400 volts vs 800 volts charging speeds question

[SageBrush]

To keep it simple, let's assume 200 cells in both systems. In the 400V case, you have 2 parallel chains of 100 cells in series, and in the 800V case you have a single serial chain of 200 cells. And let's assume you are trying to get 200kW into the battery. This implies 500A in the 400V case, and 250A in the 800V case.

However, in the 400V case the current splits between the two parallel chains, resulting in 250A through each chain.

In other words, it's basically equivalent to the 800V case.

So no, I don't think there is any inherent advantage to the 800V architecture in that respect.

This mirrors my understanding.

As for 800v before the pack, I gather that boils down to two things:
Money: more copper in 400v systems, more expensive electronics for 800v. For now, 400v wins
Charging cable: lower currents mean longer, lighter, and less clumsy. Water cooled 400v cables has brought the comparison closer, for up to 250 kW.

It is true that the 800v cabling will have less heat losses but I don't think the difference carries the day.

 

[Earl]

hard to pin point the reason down to a single factor

I think the simplest single (or double) factor answer is:
Safety vs efficiency or cost
high voltage = more dangerous
high current = less efficiency or more metal conductors

 

[RTPEV]

It is true that the 800v cabling will have less heat losses but I don't think the difference carries the day.

Worth pointing out that when we are talking about 800V charging, we are talking about non-Tesla charging stations, and while Superchargers can have relatively short cables (due to consistent charge port location), non-Tesla charging stations need longer cables to reach charge ports on a variety of cars. So actually this can end up being a more significant difference than it would with a Supercharger-like cable.

 

[RTPEV]

I think the simplest single (or double) factor answer is:
Safety vs efficiency or cost
high voltage = more dangerous
high current = less efficiency or more metal conductors

I think you can even get rid of the "safety" element. It's not like you can simply call 800V "less safe" and call it a day. You will have to achieve whatever safety standards are in place whether you're at 400V or 800V. To make the 800V adequately "safe" adds cost. So really the first one should read:

high voltage = more costly

(but of course it's less costly at the same time because it needs less metal). Which one wins?

 

[Earl]

high voltage = more costly

Definitely true.
However, if you cheap out with high voltage, the penalty is people and things can get fried, not just an overload of trouble tickets and bad online reviews like with low-voltage systems (not that 400v is low voltage either).

 

[Genie]

Safety wise any contact with 400vdc should have a similar outcome to contact with 800vdc, I wouldn’t call 400v safer- no touchy either one. More copper with 400, higher insulation class with 800.

 

[moa999]

One place where it can make a difference is charging.

In Australia, a local fuel company Ampol has just started rolling out a network using ABB equipment.

So dual CCS2/ CHAdeMO - but capped at 200A (which I assume is mostly the limit of a thinner and non-cooled cable).

So this nameplate 180kW charger, can only actually charge Tesla's at 80kW
(Less losses and battery heating)

 

[RTPEV]

Yes, at any particular 800V capable charging station, an 800V vehicle will likely receive more power than a 400V vehicle, provided both vehicles are capable of receiving the max current of the charger.

It's actually even more complicated than that though. Here is the operational curves from the ABB Terra spec sheet (not quite the one you showed, but in the same family):

So any given charger is going to have 3 separate limits: the voltage (X axis), current (Y axis) and power, which is the reason the upper right corner is cut off on these curves.

Unfortunately a lot of these details are going to be lost on the average consumer that pulls up to a charging station labeled "350kW" and when they plug in their vehicle they are only pulling in 200kW.

Anecdote: I was at an ElectrifyAmerica station, having difficulty with the 150kW station that we were plugged into (it was bouncing between delivering about 50kW and dropping down to almost 0). The adjacent 150kW stall opened up (but was showing unavailable on the app, and we needed to initiate with the app to get free charging), and then one of the two 350kW stalls opened up. We went to move the car to the 350 when a Bolt drives up and starts pulling into the now open 350 spot. I try to explain the situation and ask them if they can use the 150kW station (that showed unavailable for us, but was working fine). They pull into that spot, and as my wife starts to move the car over to the 350, they grab one of the cables from the 350 (EA stations have 2 cables, not to accommodate dual charging, but rather to provide access to cars with different charge port locations) that we were going to use and plugged in.

I told them that that was the station we were going to use, but they insisted that we could both charge because there were too cables. I told him it doesn't work like that (that only one of the cables could be used at any given time), and that their car could only charge at 50kW anyway, so they should just use the 150kW station they were parked at (and now blocking! -- there had been a 3 car queue when we arrived at this busy station). They would have none of this, however, explaining that they had a new, special Bolt that charged at "259" (this was the range, not the power!) and that I didn't know anything about Bolts (yeah, well my wife had owned one just prior!). It actually got a bit heated, and as that was happening, fortunately the other 350 opened up, so my wife pulled into that spot and we got a nice fast charge, but meanwhile, a Polestar pulls into the vacant 350 spot that the Bolt owner was trying to use. He plugs in and starts charging and I walk away from the Bolt people and let them figure it out themselves.

