Tech charging question

[DonTom]

Something I never understood about EV charging:

As we all know, EVs reduce the charge rate at above 90% SOC or so. No questions about that except for . . . .

I normally charge Electric motorcycles so these numbers below could seem low, but the same happens with cars, just larger numbers.

If I start the charge say at 24 KW, it will drop to perhaps 12 KW at 90% SOC. Still, I have no questions about that. Perhaps down to 8 KW by 95% SOC and lower and lower after that until the charge is completed. Still no questions . . .

But . . . if we start the charge 8KW, why does it need to drop at all if 12KW is okay when we start the charge at 24KW? The drop in charge power will be around the same percentage of drop regardless of how low we start with.

IOW, the charge drop is regardless of what how low we start with. Why?

I mean either 12KW is safe for the battery at 90% SOC or it is not, but which is it? I just cannot make sense out of this.

-Don- Auburn, CA

 

[lpickup]

The tapering is not a "safety" thing (well, not directly). It's just simple electricity/chemistry.

Charging a battery takes place in two stages. In the first stage, current is "pumped" into the battery at a constant rate. This is called the constant current (CC) phase. The vehicle calls for a certain amount of current that the battery engineers have determined the battery can handle (possibly based on the battery temperature--cold batteries should not be charged really fast, or even at all in the case of REALLY cold temps--they would need to be warmed up first). The charging station complies to the best of its ability. As the battery "fills", the voltage rises slowly, and because power = current * voltage, and current is constant while voltage rises slightly, you will actually see a slight power INCREASE during this time.

Then, when the state of charge reaches a certain point, again, determined by the battery engineer that designed the battery, the BMS switches over to constant voltage (CV) charging. Now instead of "pumping" in a set amount of current, the vehicle calls for a set voltage, slightly above the battery's fully charged voltage. This allows current to flow into the battery akin to the usual Ohm's law behavior (although the resistance is not inherent to the battery material itself, but rather the chemical nature of the ion transfer process inside the battery, but nonetheless, it works the same.

Ohm's law specifies V = I * R

V is the voltage differential between the charging station voltage and the current battery voltage (which again, rises slowly as the battery charges, so as this happens, V is getting smaller. I is the amount of current flowing into the battery, and R is the internal resistance of the battery. It's helpful if we re-arrange this equation to:

I = V / R

As V gets smaller (because the battery is filling up and the voltage differential is dropping) and R is getting larger (because it's getting harder to pack more ions into the anode), the amount of current flowing into the battery drops, and as a result, power (which again, is P = I * V) drops as well (in this equation, V is the pack voltage, which IS still increasing slightly, but not as fast as I is dropping).

So as you can see, the taper off (which starts immediately as the vehicle calls for a switch from CC to CV) is not being actively controlled by the vehicle, but rather by physics (or chemistry).

An analogy that I think works pretty well is to think of the battery as a container of water and the charging station as a hose. At first you just turn on the hose and actively pump a fixed flow of water into the container. But as the container gets close to full, you stop actually pumping water into the hose, and instead let one end of the hose float on top of the water in the container, and you raise the other end of the hose (which is contained to another basin of water) above the level of the container. Gravity will do its job and water will continue to flow fairly quickly down the hose. But as the level of water in the container rises, the slope of the hose gets less and less, and eventually there is not much slope at all, and the water only trickles into the container.

So that is the stock answer to your question. Hopefully that helps explain what is going on.

However, it's not entirely accurate!

New charging techniques are coming out (Hyundai/Kia employ this kind of strategy) where the current in the CC phase is dynamically stepped down throughout the charging process to more dynamically control the charging process. So for example, instead of shifting immediately to CV from CC, it may step down the CC current in stages until eventually shifting to CV. Here is the Kia EV6 charge curve for example:

You can see the slight increase in power while in CC mode from 0 to 52% (due to the battery voltage slightly increasing during this time). Then at 52% you can see a dramatic drop in power (caused by a step down of the CC current) and again from 52% to 62% we see a slight increase in power. The same step down appears at 62% and 68%, and then finally at 77% it switches over completely to CV and we see the more dramatic taper characteristic of CV phase. In a more traditional CC/CV we would see the shift to CC to CV happen earlier, and then a shallower CV slope.

