Let's talk about charging on long distance travel?

[DucRider]

No. C is the capacity of the battery in coulombs or ampere hours. I gave the textbook definition (with the quote from the textbook) in a post way at the beginning of this thread.

That's because there is no "charge capacity" or "discharge capacity" if one interprets "capacity" correctly. They are the same (except for the, we hope, tiny fraction of ions lost to SEI and dendrite formation reactions.)

That's a fine list of the reasons we don't express capacity in terms of energy. The capacity of a battery does not change with any of those variables while the energy we can extract from it does.

It is, of course, convenient to talk about capacity in terms of energy. If the vehicle uses 500 Wh per mile and can go 400 miles on a charge its battery capacity must be 200 kWh. That is, of course, the discharge capacity under average conditions just as the 400 miles is the range under average conditions.

Sorry if it confuses you, but you seem to like to use terms not usually used in relationship to EVs and their batteries.
I keep referring to "C rate", and you get stuck on the definition of "C".

I tend to use the definitions as put forth by MIT - and discussions on forums tend to use these terms/definitions as well. If not technically correct, so be it:
• C- and E- rates – In describing batteries, discharge current is often expressed as a C-rate in order to normalize against battery capacity, which is often very different between batteries. A C-rate is a measure of the rate at which a battery is discharged relative to its maximum capacity. A 1C rate means that the discharge current will discharge the entire battery in 1 hour. For a battery with a capacity of 100 Amp-hrs, this equates to a discharge current of 100 Amps. A 5C rate for this battery would be 500 Amps, and a C/2 rate would be 50 Amps. Similarly, an E-rate describes the discharge power. A 1E rate is the discharge power to discharge the entire battery in 1 hour
• Capacity or Nominal Capacity (Ah for a specific C-rate) – The coulometric capacity, the total Amp-hours available when the battery is discharged at a certain discharge current (specified as a C-rate) from 100 percent state-of-charge to the cut-off voltage. Capacity is calculated by multiplying the discharge current (in Amps) by the discharge time (in hours) and decreases with increasing C-rate.
• Energy or Nominal Energy (Wh (for a specific C-rate)) – The “energy capacity” of the battery, the total Watt-hours available when the battery is discharged at a certain discharge current (specified as a C-rate) from 100 percent state-of-charge to the cut-off voltage. Energy is calculated by multiplying the discharge power (in Watts) by the discharge time (in hours). Like capacity, energy decreases with increasing C-rate.
http://web.mit.edu/evt/summary_battery_specifications.pdf

The only difference being that "capacity" in discussions is actually referring to "energy", but I have as much hope in changing that as getting people to stop calling an EVSE the "charger".

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[ajdelange]

Sorry if it confuses you, but you seem to like to use terms not usually used in relationship to EVs and their batteries.

No confusion this end. This is pretty standard EE stuff. But my EE background is not in BEVs. Nor in the real world would I go into the marina and say I want a 7200Wh battery (600Ah) for my boat. Maybe it's just that I'm an old engineer.

I do sometimes get misled when non engineers borrow the terminology without really understanding what it means. When you write

Capacity is a standard rating for amount of discharge energy available at a given temperature and C rate.

the first thing I see is capacity and discharge energy followed by reference to a C rate which leads me to believe you are not straight on what C and C-rate mean because you have tied them to energy and C and C- rate refer to charge (though energy capacity depends on C - rate which is one of the reasons it is deprecated). Apparently you confuse them with what MIT calls E-rate

• C- and E- rates – ... Similarly, an E-rate describes the discharge power. A 1E rate is the discharge power to discharge the entire battery in 1 hour

The only difference being that "capacity" in discussions is actually referring to "energy", but I have as much hope in changing that as getting people to stop calling an EVSE the "charger".

You want the manufacturer's to specify battery size in Ah? I understand the all the EE stuff interrelating true (coulometric) capacity, battery voltage and internal impedance but I sure as hell don't want to be messing with that when trying to figure out how much range I have left when the battery gauge says 47% SoC. The energy based capacity is plenty accurate enough for that. I agree that it would be nice to know the battery capacity in Ah but obviously the average consumer wouldn't find that bit of information useful.

 

[CappyJax]

No. C is the capacity of the battery in coulombs or ampere hours.

A 1C battery can be 500mAh or it could be 100kWh. The C rating of the battery has absolutely nothing to do with the capacity of the battery.

 

[DucRider]

No confusion this end. This is pretty standard EE stuff. But my EE background is not in BEVs. Nor in the real world would I go into the marina and say I want a 7200Wh battery (600Ah) for my boat. Maybe it's just that I'm an old engineer.

