Fisher99
Posts: 52
Joined: Sun May 12, 2019 6:24 pm

Charge Percentage

Sun Jun 09, 2019 6:12 pm

So, as a new i3 owner, and wanting to get every mile possible of electric range, my gut feeling is to charge it to 100%. However, I just read a blurb where Elon Musk says that Tesla owners should only charge to 80%, to extend the service life of the battery. What do you guys do? 80% 100%? And if 80%, how do you limit it to 80% without having to keep checking on it? Seems like that would be a major pain. I want to just plug it in at night, unplug it in the morning, and be on my way...

bwilson4web
Posts: 704
Joined: Fri Apr 29, 2016 11:59 pm
Location: Huntsville, AL

Re: Charge Percentage

Sun Jun 09, 2019 7:52 pm

Fisher99 wrote:
Sun Jun 09, 2019 6:12 pm
So, as a new i3 owner, and wanting to get every mile possible of electric range, my gut feeling is to charge it to 100%. However, I just read a blurb where Elon Musk says that Tesla owners should only charge to 80%,
It is for regeneration efficiency. A fully charged battery does not have the 'head room' to accept the high power rates of regenerative braking. So the mechanical brake is used which wastes the energy that otherwise be put into the battery.
Fisher99 wrote:
Sun Jun 09, 2019 6:12 pm
. . . to extend the service life of the battery. What do you guys do? 80% 100%? And if 80%, how do you limit it to 80% without having to keep checking on it? Seems like that would be a major pain. I want to just plug it in at night, unplug it in the morning, and be on my way...
This is not quite accurate. It is efficiency, leaving enough head room so the battery can absorb regeneration energy. So the Model 3 control software allows me to limit the charge level from 50%-100%:
  • 100% - used only when leaving in the morning for a long distance leg. For our 240 mi range Model 3, we typically plan on 210 miles with a 30 mile reserve. Because I have my own, mph vs range chart, I can go further but now we're getting into 'Johnathan Livingston Seagul' areas.
  • 85% - seems to be a good level for Huntsville AL, a fairly flat, ~10-15 mile wide, river bottom town. With more hills like Little Rock or Birmingham, a lower charge level because of more regeneration. On the Great Plains like Oklahoma City and Tulsa, perhaps a little higher battery level because they are flat and spread out with longer distances.
With 50-55 kWh and three generations of Teslas, they incorporate 'lessons learned' in the control laws. In contrast, our 2014 BMW i3-REx reflects what they knew then, a fraction of what Tesla has learned. So just charge the car; drive it sensibly, and; enjoy. It is our backup for our Model 3, not our former Prius Prime.

Bob Wilson
20k/27k mi 2014 BMW i3-REx
10k/10k mi 2017 Prius Prime

Fisher99
Posts: 52
Joined: Sun May 12, 2019 6:24 pm

Re: Charge Percentage

Sun Jun 09, 2019 9:48 pm

So, if charging to 100% isn't hurting the battery pack, and the only downside is that mechanical braking happens until you drop 15% or so on SOC, then it seems to me that just letting it charge to 100% will be my method of operation.

Which leads me to ask: If the battery is at 100%, and can't absorb juice from regen braking, what actually happens when you back off the throttle? Does it just coast? Or does it somehow activate the manual brakes? Or does it still spin the generator, thus slowing you down, but somehow shunts off the juice that would normally go into the battery?

JohnKelly
Posts: 41
Joined: Sat Oct 07, 2017 7:02 am

Re: Charge Percentage

Mon Jun 10, 2019 9:31 am

You can hear the brake pads contacting the rotors automatically to slow the car if you let off the accelerator with a 100% charge.
2015 REX / Participating on plugshare w/level 2 Clipper Creek 32 amp, Moclips, Washington coast

vreihen
Posts: 87
Joined: Tue Apr 30, 2019 4:55 pm
Location: Orange County, NY (FN21vm)

Re: Charge Percentage

Mon Jun 10, 2019 10:01 am

I thought that I read somewhere about BMW software-limiting the top end of the battery pack for longevity reasons, where 100% full isn't really 100%? Am I confused and it was actually someone else doing it?

When I leave home every morning with a full charge, regen seems to work fine at the stop sign on end of my street. Don't know if it is the above leaving some battery buffer space or it is just a transparent use of the mechanical brakes, but it one-pedals the same AFAICT.....
2015 BMW i3 BEV, Giga World, Tech and Driving Assistant packages, 15K miles

jadnashuanh
Posts: 4488
Joined: Thu May 22, 2014 2:07 pm
Location: Nashua, NH USA

Re: Charge Percentage

Mon Jun 10, 2019 2:03 pm

Depending on the state of your brake rotor, you may never notice that the brakes are being applied versus regeneration slowing the car...the computer balances the two automatically. For many people, their rotors tend to get a coating of rust on them from little use, and on those, you do tend to hear the brakes engage. ON a new one, or one where the actual brakes are used regularly, the rotors may be smooth enough with any rust, and you'll never hear the brakes engage.