 

[DonTom]

You make it sound like a design issue caused by the BMS design. Is it?

I am very aware of the CC changing to CV near the end of the charge. But can't it be designed better to allow the exact same charge rate with a lower charge to start with as I would get as I do when I start with a higher charge? After all, it is not a safety thing and a 20KW charge is a 20 KW charge regardless of how much I start with.

It seems to me it would be possible to ONLY have the charge drop at higher charges, but should not at all be necessary at lower charge rates, it is just a poor design issue. Is that the case?

There should be a fix for such, IMO. I cannot believe we do not have the technology to fix such an issue, to simply allow the exact same charge rate regardless of the rate at the start of the charge.

-Don- Auburn, CA

 

[lpickup]

Maybe you can describe the charge curve you are talking about, because maybe I don't understand what you are talking about.

All you really said was "But . . . if we start the charge 8KW"

You didn't say why you started to charge at 8kW (charger limitation maybe? high SOC? unknown?) and then you referred to a (possible) drop in charge rate after that point, but you didn't really describe what that drop was.

But I think you've also missed a key point of what I was talking about. The BMS is not limiting the charge power in any way after it switches to CV mode (well, unless it adjusts the voltage it is calling for, but I'm not aware that that is done). It's the physics of the battery itself. For a given set charger voltage, the battery will accept what it accepts. There may be environmental factors that speed up or slow down how fast the battery is going to accept charge, but it's not the BMS. Really the only place the BMS enters the picture is determining at what point the switch from CC to CV happens. After that it's in the hands of the battery itself. And for a reasonably high SOC, it's likely the charge is starting in the CV regime.

All else being equal (battery temp, etc.) what I would expect to see in the case of a charger limited 8kW charge session is that the charge rate would hold at 8kW until whatever point it normally drops below 8kW even when starting at a higher charge rate. Are you saying that you are not seeing this?

I made a quick chart to illustrate what I would expect see. These are not accurate models, just something I hacked up quickly to illustrate the concept.

In this charts, the Blue and Red lines are from a vehicle connected to a charger with a max current of "125" (the units aren't important) and Yellow and Green are from a charger with a max current of "50".

The chart shows the charge process over the course of time (as opposed to SOC).


Looking at the Red (power) and Blue (current) lines, you can see where it switches from CC to CV and tapers off. You can see the slight increase in power during CC. The same for the Yellow and Green, although the increase in power is less noticeable (as it is happening over a longer time period). But the vehicle connected to the lower power charger does hold its CC charge rate for longer, because it took longer to reach that SOC. However, the important part to see is that once it did reach that point (at time step 90 or so), the taper looked exactly the same as the blue curve after it passed the 50A point at time 68-ish).

This is the kind of behavior I would expect to see, all else being equal. Of course all is not always equal. You might have cooling equipment that uses up some of the charging station's power, even on the lower power charger because the charge is taking that much longer. It's hard for me to comment further without knowing more about the situation you are describing.

 

[DonTom]

Maybe you can describe the charge curve you are talking about, because maybe I don't understand what you are talking about.

I will try wording it a bit differently.

As we know, CCS chargers use a rather complicated program that must take all types of things into consideration to decide the charge rate for the vehicle connected. It should be able to do just about anything to decide the best charge rate for vehicle type, battery temp. etc.

But there is a drop off in charging power at the end of the charge, always, no exceptions, AFAIK. It doesn't care if you start with one KW or a hundred KW, there will be that drop in charge power at the end. I can understand this drop off will be when we start at 100 KW. Say it drops to ten KW at 99% SOC. Okay, no issue there. No questions from me about that.

What I cannot understand why it still must happen when we charge at ten KW to begin with. Why cannot it not stay at ten KW all the way to the same 99% SOC when starting with ten KW? Why does the CCS charger still need to drop its charge rate?

What is the difference between ten KW and ten KW? It comes to Zero difference, yet the CCS charger will not allow the ten KW at the end of the charge when at CV, but ONLY when starting with ten KW.

IOW, why cannot it increase the CV to still have the ten KW near the end?

BTW, this really has nothing to do with CCS. If I use a J-plug and start the charge at 1.4 KW, it will still drop down at the end. But isn't CCS smart enough to realize the drop off is NOT at all necessary at the end when starting with a very low charge rate to start with? Is it too dumb to know it can safely increase the CV to not have the drop when very low charge rates are used?

