Charge Percentage

[Fisher99]

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]

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.

. . . 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

 

[Fisher99]

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]

You can hear the brake pads contacting the rotors automatically to slow the car if you let off the accelerator with a 100% charge.

 

[vreihen]

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.....

 

[jadnashuanh]

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.

 

[Fisher99]

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]

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 https://www.mybmwi3.com/forum/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/permalink/2728519863888524/?comment_tracking=%7B%22tn%22%3A%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.

 

[Srivenkat]

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]

If you are at 100% and start going down hill, you have used no appreciable percentage of the battery before braking is required.

 

[Srivenkat]

If you are at 100% and start going down hill, you have used no appreciable percentage of the battery before braking is required.

I understand this. I was wondering about when NOT going downhill. IOW, when all the momentum would have been drawn from the battery which should create the room for regen right?

 

[Idleup]

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 https://www.mybmwi3.com/forum/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/permalink/2728519863888524/?comment_tracking=%7B%22tn%22%3A%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.

This statement is not totally correct. On the 2019 120 I-3 it regens at full SOC without the use of brakes. According to the information I received, regen at full charge will still occur by adding charge to the batteries upper buffer on the battery for a period of time.

Stay Safe- Mike

 

[alohart]

On the 2019 120 I-3 it regens at full SOC without the use of brakes. According to the information I received, regen at full charge will still occur by adding charge to the batteries upper buffer on the battery for a period of time.

The charging power of even the 120 Ah battery pack is decreased as the charge level reaches its maximum allowed because charging a nearly full battery cell at high power could damage the cell. Certainly, this must also apply to charging via regen. I don't know what full regen power is, but it might be similar to the full DC charging power of 50 kW. The charging power as a pack nears its maximum allowed charge level is nowhere near 50 kW, so regen power when the charge level is high must be very low to protect the battery cells.

 

[Idleup]

Thanks for the reply however even at full charge there is sufficient high end buffer on the I-3 for regen when at full SOC. So everyone understands it does not damage the health of a lithium battery to charge it to 100% on occasion. I would highly suspect that BMW has a regen level at some point to prevent an owner going down a large hill after a recharge to prevent it from going over 95-100%.

Keep in mind, this big buffer thing is primarily in place just to protect the manufactures from warranty problems down the road. In fact, on our heavy military drones (345 lbs) we charge to 100% for increased range and discharge to 5% and we have drones which are 8-10 years old with no damage to the packs.

Regards Mike

 

[jadnashuanh]

WHen I get home, I plug my i3 in, and only unplug it when I'm ready to leave again. Been doing that for over five years now. The max range on the car is about what it was when I bought it. Unlike a lot of smaller batteries in things, the i3 literally removes the input power once it reaches what it is programmed as maximum, and, usually, may not turn on again for a very long time. It takes a fair amount of discharge before it turns power back on - it could be weeks. So, it's not like most things you plug in to recharge. The built-in buffers seem to work. Yes, on other things, charging up to an actual 100% regularly can degrade the battery, but you're not doing that on an i3.

My driveway is pretty steep. Probably more often when it's cold out than warm, but if I haven't driven mine for awhile, there's usually some rust on the rotors, when I lift off going down, I can hear the brakes...so, this is within a couple hundred feet of starting out and there'd be little opportunity to have used much energy, and I can hear the car applying the brakes versus slowing the car with regeneration.

Whether you can hear it or not depends a lot on how wet it is and how much you regularly use the brakes.

 

[alohart]

Thanks for the reply however even at full charge there is sufficient high end buffer on the I-3 for regen when at full SOC.

My point isn't that there's insufficient high charge level buffer to regen but that regen power at a high charge level must be reduced to avoid battery cell damage. Charging power is reduced considerably as the battery pack reaches its maximum allowed charge level for the same reason. On our 208 V AC Level 2 EVSE, the charging current drops to 6 A near full charge. That's only 1.2 kW. Would 1.2 kW of braking regen even be noticeable?

On the 2 down ramps that we must descend to exit our parking garage parking space, I hear the brake pads against our rusty disk rotors without pressing the brake pedal. If regen braking is occurring, it's not powerful enough to decelerate at the full regen braking rate that occurs at lower charge levels. If the i3 is trying to mimic the normal full regen power regardless of the charge level, the disk brakes must be engaged at high charge levels to compensate for the lower regen power.

Or maybe there's another explanation… The fact that an i3 can stop completely during regen seems unlikely using regen alone because regen power approaches 0 as the propulsion motor's rotation speed approaches 0. The i3's low rolling resistance wouldn't cause it to stop rolling as quickly as it does. Maybe the motor is switched to reverse as the regen power decreases causing reverse motor braking to occur. If true, this could be the way the regen braking is mimicked at high charge levels without the need to engage the physical brakes under all circumstances. Anyone know whether this would be possible?

 

[Fisher99]

It's an interesting theory and I don't have a definitive answer, but I do know that my manual specifically says to make sure that you are at a dead stop before switching from drive to reverse (or vice versa). Not that an electric motor can't handle switching directions without fully stopping the motor first, but I think BMW recommends this to reduce strain on the motor mounts. If so, it would seem strange that they would reverse the motor to provide braking.

 

[eNate]

There's a method called "injection braking" that applies current to the motor to stop it. This in common in higher end table saws to rapidly slow a blade. I wouldn't be surprised to learn the i3 utilizes this, as regenerative braking alone couldn't get the car to the near stop it's capable of. I suspect the electronics involved are also used as anti-rollback.

 

[Idleup]

Lithium cells still have upper and lower protection buffers at “absolute” full and low charge. Therefore, even if they were to reach this state, these buffers built into the cell to further protect it. It’s important to note, just because a pack is charged to 90-100% on occasion or even often, does not destroy the packs longevity permanently. While its difficult to explain, while using 90-100% will degrade the longevity, this lost lifespan or cycles can be added back by simply only charging to 80% at times, which increases cycles and life, so you can balance the life of a pack using your own battery management.

During missions our lithium powered military drones are charged to 100%, however we only charge to 80% for maintenance. We have machines which are 5-8 years old and still have the same packs they were built with still in specs.

There are hundreds of applications using the same chemistry as EV’s for industrial use, computers, RV’s, drones, bikes, etc. which work at or near absolute 100%. The only caution to 100% charging, is it leaves little room for over-charging and thermal run-away.

These big upper and lower buffers are in place to protect the car manufacturers from replacing batteries during the warranty period. If I could find a programmer to work on the IPace, I would modify my upper / lower buffers in a second, to increase my range for when I needed it. I would then change to 80 % for every-day charging for add back the longevity.

Stay Safe - Mike