frage #1
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Hello,

I spoke to you a bit out charging the bmw i3 after your post here (https://old.reddit.com/r/BMWi3/comments/o8ekfw/how_to_care_for_the_hv_battery/)

I am getting word from the Germans that your post is incorrect, and that the BMS does charge to 100%. I assumed you were right because of the max 42.2kw capacity there is limited amount that is useable, and I assumed that was because it never actually charges to 100%. Upon reading the SoC in the hidden menu after fully charging, the batter is indeed beyond the stated 37kwh "useable" capacity. So I believe now the ABC rule may be false, aside from doing it occasionally for the sole purpose of cell balancing, and to otherwise keep it above 50% and below 80%
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antwort #1
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Hello. I am going to respond to your concern in 3 parts.

First off with my qualifications: Senior Chief Engineer in energy division of a US based company.

    Lithium based batteries are not immune to wear and capacity loss which will lead to lower performance, the "Germans" aren't an exception.

BMU/BMS/PMC modules or segments treat lithium batteries differently depending on the design life and the application.

For example a lithium based battery in a phone is charged and discharged more deeply because it's only designed to operate for 1-3 years. Getting more performance out of the battery takes priority over it's lifespan. Smaller batteries that are fully used leave more room for other components in the device and is also cheaper to manufacture in contrast to bigger batteries that are limited in their depth of charge and discharge.

However, lithium batteries in vehicles are treated for longer life over performance. EV battery packs are topically designed for 8-10 years of operation which is usually backed by a warranty. Manufacturers go through extensive testing of battery packs to make sure these batteries last their design life. There are many ways this is achieved such as charge (voltage) limiting, power (current) limiting, and temperature management.

It is extremely easy for manufacturers to get great range out of their cars by increasing the charge depth which results in the shortening of their design life to that of a phone. As you can imagine, no one including BMW wants to replace battery packs every 1-3 years.

    Let's gather an understanding of the term "SOC". State Of Charge is most often mistaken by the indicated battery level that is usually shown to you.

SOC is derived from an SOC table written by the battery manufacturer and is directly proportional to cell voltage (non linearly).

We do not use the capacity in Wh (Watt hour) to determine the SOC. The battery capacity (energy remaining inside the battery) is calculated from SOC. We purely use individual cell voltages to determine SOC from a table.

It is typically limited from 2.3-3V @ 0% SOC and 4.2-4.5V @ 100% SOC depending on the type (chemistry) of the lithium battery.

Now we arrive at charge limiting. It is well known that cycling lithium ion batteries towards the edges of their SOC will wear them down just like we discussed in the first part of my response.

Here is an EXAGGERATED table showing the relationship between SOC and battery lifespan under testing. (This exaggerated on purpose for the sake of demonstration. Do not take these values literally).

SOC 0%-100% : 1-3 YEARS <--- consumer electronics SOC 5%-95%. : 3-5 YEARS SOC 10%-90% : 6-8 YEARS <--- BEVs/PHEVs/HEVs SOC 20%-80% : 10-12 YEARS <--- BEVs/HEVs SOC 30%-70% : 15-18 YEARS

It is the responsibility of the manufacturer to set a charge limit on their batteries depending on the design life of their products. This includes BMW and the i3. There is absolutely no question in the existence of a battery charge limit but in how much of a charge limit there is. You can go and have a look at the set charge limits on the vehicle yourself using BMW ISTA software.

    Why to follow ABC rule? Short answer: Because the manufacturer says so. Long answer: Lithium batteries are not perfectly matched to each other. Certain cells may discharge more than their neighbors. Therefore, there is a device that manages the charge of each cell or cell block to prevent them from overcharging, "balancing" the battery pack.

Most vehicles including the i3 use passive balancing. The battery cell is connected to a charger bleed resistor when it reaches it's charge limit. This means that the battery pack will not balance until it is fully charged in respect to the charge limit set by the management system.

How often does the battery need balancing? Every charge cycle. The cells will be out of balance enough to cause excessive wear in just one charge cycle.

How does battery imbalance cause excessive wear? Certain cells may discharge beyond their discharge limit. This may lead to one or more cells losing considerable capacity. This is typically observed on vehicles that fast charge more frequently than AC charge. Basically it will cause more diversity among the cell capacities in a battery pack which is not s good thing.

The "Germans" may have told you that it indeed does charge to a 100% when plugged in, and that is true. But they consider 100% as what is defined by the the engineers to be shown to the consumer. In the background that 100% is just an indicator and is not linearly related to the SOC.

The engineers have gone to great lengths to make it as easy for you to just plug it in and forget about it!

So that is my recommendation to you as well. Just plug it in as much as you can and forget about it.

Don't over think it. We already did that for you.



meine frage
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Wow, thank you for that amazing response. As you said yourself, actual SOC is derived from cell voltages. The electrify app that reads from the ODB spits out 4.2 when fully charged, which is what led to the belief that 100% is really 100%. I'm not familiar with the ISTA software, nor do I know how reliable the ODB info ist. So you think then battery still isn't at 100% when it reports 4.2v? And even if it is, it is worth keeping at 100% for the sake of balancing?

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antwort #2
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I actually had a discussion with someone about this. It seems like the top charge limit was increased to around ~90% SOC in the newer i3s compare to the ~80% SOC which is reflected in the 4.2V reading at 100% indicated. Older i3s, mime included read 4V at 100% indicated. However, 4.2V is still not 100% SOC.

Like I explained the SOC is derived from a table that the manufacturer of the cells provide. The company I work for make cells so we are very familiar with this process, it's very standard without the industry.

We come up with an SOC table based on test results. Sometimes we even offer multiple SOC tables based on how long we expect the cell to last. Basically the 100% SOC mark is at a different voltage. So in the datasheet we will write the following (again, exaggerated for demonstration, not actually values)

fully charged voltage: 4.00V @25°C for 10000 cycles fully charged voltage: 4.20V @25°C for 1000 cycles fully charged voltage: 4.40V @25°C for 100 cycles

nominal voltage: 3.7V @2.5C

Storage voltage @25°C: 3.7-4V

Of course these values are different for different chemistries as well, some might like higher voltages some might like lower. The point is that voltages change with chemistry.

It looks like two things are causing that 4.2V indication, firstly the chemistry change which allowed more energy, secondly the increased buffer or charge limit which allows more usable energy.

It gets confusing very quickly because usually the suppliers of cells aren not the ones who put it in a final product. In BMW's case, Samsung makes the cells and BMW implements them. So BMW is provided with recommended parameters to make sure their battery holds up to it's design life. That's why there are two state of charge readings. One is the one from the battery manufacturer and the other is from BMW (charge limit/top buffer excluded). We make the cells and the final product ourselves to we only have one SOC value.

Now you may ask, doesn't the higher charge limit cause more wear on my battery pack of I keep it plugged in? Yes and no. Yes if you're increasing the top buffer of an older i3 for example. No if you're talking about a newer i3. The new chemistry allows for more of the battery being used without significant wear. So basically those 80% SOC is similar to 90% SOC in the newer cars in terms of wear. The have tested this extensively just like we do.

I have actually made over half a million dollars on pack testers just for the i3 pack development program just to test them in different environments, it's not a joke to warranty battery packs that go in the hands of people for 10 years.

To summarize, 4.2V is not 100% SOC and it won't cause significant battery wear to keep it plugged in. And again, don't overthink it. The engineers have already did that for you by doing extensive testing.

I'm very happy to keep engaged with the community though, so I appreciate you reaching out. I feel like there is too much corporate secrecy going on.