-Don- Auburn, CA

 

[lpickup]

I will try wording it a bit differently.

As we know, CCS chargers use a rather complicated program that must take all types of things into consideration to decide the charge rate for the vehicle connected. It should be able to do just about anything to decide the best charge rate for vehicle type, battery temp. etc.

As I've said before, DC fast chargers do NOT have a complicated program. It's about as simple as it gets. It's the vehicle's own BMS that has the complicated program. It tells the charger whether to provide constant current or constant voltage, and how much current or voltage to supply, and the charging station does its best to comply. That's as complicated as it gets from the charging station side.

Now the vehicle side is different story. The BMS is what decides when to switch from CC to CV, possibly with different CC values (as I showed in the first chart from in this thread). Even then it's not super complicated. It usually boils down to 4 parameters:

1. Current to request during CC
2. Voltage to request during CV
3. Pack voltage at which to switch from CC to CV
4. Minimum current flow at which the battery is considered "full" and charging should be terminated

It does of course monitor battery temperature as well and if it's too hot or too cold it will either curtail charging or adjust battery heaters/coolers as necessary. But it's actually not that complicated.

Of course in the real world, it may appear complicated because there might be a technical issue with the charger that prevents it from supplying the requested amount of power, and this may appear to be some kind of "algorithm", but it's usually not.

What I cannot understand why it still must happen when we charge at ten KW to begin with. Why cannot it not stay at ten KW all the way to the same 99% SOC when starting with ten KW? Why does the CCS charger still need to drop its charge rate?

If you've observed it is charging at 10kW at 99% SOC, then there is no reason for it to drop below 10kW even if you started at 10kW. It should be able to maintain 10kW the whole way. Well, I will add one caveat that at 99% SOC there may be some other stuff going on like cell balancing, or operating HVAC for either the cabin or battery itself. It's probably best to make your statement at 95% SOC.

So again, the question is this: is this what you are observing?

What is the difference between ten KW and ten KW? It comes to Zero difference, yet the CCS charger will not allow the ten KW at the end of the charge when at CV, but ONLY when starting with ten KW.

IOW, why cannot it increase the CV to still have the ten KW near the end?

That's not what the BMS is programmed to do, nor is that how you charge a battery. You don't tweak the CV voltage to get a specific power into the battery. That's not CONSTANT voltage. That's constant power. And Li-ion battery chemistries are not charged using constant power (I'm not sure any battery chemistries do). Constant voltage simply means you hold the charger voltage at the set constant voltage and the battery absorbs as much as it will based on the difference between the CV voltage and the pack voltage. And be aware that there are environmental factors (mainly temperature), that will impact how fast the battery will accept charge at the tail end.

BTW, this really has nothing to do with CCS. If I use a J-plug and start the charge at 1.4 KW, it will still drop down at the end.

Correct. The only difference there is the charger when using a J-plug (AC charging) is on board the vehicle, but it's essentially doing the same exact thing the CCS charger is, just at lower current. That is, it responds to the BMS's commands for current and voltage and provides as much current as it can during CC (it should always be able to supply the requested CV as it was designed to work with the battery's designed CV voltage).

But isn't CCS smart enough to realize the drop off is NOT at all necessary at the end when starting with a very low charge rate to start with? Is it too dumb to know it can safely increase the CV to not have the drop when very low charge rates are used?

I guess the answer to this question is "yes". It's not programmed to do anything with the CV voltage. The vehicle's BMS tells it what the CV voltage should be and it just complies. Your followup question is whether the BMS is too dumb, and again, the answer is also a "yes" in that it's not programmed to tweak the CV at the end to achieve constant power.

The problem with using constant power charging is that you won't be able to detect when the battery is full. With li-ion batteries, you do this by monitoring the current flowing into the battery while holding the voltage constant. When it drops below a certain minimum value (parameter #4 above), the battery is deemed "full" and you terminate the charging session. If you're going to keep tweaking the CV voltage to achieve a set amount of current into the battery, you will overcharge the battery and bad things will happen.

 

[DonTom]

IMO, if the CCS charger was simple, there would be no compatibly issues at all. And as you know, there often is. As Dave and I had yesterday in Yuba City, CA. And then the old ChargePoint FW issue.

