Oleksiy
Posts: 63
Joined: Fri Mar 23, 2018 4:30 am
Location: Kyiv, Ukraine

Re: Battery capacity issue

Sat Jan 26, 2019 1:34 am

alohart wrote:It is true that there is no instantaneous way to measure a capacity of any battery pack (many different capacities exist depending on conditions). So it would be difficult to provide an i3 driver with a definitive pack capacity.

This is a very fair point. It is sad though, that it's true for BMW that there is no instantaneous way to measure a capacity of the battery pack. It is not necessarily true for other brands though. E.g., Nissan LeafSpy provides a very accurate estimate of battery capacity at any moment, and this estimate closely matches the dashboard battery capacity indicator in bars, the latter triggering a battery warranty whenever there's less than 9 bars remaining. All these also match the outcome of battery capacity assessment using more rigorous tests via slow discharging and charging of the battery. And Leaf has had this capability from the onset, it's not that BMW had to invent a wheel.

I doubt BMW don't have algorithms to estimate the battery capacity by voltage range etc. at any given moment, since Nissan has used them since 2011. I just think it's a lot more convenient for BMW to communicate they don't. And to obscure the process of battery capacity verification and its outcomes in every possible way.

MKH
Posts: 138
Joined: Fri Nov 02, 2018 5:48 am
Location: Dallas

Re: Battery capacity issue

Sat Jan 26, 2019 5:21 am

Interesting article by an i3 owner, and the formula he uses to track his battery capacity, since both the GOM range and the battery kappa/max aren't reliable measures.

https://www.bmwblog.com/2017/04/24/bmw- ... -expected/

Since BMW claims the car will alert them when the battery capacity drops below 70%, they should not have a problem letting owners know what that trigger actually is.

I suspect it isn't a hidden "number" that can be constantly monitored. More likely something built-in to the battery management system, for example. when cell balancing to or above a set parameter fails.
Mark H.
2015 i3 Rex, Capparis White, Tera World, Technology & Driving Assistant, Parking Assistant, Harman Kardon Audio System, 20 inch wheels, EVoInnovate EVSE

vinc
Posts: 13
Joined: Fri Nov 09, 2018 10:59 am

Re: Battery capacity issue

Thu Feb 07, 2019 11:26 pm

I find very interesting that there isn’t a single member who can share a first hand experience of having the battery replaced under warranty. If you go to a Tesla forum, for instance, you quickly hear from people who did.

It certainly does sound like BMW really, really, really doesn’t want to replace the battery. Or the i3 is unusually reliable, which might be true all jokes aside.

Oleksiy
Posts: 63
Joined: Fri Mar 23, 2018 4:30 am
Location: Kyiv, Ukraine

Re: Battery capacity issue

Fri Feb 08, 2019 6:25 am

MKH wrote:Interesting article by an i3 owner, and the formula he uses to track his battery capacity, since both the GOM range and the battery kappa/max aren't reliable measures.
https://www.bmwblog.com/2017/04/24/bmw- ... -expected/

I've been tracking estimated SOH both using a similar formula (I use efficiency and miles driven to get kWh used for the trip, and then divide this number by depth of discharge to obtain SOH) and a Batt.Kapa reading. They are more or less close in terms of monthly averages - summer SOH is 0.1-0.4 kWh higher for calculated values and Batt.Kapa outcomes are higher in winter (they converge in moderately cool weather in Sep and Oct):

month * calculated SOH, kWh * Batt.Kapa SOH, kWh
May-18 * 18.2 * 18.1
Jun-18 * 18.6 * 18.1
Jul-18 * 18.1 * 18.0
Aug-18 * 18.0 * 17.9
Sep-18 * 17.8 * 17.8
Oct-18 * 17.6 * 17.6
Nov-18 * 17.2 * 17.5
Dec-18 * 16.9 * 17.4
Jan-19 * 16.7 * 17.2

I also contrast these with Batt.Ladung reading - this is the remainder of kWh in the battery at any given time. One could calculate battery's full capacity if this number is divided by SOC %. This one is not even close. When at or around 100% SOC, it matches Batt.Kapa.max, and at lower SOC it severely underestimates the capacity of the pack. E.g., my winter average estimated SOH using this metric is 15.3 kWh or 81% (I usually take readings in the end of the day at lower SOC).

As far as I understand (I read this in many places), Batt.Kapa.max is calculated on the basis of the amount of kWh that sneaked into the battery during the last charging session. It corresponds with my observations - the number very rarely fluctuates between charges, but often shifts to some new value immediately following a new charging session. Should this assumption be correct, I would use Batt.Kapa as the true indicator of battery health, since the capacity of a bottle is the volume of liquid you could pour into it.