I do sometimes get misled when non engineers borrow the terminology without really understanding what it means. When you write



the first thing I see is capacity and discharge energy followed by reference to a C rate which leads me to believe you are not straight on what C and C-rate mean because you have tied them to energy and C and C- rate refer to charge (though energy capacity depends on C - rate which is one of the reasons it is deprecated). Apparently you confuse them with what MIT calls E-rate



You want the manufacturer's to specify battery size in Ah? I understand the all the EE stuff interrelating true (coulometric) capacity, battery voltage and internal impedance but I sure as hell don't want to be messing with that when trying to figure out how much range I have left when the battery gauge says 47% SoC. The energy based capacity is plenty accurate enough for that. I agree that it would be nice to know the battery capacity in Ah but obviously the average consumer wouldn't find that bit of information useful.

Specifying battery capacity in kWh (not C) is the standard and is used in tax credit and rebate calculations, what manufacturers use in their specification when battery "capacity" is indicated, and generally all things EV.
Using kWh as a capacity is easy for people to calculate things like miles/kWh (or in Tesla's case watt hours per mile) and remaining range.
It is very rare that any discussion of Ah occurs, and C rate is only used to describe the relative stress placed on a battery during charging and discharging (3.2 C on a Model 3 charging @ 250 kW vs 1.4 C for a Rivian @300 kW).

Once again, sorry if these "incorrect" terms bother you, but that is the common and accepted usage:
Electricity is sold by the kWh
Battery capacity (the fuel tank) is expressed in kWh
Charging power is expressed in kW
Charging stress (rarely used) is expressed by C rate (kW/kWh)

Most EV buyers drivers will not be EEs, and therefor the above work just fine for what they want to know about a particular vehicle.
Charging stations have a kW rating in Plugshare and other apps (Tesla promotes the V3 Superchargers 250 kW capability).
Vehicle charging capability is expressed in kW (Tesla announces upgrades to charging speed in kW like the recent upgrade to the Model S to 250 kW).
Battery size is expressed in kWh. These terms are adequate for people to figure out relative specs between vehicles and most definitely sufficient for the purposes of this discussion about charging on road trips.

The next step in detail is to look a charging taper (once again using only the kW measure to remain consistent). This is where a graph can be convenient:

Easy to visualize which of the two will recharge the battery quicker, and see at what SOC% (and range of SOC%) you get the most "bang for your buck".

Bringing up the battery specs/terms like Ah, coulombs, "C" etc confuses the issue for those just wanting to know how long it takes to charge on a road trip

 

[ajdelange]

Charging taper information isn't generally useful to the user as he doesn't have it until he does the charge. The actual taper experienced depends on various things such as the capacity of the charging station, how loaded it is, temperature (of the charging equipment and the car), the initial and desired SoC of the car, the car's taper algorithm and who knows what else. A user will do better to collect some statistics (or use one of the programs that does that) to get an idea of what he can expect. As an example here are the records of 5 fast charges:

From driving an X around for a while I know that I can expect to spend 10 or 20 minutes at an SC for a partial top up to insure that I'll get home with plenty of margin but more like 40 minutes to charge for a long leg in the middle of a trip. I also know that a 120 kW charger (the most common at this point) will give me an average rate of about 90 kW.


If one is interested in such things he can deduce something about Tesla's taper philosophy which appears to be full bore (or actually quite a bit more than the rated level for the charger) as long as SoC is below 30 - 40% with a pretty rapid taper above that down to about 70% of the charger's rating followed by another taper as the vehicle approaches the desired charge rate set by the user. Note that the Paramus charge taper had more dramatic onset and that the initial power delivered was not above the stations rated (by PlugShare) capacity. Those of you who drive Teslas on the east coast know the reason for this. Paramus is a very busy station. Every stall was full when I made this charge. For all the others I was the only vehicle at the station. So sure the taper curve tells me that I'll complete my charge faster at Brattleboro than at Paramus but I already knew that because I had to do the charges to get the curves.

As Rivian drivers we are going to have charging experiences that are the same but different. The same in the sense that the we will soon collect some statistics, if informally, on how long the average Rivian charging session will last and what the average delivered power will be.

I'll add, as I often do, that it is natural to obsess over this when contemplating buying a BEV or when in anticipation of the delivery of one but in most cases the new owner will discover that he isn't going to run out of juice every day and that charging isn't much more, if any burden, than fueling with petrol. This should especially be the case with a vehicle that is marketed for "adventure". The difference between 39 minutes and 43 minutes charging time may be a burden to someone who makes his living on the road but a Rivian probably isn't the vehicle for this man.