I have even seen the opposite, where my Energica motorcycle will charge fine, but the Chevy Bolt right after me would not charge at all. The happened at the west bound Donnor Sumit Rest area. I tried to help the guy with his Bolt, to no avail. Thís was during the time this CCS free charger (Cal-Trans) was working, last year. Now broken for around a year. The strange thing is, this charger worked well on my 2022 Chevy Bolt as well as my Energica, but not his Bolt that was only a few years older. He could charge his Bolt anywhere except for there.

I just noticed the West Bound has been removed from Plugshare. The East bound, that also has not worked for more than a year, is still shown.

But I think you may have answered my question. Thanks. I also wonder if it has something to do with the cell balancing at the end of the charge. Something I forgot about. But I wouldn't think so, because of the same reasons, ten KW is still ten KW regardless of what we start with.

And I wonder if there is a way for the BMS to report to the charger when full. Seem so simple, as the SOC% is reported back to the CCS charger anyway, as we can read it right on the CCS charger. Have the slowdown at 99% SOC to be safe.

I wonder if this issue can be "fixed". I would think so. It just seems so unnecessary to lower the charge rate when it is already very low.


-Don- Auburn, CA

 

[lpickup]

IMO, if the CCS charger was simple, there would be no compatibly issues at all. And as you know, there often is. As Dave and I had yesterday in Yuba City, CA. And then the old ChargePoint FW issue.

I am not saying the CCS charger is simple. I am saying the algorithm it uses (or lack thereof) for charging is simple. What you are experiencing are things like payment systems, activation systems, and vehicle communication issues, or even technical issues with the station itself that prevent it from delivering its nameplate power. I totally agree that the situation there is a real mess.

But I think you may have answered my question. Thanks. I also wonder if it has something to do with the cell balancing at the end of the charge. Something I forgot about. But I wouldn't think so, because of the same reasons, ten KW is still ten KW regardless of what we start with.

Yes, it's possible that there is some cell balancing or other activity happening at the very tail end. That's why I would suggest looking at the 95% level and just accepting that at 99% things may get "weird" and unpredictable.

And I wonder if there is a way for the BMS to report to the charger when full. Seem so simple, as the SOC% is reported back to the CCS charger anyway, as we can read it right on the CCS charger. Have the slowdown at 99% SOC to be safe.

No, it's not simple at all. It's not possible to accurately measure the SOC during the actual process of charging. It's all based on a rough estimate. As I said, the BMS considers the battery "full" when the charge current during CV drops below a set point. It then terminates the charge session and a more accurate reading of the state of charge can then be done (probably coincident with any dynamic cell balancing). This is why some vehicles will give a slightly different range/SOC estimate after unplugging.

And then as the battery is being used (while driving) kind of the opposite happens and the SOC is estimated by using Coulomb counting (an attempt at estimating how much charge is leaving the battery while in use). It's not until you turn the vehicle off and the battery contactors are opened that a more precise calculation of SOC can be done.

I wonder if this issue can be "fixed". I would think so. It just seems so unnecessary to lower the charge rate when it is already very low.

No. Nobody is lowering the charge rate. Again, it's the physics of the battery itself! It just naturally accepts less current at the given set CV voltage as the battery fills. I know you keep thinking that it should be possible to just tweak the CV voltage, but you really can't. The battery is designed to charge at a specific CV voltage. And actually, once you do get up to 99%, the current does reach a "knee" in the curve where it quickly drops off. So I don't think there would be much to be gained anyway.

 

[DonTom]

I know you keep thinking that it should be possible to just tweak the CV voltage, but you really can't. The battery is designed to charge at a specific CV voltage.

But if you're charging at a higher KW rate, that voltage will be higher and vice versa.

IOW, at 95% SOC a fast charger will be charging a lot faster, perhaps 8KW, in the case of my motorcycle, that started out at around 24 KW. But if I am using a J-plug, using the bike's charger, starting at 6KW I will be at less than 2 KW at that same 95% SOC.

I know you keep thinking that it should be possible to just tweak the CV voltage, but you really can't. The battery is designed to charge at a specific CV voltage. And actually, once you do get up to 99%, the current does reach a "knee" in the curve where it quickly drops off. So I don't think there would be much to be gained anyway.

