Can I charge my i3 always on DC up to 80%?

[ethelberttaylo]

Hello, I have just gotten my BMW i3 2016 model and I have been wondering what my best strategy is to charging it. I have been reading that DC charging is bad for the battery's health and so is charging it to 100%. Therefore, is it "ok" to always charge my car with DC if it's just up to 80%? If not, how often can I charge with DC?

 

[mojo]

It's probably best to put your question into Google (replacing the "i3" with "EV") and you will get a huge number of well reasoned answers, already out there, without any need for someone here to type it out from scratch.

From what I can tell you don't need to worry too much or be over-cautious. Battery wear rates to date suggest that BMW's forecast of capacity decline is conservative.

Heat is the main enemy of the lithium based cells in most current EVs. The faster you charge the hotter the battery gets - so slower AC charging is better if you're not in a hurry.

Charging to 100% is not considered especially bad if you are just about to use the car, but it's not good to leave the car unused for long periods with the battery either fully charged or at a very low state of charge.

 

[Arm]

To give you an idea of my charging schedule....which I've been doing thus far for the past 5 i3's I've owned and since 2017....I charge fully once a week (and leave it plugged in) using a wallbox at home OR charge it fully once the charge hits 20-30%.

I would do that regardless of whether it's a wallbox or a fast charger outside. I've seen no battery degradation in any of the i3's I've had. If anything, it's kept the batteries healthy and my mind at peace that I have a full tank.

 

[alohart]

Hello, I have just gotten my BMW i3 2016 model and I have been wondering what my best strategy is to charging it. I have been reading that DC charging is bad for the battery's health and so is charging it to 100%. Therefore, is it "ok" to always charge my car with DC if it's just up to 80%? If not, how often can I charge with DC?

Because your i3 has a 60 Ah battery pack, I suggest being very conservative with your charging. Charge to an indicated 90% unless you might need its full range. Unfortunately, an i3 doesn't support limiting the maximum charge level with a setting as many EV's, apparently including BMW's newer EV's, do, so you'd need to do this by stopping charging yourself (I set an alarm on my watch to remind me when to stop charging).

The i3's maximum DC fast charging rate is a nominal 50 kW. That's a C rate of 50 kW/21.6 kWh = 2.3 which isn't terribly high compared with other EV's but is twice as high as with an i3 120 Ah battery pack. The C rate indicates the level of stress on battery cells during charging or discharging. I charged our 2014 with a DC fast charger maybe 5 times total and almost always only a 10-minute session to add enough energy to be able to return home comfortably. This shouldn't have degraded the battery cells noticeably. However, if I had only DC fast charged and always did so to an 80% charge level, the cell degradation rate might have increased. So it's best to use AC Level 1 (120 V) or AC Level 2 (208-240 V) charging routinely with DC fast charging only when far enough away from home to need to add some extra range quickly.

More than half of the local 2014 i3's whose owners I know have suffered such serious battery pack capacity degradation that BMW has replaced their battery packs under warranty. This is now happening with some of the local 2015 i3's. Our climate is not extreme and our driving conditions are ideal for EV's (no high speeds, relatively short distances).

I sold our 2014 BEV with ~25% capacity loss after less than 13k miles 6 months before its battery pack warranty expired. I wasn't confident that our battery pack would degrade enough to be replaced under warranty and didn't want to have an i3 with limited range. I sold it to someone who didn't need more range than it had with the recommendation that he ask for a battery pack capacity test just prior to the expiration of the battery pack warranty.

That said, many owners of i3's with 60 Ah battery packs report little capacity degradation, so it seems to be a craps shoot. Also, 94 Ah (2017-2018) and 120 Ah (2019 and later) battery packs seem to be very much less likely to degrade.

 

[chrishall212]

Hello, I have just gotten my BMW i3 2016 model and I have been wondering what my best strategy is to charging it. I have been reading that DC charging is bad for the battery's health and so is charging it to 100%. Therefore, is it "ok" to always charge my car with DC if it's just up to 80%? If not, how often can I charge with DC?

Hello, is this statement still the case please?: "Because your i3 has a 60 Ah battery pack, I suggest being very conservative with your charging. Charge to an indicated 90% unless you might need its full range. Unfortunately, an i3 doesn't support limiting the maximum charge level with a setting as many EV's, apparently including BMW's newer EV's, do, so you'd need to do this by stopping charging yourself (I set an alarm on my watch to remind me when to stop charging)."

 

[jfran2]

FWIW, I've always trusted my 2015 REX's BMS (Battery Management System) to take care of the battery, and regularly charge to 100% on my level two wallbox. Whenever I do DC charging, it's usually just to 80%, mostly because the charge rate slows down so much after that, and 80% gets me home from the charger I use most. Assuming my car started life with an 80 mile range, I've lost around 2% per year - currently down to about 65 miles of range on a warm day.

John Francis
Rolla, MO

 

[iFrog]

FWIW, I've always trusted my 2015 REX's BMS (Battery Management System) to take care of the battery, and regularly charge to 100% on my level two wallbox. Whenever I do DC charging, it's usually just to 80%, mostly because the charge rate slows down so much after that, and 80% gets me home from the charger I use most. Assuming my car started life with an 80 mile range, I've lost around 2% per year - currently down to about 65 miles of range on a warm day.

John Francis
Rolla, MO

Keep in mind that the percentage of charge shown by the car is not the actual state of charge. The i3 real battery charge is, by default, internally limited to 80% by the car battery management system. So when you see 100% on the dashboard/app, it is in fact 80%. By doing so, contrary to Tesla and others that let you charge up to 100%, the BMW is controlling the maximum charge you can input. This is why they can increase the battery effective capacity by software in order to get away from most battery replacement under warranty...

 

[alohart]

Keep in mind that the percentage of charge shown by the car is not the actual state of charge. The i3 real battery charge is, by default, internally limited to 80% by the car battery management system. So when you see 100% on the dashboard/app, it is in fact 80%. By doing so, contrary to Tesla and others that let you charge up to 100%, the BMW is controlling the maximum charge you can input. This is why they can increase the battery effective capacity by software in order to get away from most battery replacement under warranty...

From David Bricknell's Electric Vehicles and the BMW i3 (60Ah, 94Ah, & 120Ah):

“Between the bottom 10% and the top end margin (between 89.9 and 92.3% or higher) is the nominal usable battery capacity and this is the one that the car displays as relative %battery capacity.”

So a displayed 100% charge level is actually ~90%-92% of the absolute maximum charge level without the BMS limiting the charge level, depending on temperature and battery pack version, not 80%.

To avoid battery pack replacements under warranty, BMW could theoretically reduce the size of the upper and/or lower charge level buffers to increase the usable capacity. However, doing this would increase the battery cell degradation rate. If discovered, this could be quite harmful to BMW. BMW apparently didn't do this for 4 local i3's whose battery packs were replaced under warranty.

 

[airman2482]

Yup.

80% is “an urban myth”.

BMS systems charge by a few factors including temperature, current level, voltage, cell balancing and a few more factors depending on the type of cell, and BMS system.

The less control the layman has, in many cases, the better. Display something which psychologically appeases the layman.
—-

Off topic: (indirectly)

I remember when we got the first B787s. A large difference between Li, NMH, refillable NiCad and Lead Acid batteries…

The first B787s (of another company, not the one I was flying for) had problems.. the BMS system was “way different”…I was told the first few that had a battery “meltdown” (or battery fire) was because the BMS system wasn’t the proper one for the Li batteries…

( don’t know what the public media proclaimed, but the engineering bulletins from the manufacturer were accurate…)