The car definitely won't let you actually discharge the batteries flat, and I thought I read that it leaves a buffer at the top, but unlike a typical cell phone charging system, the car literally does disconnect the acv inputs when it thinks the car is charged and only will re-enable it when the battery level drops some. So, while it may still be physically plugged in, there is no power being applied once the car determines they're full.

Battery tech on LiOn is not all the same...there is more than one way to build the cells. Samsung reported that the ones used in the i3 are designed for over 4000 recharge cycles to full before significant degradation begins to rear its ugly head. Say you did that daily, that's way beyond the 8-year, 100K mile warranty before problems occur.
Jim DeBruycker
2011 535i x-drive GT, 2014 i3 BEV

Fisher99
Posts: 52
Joined: Sun May 12, 2019 6:24 pm

Re: Charge Percentage

Mon Jun 10, 2019 6:37 pm

Very interesting information. Thanks to all who contributed! Sounds to me like I'll just plug it in at night and not worry about it. Well, probably not every night, but whenever it looks like it will need juice for the next day anyway.

Oleksiy
Posts: 82
Joined: Fri Mar 23, 2018 4:30 am
Location: Kyiv, Ukraine

Re: Charge Percentage

Tue Jun 11, 2019 12:24 am

vreihen wrote:
Mon Jun 10, 2019 10:01 am
I thought that I read somewhere about BMW software-limiting the top end of the battery pack for longevity reasons, where 100% full isn't really 100%? Am I confused and it was actually someone else doing it?

When I leave home every morning with a full charge, regen seems to work fine at the stop sign on end of my street. Don't know if it is the above leaving some battery buffer space or it is just a transparent use of the mechanical brakes, but it one-pedals the same AFAICT.....
The top of the pack is indeed limited at 94% of actual capacity for a healthy battery. In case of 60Ah it's about 4.09V cell voltage (actual 100% should be about 4.15-4.20V cell voltage, haven't seen a SOC / cell voltage curve for i3, let me know if you saw it). If you check your pack via ISTA+ software, you'll see that the upper buffer may exceed 6% in case of battery degradation. E.g., in my case the upper buffer is almost 16% of the gross pack (21.8 kWh). But that's on paper, in SOC % of the gross capacity. ISTA+ also reports cell voltage, and it's 4.09V for my car with over 10% of the battery degradation, the same as for any healthy pack out there. Judging by cell voltage alone, once your battery degrades, the bottom buffer (usually at 8%) is effectively increased, i.e. cell voltage is still quite high for very low SOC. I provided details with screenshots on this in this thread on cell balancing viewtopic.php?f=11&t=13535&p=56484#p56484.

Now, regarding the regen, it's not effectively working at 100% SOC, which is expected. The car decelerates as usual, but you can hear the brake pads being applied if you turn the music off and there is no street noise. There's a quirk there as well, I tried to find out details asking the FB community on the subject here https://www.facebook.com/groups/BMWi3/p ... %22O%22%7D. The thing is, even when you are leaving at 100% SOC and there's no actual regen (brake pads are applied instead), the car still reports regen in the trip computer data on the dashboard, just check my screenshot below with a whopping 11 miles/kWh efficiency at 100% shortly after I departed. I'm thinking the BMS just doesn't have a clue what's going on there with the batter, efficiency etc., the model that produces these estimates is very flawed.

Image

Srivenkat
Posts: 78
Joined: Sat Apr 21, 2018 11:27 am

Re: Charge Percentage

Tue Jun 11, 2019 5:17 am

I am confused by the regen not working if the car started with 100% SOC. Wouldn't the regen charge be always less than what was taken from the battery to begin with, to get the car moving (unless one starts with a downhill right after charging to 100% SOC), thus always leaving a buffer for regen?

JohnKelly
Posts: 41
Joined: Sat Oct 07, 2017 7:02 am

Re: Charge Percentage

Tue Jun 11, 2019 7:21 am

If you are at 100% and start going down hill, you have used no appreciable percentage of the battery before braking is required.
2015 REX / Participating on plugshare w/level 2 Clipper Creek 32 amp, Moclips, Washington coast

Return to “General / Main i3 Owners Forum”