First, did I hear somewhere that the switch to CV from CC is at around 90% SOC?

Yes, so why cannot it have the exact same charge voltage at 93% SOC regardless of charger used to start the charge at? But yeah, you understand my question, which is why it cannot be tweaked up with the CV at the end with the same CV voltage in each case?

I will use my real numbers here based on my Enegica motorcycle under perfect weather conditions:

The CCS charge will usually start at 24 KW. It will be at around 8 KW when at around 95% SOC and dropping more above that 95% SOC when using a CCS fast charger.

It is easy for me to understand why it must lower at the end to 8 KW when I start the charge with 24 KW.

But say I start the charge with a J-plug using 8 KW direct to the battery from the vehicle charger. I will be at 2 kW at that same 95% SOC. Why cannot it not it be designed to increase the CV to have the exact same charge rate as if I were using CCS?

You say in above :

The battery is designed to charge at a specific CV voltage."

And that is exactly what I am asking. Why can't both charges be at that exact same CV at the exact same SOC?

I am just trying to better understand.

-Don- Auburn, CA

 

[lpickup]

But if you're charging at a higher KW rate, that voltage will be higher and vice versa.

No, not necessarily. P = I * V. The voltage can (and does during CV) remain constant. It's the current that is higher. And the amount of current that is flowing is a function of the voltage differential of the charger voltage and the pack voltage, as well as the effective resistance of the battery pack.

IOW, at 95% SOC a fast charger will be charging a lot faster, perhaps 8KW, in the case of my motorcycle, that started out at around 24 KW. But if I am using a J-plug, using the bike's charger, starting at 6KW I will be at less than 2 KW at that same 95% SOC.

I can only say if this is what you are seeing that either the BMS is calling for a higher CV voltage when using a fastcharger versus using AC charging, or the battery itself is allowing more current to flow at the tail end of a fast charge.

First, did I hear somewhere that the switch to CV from CC is at around 90% SOC?

That's dependent on the specific battery we are talking about. For my Tesla Model 3 the switchover is at a remarkably low SOC (like 20-25%) (likely because the amount of current it calls for in CC mode is massive: 625A). My original Nissan LEAF switched over at about 60%, and my 30kWh LEAF switched over at close to 80%. It looks like the Hyundai/Kia packs as well as Audi & Porsche I think switch over closer to 90%, but their peak current during CC mode is not that much.

Yes, so why cannot it have the exact same charge voltage at 93% SOC regardless of charger used to start the charge at? But yeah, you understand my question, which is why it cannot be tweaked up with the CV at the end with the same CV voltage in each case?

It should have the same voltage. Is there some kind of scan tool that you could use during AC charging to validate this?

I will use my real numbers here based on my Enegica motorcycle under perfect weather conditions:

The CCS charge will usually start at 24 KW. It will be at around 8 KW when at around 95% SOC and dropping more above that 95% SOC when using a CCS fast charger.

It is easy for me to understand why it must lower at the end to 8 KW when I start the charge with 24 KW.

But say I start the charge with a J-plug using 8 KW direct to the battery from the vehicle charger. I will be at 2 kW at that same 95% SOC. Why cannot it not it be designed to increase the CV to have the exact same charge rate as if I were using CCS?

It likely is calling for the same voltage (unless as I said the BMS is programmed to call for a lower voltage when using the onboard charger). If it is the same voltage, it might be some characteristic of the battery itself that is giving you the lower power.

 

[DonTom]

No, not necessarily. P = I * V. The voltage can (and does during CV) remain constant. It's the current that is higher. And the amount of current that is flowing is a function of the voltage differential of the charger voltage and the pack voltage, as well as the effective resistance of the battery pack.

Yes, but I meant with everything exactly the same except for the higher charge voltage. Higher voltage will increase the current and that will also increase the wattage if there is no other change at the battery or BMS.

I had no idea that Tesla would switch to CV at 25%. I also recall hearing that Tesla can balance the cells well before a full charge. By any chance are these two things related?

-Don- Auburn, CA

 

[lpickup]

Yes, but I meant with everything exactly the same except for the higher charge voltage. Higher voltage will increase the current and that will also increase the wattage if there is no other change at the battery or BMS.

