well under 15kWh but be perfectly fine.


Orange Machine. Battery safety tester (including pressure). Discharge charge full cycle.

 

[mfogarty]

I was going to reply but Gonville's answers are spot on.