Sure enough the guy eventually walks over with his tail between his legs and admits that yes, two cars can't charge from the same EA station and yes, I was right about the car only pulling 50kW.

And this was a relatively simple case. Now we're throwing sites with different power stations, variable battery conditions, unreliable hardware, and overly simple nameplate specs into the mix and it's going to be a huge mess out there.

 

[Jeremy3292]

Yes, at any particular 800V capable charging station, an 800V vehicle will likely receive more power than a 400V vehicle, provided both vehicles are capable of receiving the max current of the charger.

It's actually even more complicated than that though. Here is the operational curves from the ABB Terra spec sheet (not quite the one you showed, but in the same family):

View attachment 845391

So any given charger is going to have 3 separate limits: the voltage (X axis), current (Y axis) and power, which is the reason the upper right corner is cut off on these curves.

Unfortunately a lot of these details are going to be lost on the average consumer that pulls up to a charging station labeled "350kW" and when they plug in their vehicle they are only pulling in 200kW.

Anecdote: I was at an ElectrifyAmerica station, having difficulty with the 150kW station that we were plugged into (it was bouncing between delivering about 50kW and dropping down to almost 0). The adjacent 150kW stall opened up (but was showing unavailable on the app, and we needed to initiate with the app to get free charging), and then one of the two 350kW stalls opened up. We went to move the car to the 350 when a Bolt drives up and starts pulling into the now open 350 spot. I try to explain the situation and ask them if they can use the 150kW station (that showed unavailable for us, but was working fine). They pull into that spot, and as my wife starts to move the car over to the 350, they grab one of the cables from the 350 (EA stations have 2 cables, not to accommodate dual charging, but rather to provide access to cars with different charge port locations) that we were going to use and plugged in.

I told them that that was the station we were going to use, but they insisted that we could both charge because there were too cables. I told him it doesn't work like that (that only one of the cables could be used at any given time), and that their car could only charge at 50kW anyway, so they should just use the 150kW station they were parked at (and now blocking! -- there had been a 3 car queue when we arrived at this busy station). They would have none of this, however, explaining that they had a new, special Bolt that charged at "259" (this was the range, not the power!) and that I didn't know anything about Bolts (yeah, well my wife had owned one just prior!). It actually got a bit heated, and as that was happening, fortunately the other 350 opened up, so my wife pulled into that spot and we got a nice fast charge, but meanwhile, a Polestar pulls into the vacant 350 spot that the Bolt owner was trying to use. He plugs in and starts charging and I walk away from the Bolt people and let them figure it out themselves.

Sure enough the guy eventually walks over with his tail between his legs and admits that yes, two cars can't charge from the same EA station and yes, I was right about the car only pulling 50kW.

And this was a relatively simple case. Now we're throwing sites with different power stations, variable battery conditions, unreliable hardware, and overly simple nameplate specs into the mix and it's going to be a huge mess out there.

And this entire story is why it is going to be awful once Tesla opens up their SuC network to non-Tesla EV's in the USA. A dreadful day indeed.

IMO

 

[RTPEV]

And this entire story is why it is going to be awful once Tesla opens up their SuC network to non-Tesla EV's in the USA. A dreadful day indeed.

IMO

Oh wait, you think that new Tesla owners (or even seasoned ones) are infallible when it comes to knowing how to user Superchargers properly?

 

[Jeremy3292]

Oh wait, you think that new Tesla owners (or even seasoned ones) are infallible when it comes to knowing how to user Superchargers properly?

No, of course not. I think you missed the point entirely. I was merely commenting on the confusion that ensues at EV chargers when there are many different makes, models, port locations, etc. Those issues do not exist at SuC. There's only one way to charge: back in and plug in.

 

[RTPEV]

No, of course not. I think you missed the point entirely. I was merely commenting on the confusion that ensues at EV chargers when there are many different makes, models, port locations, etc. Those issues do not exist at SuC. There's only one way to charge: back in and plug in.

Unless there is someone towing and doesn't want to unhitch and decides to park sideways.

Or someone not towing using the pull-in spot.

Or someone plugging into a shared V2 stall when there are tons of other open stalls. I even did this myself (unintentionally) due to a very strange stall arrangement.

Or a mixed 150/250kW site like we now have in Raleigh.

Believe me, there are plenty of ways that Tesla owners can screw up Supercharging!