This is kind of like the "turn the amps up to 11" thing. You really can't do that. If a battery has been designed such that a CV voltage of 420V will result in the current dropping below 10A (thus signaling the battery being full), you can't just say "well let's tweak the voltage up to 430V to get a little more power into the battery" because then you will likely overcharge it. It is vital that you maintain the specified CV voltage, especially at the tail end of the charge, as this is the only way to accurately determine when the battery is full according to the characterization that the battery engineers likely spent tons of hours testing and characterizing. You just have to live with the fact that the battery is going to take what the battery is going to take.

One other possible cause for the behavior you've described occurred to me, however. As I said before, estimating SOC while the battery is actively charging is tricky. The way it's done is by measuring the pack voltage while charging, but this is not super accurate. For one thing, with Li-ion chemistries, the slope of the voltage vs. SOC curve is extremely shallow, meaning only a few millivolts can represent a huge swing in SOC. But also, it's hard to get an accurate reading of a charging battery. Think of filling a container of water with a high pressure hose and trying to measure the height of the surface of the container. You will have waves/ripples in the surface of the water making it hard to get a real accurate measurement. With a battery, you have a number of different "stacks" of cells connected in parallel, each stack filling at a different rate. You can average them to get a reading, but they are constantly shifting with respect to each other as the battery fills, not unlike ripples in the container of water. So you need to take the SOC estimate while charging with a bit of a grain of salt. And obviously when charging at higher power, the accuracy is less than at slow power. You will get a much more accurate measurement when the contactors are open and the battery voltage settles down and stabilizes.

So I wonder if what is happening is that when fast charging, when whatever you are looking at says it's 95% (or 99%) full, that that's not really the case, that it's more like 92% or 93%, while when slow charging the reading is more accurate and you really are at 95%. That would explain some of the discrepancy you are seeing. You could test this hypothesis by seeing how long it takes to fill the battery after you hit "95%" on the fast charger. Is it taking longer than you would expect at the higher charge rate?

Using your numbers for a moment...if you time the time it takes to complete a charge while AC charging once the power drops below the 8kW (or whatever) the AC charger is providing, I would expect that to be the same as the time it takes the fast charger to complete the charge once it drops below that same power level. All regardless of what it's saying the SOC is.

A couple of things could invalidate this experiment of course. One is cell balancing that may be occurring, and more cell balancing may be required in one scheme vs the other. And the other would be if there is any battery conditioning (cooling) going on which may sap power from the system. And even the power meters you are looking at may not be reporting the same quantity. The fast charger may be reporting the power that it is putting out, but doesn't include losses in the cable and its own internal cooling systems.

I had no idea that Tesla would switch to CV at 25%. I also recall hearing that Tesla can balance the cells well before a full charge. By any chance are these two things related?

No, and not all Teslas switch to CV at 25%. Older Model S & X switched over at closer to 50%. But they also only had a max current of 375A, and with larger batteries they could take 375A for longer. Cars that can accept 250kW (at 400V) cannot withstand the higher current for as long, and that's why they switch earlier. 800V vehicles can switch at much higher SOC because the current (vis-à-vis 400V vehicles) is halved (of course they have fewer stacks in parallel which means they can't accept as much current either, but all things considered, they do still hold CC for longer).

And while it's true that Tesla can balance cells before a full charge, I think that still happens above 90% or so. So not related.

 

[DonTom]

You just have to live with the fact that the battery is going to take what the battery is going to take

But that is my question, why the battery canNOT take the exact same conditions as it did before with the higher charge, when a lower charge rate is being used to start with.

In a way, I am wondering if you're saying the battery will overcharge, say at 30 amps near the end, but only when we start with a low charge rate, but not when we charge with a high charge rate. IOW, 30 amps is fine if we start with a higher charge, but not the exact same 30-amp battery draw when we start with a lower charge rate at the exact same SOC%. That is the thing that confuses me.

And while it's true that Tesla can balance cells before a full charge, I think that still happens above 90% or so

You just have to live with the fact that the battery is going to take what the battery is going to take.

Well, this is just a curiosity thing. I just like to know how things work. I have to accept the way it is, as I do not know enough to try to change anything. I understand countless hours had to be spent figuring out EV batteries by some of our best EV battery engineers.