Now, to avoid extrapolation errors, it's best to discharge the battery to zero (this would mean 8% of the true battery SOC), and then charge it to 100% SOC (94% true SOC of a healthy battery) and read Batt.Kapa.max. In this case the indicator will represent the most correct SOH possible, I assume. Well, unless the BMS messes things up due to some reason, actively precluding the battery to be filled up to its full.

JLB2
Posts: 6
Joined: Fri Jan 25, 2019 9:41 am

Re: Battery capacity issue

Fri Feb 08, 2019 1:24 pm

My home charger (Chargepoint) gives me the amount of KWH's that was transferred to my i3 during a charging session. Starting at an indicated range of 5 miles, Chargepoint said that about 15.5 kwh's were required to charge my car to 100%. That about matches what kappa.max indicates. I suppose 2nd source of data is a good validation of BMW's number.

We are all experiencing degradation to one extent or another. Degradation happens at an increasing rate so I expect more and more i3's will start to hit kappa.max of 12kwh's or less in the next 2 years. The 2013's and 2014's will be 8 and 7 years old at that point, still within the warranty period.

It would be helpful to all if we continue to report degradation issues and BMW responses. I only hope BMW doesn't stiff us out of a warranty replacement.

If anyone learns of a battery replacement under warranty, please do share it here.

jadnashuanh
Posts: 4339
Joined: Thu May 22, 2014 2:07 pm
Location: Nashua, NH USA

Re: Battery capacity issue

Fri Feb 08, 2019 3:40 pm

From an engineering teardown (Munroe sp?) they indicated that BMW chose a higher cost, longer lived battery chemistry versus that used in the Tesla...time will tell. Now, whether that is also true on the subsequent capacity increases, I don't know, as the analysis was done on the original battery chemistry/configuration.
Jim DeBruycker
2011 535i x-drive GT, 2014 i3 BEV

alohart
Posts: 1537
Joined: Sat Nov 01, 2014 7:36 pm
Location: Honolulu, HI

Re: Battery capacity issue

Fri Feb 08, 2019 5:36 pm

JLB2 wrote:My home charger (Chargepoint) gives me the amount of KWH's that was transferred to my i3 during a charging session.

That's not the energy actually used to charge the battery pack due to some of the energy being converted to waste heat during charging. The Idaho National Labs measured a 2014 i3's charging efficiency at various charging powers. The most efficient charging, 93%, was 30 A @ 240 V. Lower charging powers were less efficient.

Whereas, Batt. Kapa. max is the calculated maximum energy that could be added to the battery pack (i.e., excluding wasted energy).

So to make these equivalent, you'd need to correct the Chargepoint energy for energy lost to heat (i.e., at least 7% less than Chargepoint reports).
Last edited by alohart on Fri Feb 08, 2019 6:27 pm, edited 1 time in total.
Aloha,
Art

2014 BMW i3 Arravani Grey, Giga World, Tech + Driving Assist, Parking Assist, DC Fast Charging, JuiceBox EVSE

jadnashuanh
Posts: 4339
Joined: Thu May 22, 2014 2:07 pm
Location: Nashua, NH USA

Re: Battery capacity issue

Fri Feb 08, 2019 6:01 pm

And, depending on the ambient conditions, some of that power could easily be used to condition the battery (heat or cool) and not actually get stored in the battery.
Jim DeBruycker
2011 535i x-drive GT, 2014 i3 BEV

JLB2
Posts: 6
Joined: Fri Jan 25, 2019 9:41 am

Re: Battery capacity issue

Mon Feb 11, 2019 8:54 am

Good points regarding Chargepoint. This further proves that my battery is degrading faster. My 2014 i3 is now 5 years old with about 48k miles driven. That's about 80% of original capacity. It degraded by about 5% per year in the last 3 years. In 2 years, it'll probably be down to 70%. I just hope BMW honors the warranty.

I wonder what they will do in 2023 when the 2014's are out of warranty and need new batteries. I'm sure many of the cars will still be functioning perfectly but needing new batteries. Will they allow owners to buy battery replacements?

TheMK
Posts: 126
Joined: Tue Sep 18, 2018 8:57 pm

Re: Battery capacity issue

Mon Feb 11, 2019 11:02 am

JLB2 wrote:I wonder what they will do in 2023 when the 2014's are out of warranty and need new batteries. I'm sure many of the cars will still be functioning perfectly but needing new batteries. Will they allow owners to buy battery replacements?



Hopefully they'll let you do a swap for a nice price, though I'm sure "nice price" will be $5000. Perhaps they will let you buy the newer 94Ah and 120Ah battery packs used in the 2017+, and 2019+ respectively. All jokes aside, I'm not sure if I will still be in my '14 in 2023, I'd like to get a '19 i3 sport BEV, or maybe a used Tesla depending on cost of ownership (Tesla has ridiculously high out of warranty costs). A lot of it comes down to how rapidly 2019 models depreciate.

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