Plus, I think it's a bit too soon to see how Tesla plans to implement this with respect to cable length, position of the station, available stalls with CCS, etc.

And I'll take my own advice on not make assumptions about what's going to happen when the Cybertruck starts rolling into charging stations, but that's something I'll be interested in seeing how that plays out!

 

[Earl]

but that's something I'll be interested in seeing how that plays out!

Yep. After 18 seasons, Tesla is just one continuous nail-biting performance.
Keep your popcorn handy and keep enjoying the show.

 

[Droschke]

@RTPEV -

In your opinion, as it relates to charging, between a lower voltage (lower than 4.2Vc as vMAX) and lower current (lower than what the potential max current is set to be), which one has a bigger role in helping with the battery longevity?

 

[RTPEV]

@RTPEV -

In your opinion, as it relates to charging, between a lower voltage (lower than 4.2Vc as vMAX) and lower current (lower than what the potential max current is set to be), which one has a bigger role in helping with the battery longevity?

Good question, but I'm not qualified to answer that. I imagine that for a given battery, it was characterized at many different charging regimes to find the sweet spot so that by the time you wind up with a pack with a certain Cmax and Vmax, those parameters are in fact already optimized.

 

[Droschke]

Good question, but I'm not qualified to answer that. I imagine that for a given battery, it was characterized at many different charging regimes to find the sweet spot so that by the time you wind up with a pack with a certain Cmax and Vmax, those parameters are in fact already optimized.

I agree. I've been curious about the harm of supercharging, or any DCFC fot that matter, when higher current is involved. It's no secret that Tesla throttles the charging speed after a certain amount of DCFC (I used to know the amount). To this date, and being the first owner of my S85 for over 7 years and a graduate of the Batterygate fiasco, I'm yet to be convinced that the DCFC charging is harmless to the health and longevity of our battery packs.

DCFC: Is it necessary? Yes. Is it a good Marketing? Yes. Is it good for your battery? No.

 

[RTPEV]

I agree. I've been curious about the harm of supercharging, or any DCFC fot that matter, when higher current is involved. It's no secret that Tesla throttles the charging speed after a certain amount of DCFC (I used to know the amount). To this date, and being the first owner of my S85 for over 7 years and a graduate of the Batterygate fiasco, I'm yet to be convinced that the DCFC charging is harmless to the health and longevity of our battery packs.

DCFC: Is it necessary? Yes. Is it a good Marketing? Yes. Is it good for your battery? No.

There were some studies (about a decade ago at this point) that showed that occasional DC fastcharging was actually beneficial to battery health, at least in the early Nissan LEAFs.

Like any good engineering problem, there is likely not a point that jumps out and says, yes, this is 100% without a doubt the best Cmax and Vmax for this battery. More likely there is a curve (or set of curves) that the engineers traded off battery longevity to vehicle usefulness to arrive at their final BMS parameters. That is, they realized that customers are going to demand a certain amount of fastcharging in order for their vehicles to be practical in their lives, and so set up the charging parameters such that battery degradation was minimized for that level of fastcharging. And perhaps they even monitor statistics returned from the fleet to determine what the "average" case is.

But you do raise an interesting thought: would it make sense for customers to be able to "tune" these parameters (within an allowable range) to their own usage patterns (either higher or lower Supercharging use). My guess is that the benefit gained by that would be too small to warrant the excessive training/knowledge necessary to know how to tune it though.

 

[Droschke]

There were some studies (about a decade ago at this point) that showed that occasional DC fastcharging was actually beneficial to battery health, at least in the early Nissan LEAFs.

Actually in 2015-2016 there were some observations (I would not call those research per se) in Europe based on the data which at least one monitoring portal had gathered at that time, the supercharged cars were showing less degradation than AC fed Teslas. Never heard any followup to see if the conclusion was holding up in the later years.

And perhaps they even monitor statistics returned from the fleet to determine what the "average" case is.

That's exactly what Tesla has been doing all along and a justification for the Chargegated cars (older S85 variations mostly), where the supercharging speed at the low SOC is about 40kW at best.

But you do raise an interesting thought: would it make sense for customers to be able to "tune" these parameters (within an allowable range) to their own usage patterns (either higher or lower Supercharging use). My guess is that the benefit gained by that would be too small to warrant the excessive training/knowledge necessary to know how to tune it though.

The idea of giving owners the choice to select the supercharging speed has been brought up by the old timers. There are use cases when the drivers are not in such a hurry to open that floodgate of electrons and let it pour into their cars. Or, even better, to have messages popping up before the charging sessions start advising the owners that based on the history of their charging habits the X speed of charging at this time is what's "recommended" for the good health of their packs. The owner then can accept and charge slower or reject to opt for the faster (Fastest) speed.