And while it's true that Tesla can balance cells before a full charge, I think that still happens above 90% or so.

I own a 2018 Tesla M3, AWD, LR. It gets very little use. I now have only around 12,000 miles on my Tesla. But I normally have it charge to 80% SOC. Should I charge it above 90% once in a while just to balance the batteries?

The next time I home charge it (48 -amp Tesla Wall connector) I will check the charge rate near the end and compare it at the same SOC when a Tesla Supercharge is used. I don't recall ever looking at it, as I have only charged it to full two times since I owned the car from new in year 2018.

I own many vehicles, including ICE, so none of them get much use.

My current collection of EVs:

2017 Orange Zero DS ZF 6.5 (with the 7.2KWH battery since it was replaced in warranty).
2017 Red Zero SR ZF13 w/ Pwr Tank
2020 Blue Energica Esse Esse 9
2023 Bormio Ice Energica Experia
2023 Green Zero DSR/X

2018 Blue Tesla M3, AWD, LR
2022 Red Chevy Bolt EV (not EUV).

Other weird things I have been curious about:

When I CCS charge my Energica Esse Esse 9, at around 80% SOC the screen says "cell balancing" . BTW, does the cell balancing slow down the charge rate significantly? This battery is 11.7 KWH (max capacity).

But the owner's manual for my 2023 Energica Experia says the bike will only balance the cells when the internal AC input charger is used. Says it will NOT balance with a CCS charge. This battery is a 22.5 KWH (max capacity). Useable capacity is around 5% less.

I wonder what the difference could be.

I think I heard my 2018 Tesla M3 LR, uses a 70 KWH battery. Do you know if that is the max or useable rating?

-Don- Auburn, CA

 

[lpickup]

But that is my question, why the battery canNOT take the exact same conditions as it did before with the higher charge, when a lower charge rate is being used to start with.

Somehow I just don't think I am getting through.

It's not whether the battery CAN or CANNOT take the same conditions. It's whether the battery WILL or WILL NOT take them. For whatever reason, the battery you are looking at is taking more current at the end of a fast charge when the SOC reads 95% than when it reads 95% at the end of a slow charge. But I can almost guarantee you that the CV voltage is the same. It's just that for whatever reason (and there are many reasons I have put forth), the battery is accepting more current in the former case than the latter.

In a way, I am wondering if you're saying the battery will overcharge, say at 30 amps near the end, but only when we start with a low charge rate, but not when we charge with a high charge rate. IOW, 30 amps is fine if we start with a higher charge, but not the exact same 30-amp battery draw when we start with a lower charge rate at the exact same SOC%. That is the thing that confuses me.

Remember, at the end of the charge, we are not "forcing" 30A into the battery. The battery is simply taking whatever current it will take when we set the voltage at a given set point (CV voltage). Like I said before, for whatever reason, it is taking more current at the tail end of the fast charge than it is at the tail end of the slow charge. I've put forth a few possible reasons why this may be. My comments about overcharging are just to say that if we artificially raise the CV voltage in order to obtain 30A at the tail end (when the battery would not otherwise accept it), then yes, we risk overcharging it.

I own a 2018 Tesla M3, AWD, LR. It gets very little use. I now have only around 12,000 miles on my Tesla. But I normally have it charge to 80% SOC. Should I charge it above 90% once in a while just to balance the batteries?

It wouldn't hurt to occasionally do so. It's not just to balance the batteries, but it will also help recalibrate the state of health of the battery.

When I CCS charge my Energica Esse Esse 9, at around 80% SOC the screen says "cell balancing" . BTW, does the cell balancing slow down the charge rate significantly? This battery is 11.7 KWH (max capacity).

It depends on how far out of balance the cells are and what you are considering "charge rate". What is happening during cell balancing is that it shunts charge from cells that are near "full" to those that are closer to "empty".

To put forth another analogy, imagine filling an ice cube tray with water. To get the"fullness" (SOC) of the tray, you measure the height of the highest cube in the tray (it would be very bad if you "overfilled" this in a battery, so the BMS will always take the value of the "fullest" stack in the battery). When that reaches a certain point (apparently 80% in the case of your bike), the water is shunted to the emptier cubes. You are still getting the same amount of water flowing into the ice cubes, but the measured SOC (the height of the tallest cube) will be saying the same because it won't be getting any water. What this looks like to an observer is that while you are still pumping 5kW into the battery, the SOC is not increasing (or it's increasing very slightly). If you are measuring the power going into the battery by how fast the SOC is increasing (which is certainly one way to do it), it may appear much lower than what is actually being transferred to the battery. And voila, we have yet another hypothesis as to why you are seeing the results you are.