 

[kccougar]

Given this the best you can do is try to come up with reasonable estimates. The fact of the matter is that you won't know how a Rivian will perform on such a trip until you actually drive it. It makes sense to base the estimate on what you know and that is that an R1T will have a 180 kWh battery and a nominal (EPA) range of 400 miles for consumption of 180000/400 = 450 Wh/mi. Those of us with experience with BEVs realize that this is probably a reasonable number to start with based on the size and shape of the vehicle but we all know that whatever number you observe when you actually drive the truck it won't be 450. But let's start with that. You want to drive 600 miles. At 450 Wh/mi that will require 600*0.450 = 270 kWh energy. Assuming you handle the trip the way most people do you would leave home in the morning with 90% charge (162 kWh) on board and return home with perhaps 20% (36 kWh) so that you would be replacing 270 - (162 - 36) = 144 kWh on the road. The rate at which you will charge also depends on things we don't know but there are several of the big EA stations in Utah along the main roads (and given that you apparently average 85 mph or so on this trip I am assuming that you will be on the main roads). Given this I think it safe to assume that you will be able to charge at at least 144 kW which means that you will require an hour for charging. This, of course, depends on where and how you charge. Were I doing it I'd probably stop about half way, take on enough charge to get me to the destination with about 20% in the battery, charge at the destination with enough to get me back to the charger I used on the way up and then charge there with enough to get me home with 20% on board. Among other things this means starting at low SoC which will give me faster charging. Now an hour of charging at an EA station is going to cost you 0.99*60 + 3 = $62.40 = $0.43/kWh. That's really not too bad. And it might be a bit less than that as 144 kW may be a conservative estimate depending on the mood of the particular charger when you arrive and Rivians taper algorithm. You may also wish to include the cost of getting your truck back up to your nominal around town charge level. If you took it back up to 90% (most Tesla owners don't do that) you'd be adding 126 kWh at $16.38 (based on the 13¢ average cost of a kWh in the US) for a total energy cost of $78.78 equivalent to 13.13¢ per mile. Assuming petrol costs $2.50, any ICE vehicle that does better than 19 mpg will be less expensive to operate in this mode.

Now let's quickly rework the problem assuming that you can get the full 350 kW out of an EA charger (you can't). Your 144 kWh would be picked up in 25 minutes and your total bill from EA would be 3 + 0.99*25 = $27.55 for 19.2¢/kwh. That is a bargain! Your total energy cost for the trip would be $44.13 or 7.3¢/mi and an ICE car would have to deliver more than 33.9 mpg to best it in fuel cost.

So we have sort of bounded the problem. You are going to encounter charging time of more than 25 minutes but less than an hour on the road and give EA more than $27.65 but less than $62.40 for electricity. The fact that you plan to drive fast will push up the Wh/mi and cost but the fact that you may get better than 144 kW charging will push the charging time and cost down. Tail winds will help. Head winds will hurt. Cold weather will hurt. Use of the A/C in hot weather will hurt.

It's probably worth mentioning that 80% of BEV charging is done at home. With the average cost of a kWh and 450 Wh/mi consumption that's 5.85¢/mi.

This is a good way to look at it. Bottom line for my individual circumstance is that a Rivian won't be a great replacement for my Audi TDI in terms of cost and time on these trips the way I originally hoped, but there are certainly other benefits that I'm still looking forward to.

 

[azbill]

BMW is one manufacturer that publishes both actual and usable capacities - as an example the 2020 i3 has a 33 kWh battery with 27 kWh usable. This means a 100% charge is 27 kWh. Tesla does not publish battery capacity at all (they used to have a number on their models that was loosely correlated to battery capacity, the they never claimed a P100D had a 100 kWh pack).

Chevy also advertises the Bolt as 60KW, but actual usable is 57.4KW. They do not have any recommendation against charging to 100%, but they do have a mode called "hilltop reserve" that cuts off charging at 87%. The purpose of that is to allow regenerative braking immediately, when leaving home. At full charge, regenerative braking is severely cut back in order to not overcharge the battery.

 

[azbill]

Here is a guesstimate of what the charging taper may look like for the Rivian. do nto take this as exact, but it is based on a comment form RJ that stated one could charge form 10% to 90% in 35-40 minutes with 300KW charging.

My assumptions:
- Initial 18KW on battery
- Add 81KW in 16 minutes to get 55%
- Assume charge rate drops to 50KW @ 90% (.25-.3 C rate)
- Add 63KW in 19-24 minutes after 55%
- End at 162KW (90%)

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