But the owner's manual for my 2023 Energica Experia says the bike will only balance the cells when the internal AC input charger is used. Says it will NOT balance with a CCS charge. This battery is a 22.5 KWH (max capacity). Useable capacity is around 5% less.

This is not surprising. Shunting charge around is a fairly delicate process best done at lower charge rates.

I wonder what the difference could be.

I think I heard my 2018 Tesla M3 LR, uses a 70 KWH battery. Do you know if that is the max or useable rating?

I have the same car. It's a 78kWh battery. Usable is only slightly less.

 

[DonTom]

Somehow I just don't think I am getting through.

But it is my fault. I will read more carefully your previous messages. I think you probably covered all I was asking, but I need to take a bit more time to understand it all than just the quick reading I was doing previously.

I have the same car. It's a 78kWh battery. Usable is only slightly less.

It looks like the Tesla gets around 4 miles per KWH average. That's pretty good considering I am lucky to get 7 miles per KWH from my electric motorcycles with normal use.

-Don- Auburn, CA

 

[lpickup]

It looks like the Tesla gets around 4 miles per KWH average. That's pretty good considering I am lucky to get 7 miles per KWH from my electric motorcycles with normal use.

Yes. On trips I'm actually getting closer to 5 (at least 4.5). It's the around town with starts/stops and having to heat/cool the cabin to comfortable ambient temp that bring it down to 4. My lifetime efficiency is 258 Wh/mile or 3.88 mi/kWh.

 

[DonTom]

. It's whether the battery WILL or WILL NOT take them.

I finally got around to reading this entire thread a bit more slowly, trying to be sure I understand all.

But I am still not sure if I do.

WRT what I just quoted above in this message . . . . . . .

What the battery will accept near the end of the charge, when in CV. . . . is somehow dependent on what the charge started with when in CC?

It sure seems like it.

Is the guesswork of the true SOC% while charging involved in this slowdown?

-Don- Reno, NV

 

[lpickup]

I finally got around to reading this entire thread a bit more slowly, trying to be sure I understand all.

But I am still not sure if I do.

WRT what I just quoted above in this message . . . . . . .

What the battery will accept near the end of the charge, when in CV. . . . is somehow dependent on what the charge started with when in CC?

No, I'm not saying that. I guess what I am saying is that a possible explanation is that the estimates done on the SOC while in the process of being charged are not necessarily accurate, as errors may accumulate over time, and may be affected by either the duration or the speed at which the charging is taking place.

So in other words, when you say that at 95% SOC you are seeing 20A current draw in one case and 8A current draw in the other, I would suggest that in both of those cases the true SOC is not 95%. That's just an estimate that something (whatever it is that you are looking at that says 95%) is giving you, and the true SOC might be 90% in one case and 97% in the other. But in both cases, when the current draw gets down to 8A, the true SOC is the same.

So yes, indirectly, the guesswork of the true SOC% is involved. Not necessarily in the slowdown itself (which is determined by the physics of the battery), but it's the guesswork (and relatively inaccuracy of that) that is responsible for the apparent (but not actual) difference in current draw between the two cases.

Now whether this is actually the case or not, I don't know. This is just speculation on my part. But I am reasonably certain that it's not the BMS actively changing the charging profile based on whether you are doing a fast charge vs. slow charge, or started at 10% SOC vs. 50%.

 

[DonTom]

I guess what I am saying is that a possible explanation is that the estimates done on the SOC while in the process of being charged are not necessarily accurate, as errors may accumulate over time, and may be affected by either the duration or the speed at which the charging is taking place.

Let's say we had some magic that make the SOC 100.0% accurate in every case.

Would then the high current charge lower to the same charge current as with the lower current charge on the same battery at the same accurate SOC?

-Don- Reno, NV

 

[lpickup]

I suspect that would be the case, at least within a few percent. Nothing in nature is ever perfectly the same every time.