Push for Level 3 chargers providing coolant.

Certainly Nissan will have some input into the standardization for Level 3 (rapid) charging. I'd be interested in seeing the standard involve the *charger* providing coolant for the battery pack. That way, the vehicle doesn't need a massive cooling system for when you get to really high power (60kW on up) -- just ductwork. Why should the vehicle have to haul around such a big refrigeration system? Why should each vehicle have to have one? And level 3 chargers cost so much, it shouldn't make a big price difference for them.

And it gets better: if you have your power wires go down the center of the charging cord and the outbound coolant go around them, your cable gets actively cooled. So you don't need to have as heavy of conductors -- a really big deal at high currents. As for coolant, I like the thought of using supercritical CO2 -- there should be no issues with disconnection or contamination or loss of coolant that way. It's lightweight, so that should further help reduce cable weight, but still enough density to store a significant amount of heat. It's extremely low viscosity, so it should be able to go through even the smallest spaces in the battery pack. And since it's nonconductive, you could probably just pump it freely into the pack on one side and collect it on the other. Downsides include a relatively high minimum temperature and high pressure.

Providing external cooling for the pack for quick charges does make sense. Not sure about the CO2 choice.
Isn't that only supercritical above room temp? Also, in that case, the ducting would need to support moderately high pressures (1000 psi?).

Possibly just use dry, cold air, since the battery is designed to run in air?
Then, recovery is not an issue and supply storage is easy.

Condensation around the battery box, and even inside the battery box, will be a problem once "normal" cooling air resumes through the battery box.

But, available (or not) cooling would be a great addition to the standard.

Cars that do not need the cooling can just block the input vents.

The Charging Station can indicate to the vehicle if the cooling is (or is not) available, and the car could charge slower if no cooling is available.

In any case, like when the expected coolant is not "available" (or cool enough), the car needs to monitor its own battery temperatures, and slow (hopefully not abruptly stop) the charging as the temperatures rise.

It's supercritical at 32C. So yeah, it's not super-cold, but it could prevent very high, damaging temperatures from being reached. "Gentle" charge/discharge cycles in most battery spec sheets are 25C, while "harsh" ones are generally at 45C or more.

Dry, cold air is problematic in that it has low density, so not very much specific heat capacity or rate of conduction.

Either way, there are half a million coolant choices, and the key factor is having the charger provide the coolant. There are other supercritical gasses (for example, methane at -83C and a little over half the pressure, albeit at a third the density -- but still over 3x the density of air at that pressure), and countless liquids. Nontoxic, nonconductive liquids would have most of the same benefits as supercritical CO2 without the pressure or temperature requirement -- albeit at higher viscosity and requiring a better flushing of the system when you're done.

You know, a flushing of the system wouldn't be that bad if you designed the ducting properly (avoiding channels that are too large). Just have a cycle at the end, right before disconnect, a couple seconds of high pressure air should get the overwhelming majority of it out.

Anyway, the key point is that the vehicles shouldn't have to haul around high power cooling systems. Cooling should be the charger's job -- and by providing the vehicle coolant through the cable, they cool the cable's wires as well, thus reducing the required cable mass.

I work with cylindrical LiFePO4 cells that are 10C capable (100A out from a 10Ah cell) and 4C charging without cooling. The flat cells Nissan is using run much cooler than the cylindrical cells, and 4C charging is easy.

The Leaf cells are 33Ah. If they're not paralleled, and if they can handle 4C charging without cooling, that's 132A per cell and a 15 minute charge.

Interesting concepts, folks, but is this really necessary?!

First off, you're talking about the charging profile of individual cells. Individual cells do not a pack make. In a pack, your cells are all jammed tightly together. Cooling is a much more difficult and much more important. Nissan limits charging rates to an 80% charge in 30 minutes for a reason. There's even a bit of skepticism about that, although, personally, I suspect that the average person won't rapid charge often enough for that to be a problem.

Secondly, while a pack fan may be good enough for occasional charges at 50kW, it's certainly not enough for more frequent charges at 300kW. We do want EVs to eventually replace gas cars, right? That's not going to happen at 50kW. 30 minute charges for an (optimistic) 80 miles range won't cut it for the average American. 5 minutes per 80 miles, however, that might just do it.

50kW at 96% efficiency is 2kW waste heat -- a little more than the heat of a plug-in space heater.
300kW at 96% efficiency is 12kW waste heat -- the output of a furnace for a 1,000 square foot home.

A pack fan might be good enough in the former case, but not in the latter. You need serious cooling for that. And if people started doing this often, you *definitely* need to keep the temp down. Cooling of the cable and connectors also starts to become an issue at high currents. You know how thick of a cable you need to handle 600A or so if you're not cooling it? Even if just for minutes at a time? That's some serious copper right there!

One thing I do find interesting is that in the very long term scenario, rapid charging suddenly becomes unimportant. A car that can go 300 miles on a charge needs to be able to fill up those 300 miles really quickly to be a direct replacement for a gasoline car. But a car that can go 1000 miles on a charge... well, that's a full day's driving. So it only needs to be able to fill up overnight and during meal breaks -- 240V/80A should do the trick.

I took carpet out of the rear of my Ford Escape Hybrid yesterday and made a quick trip to dinner. Apparently, Ford has a powered fan to cool the batteries. I never noticed it with the carpet in place as it deadens the noise, but with it out, the fan was really noticable.

I don't really care what Nissan uses, whether fan or passive or whatever, so long as it does the job.

Karen, if active cooling is definied in the Level 3 standard, I think the battery pack / cabling, is already definied for the Leaf, you think they would actually modify that now, so close to actual production? They are probably going to accept the charging limitations that exist for the current design, and perhaps design an activly cooled pack for the next version. As long as the connector is defined, and standardized it can come later.. I think the active cooling is great idea, I just don't think it's going to happen, today. Putting it in the spec would be a good idea though, for the next go around of EV packs, and to get the Level 3 connector standardized.

mitch672 said:

Karen, if active cooling is definied in the Level 3 standard, I think the battery pack / cabling, is already definied for the Leaf, you think they would actually modify that now, so close to actual production?

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No. But there currently is no standard. Nissan is just using one possible Level 3 connector (TEPCO).

They are probably going to accept the charging limitations that exist for the current design, and perhaps design an activly cooled pack for the next version.

Click to expand...

Very likely. I don't expect Nissan to make such significant changes this close to launch. But I care about the future. And as a consequence, I want to see a charging standard that can scale to the point where even the average consumer would see electricity as a valid complete replacement for gasoline: multi-hundred kilowatt chargers spaced every 70 miles or less along every interstate and major state highway in the US, so that there's few places in the country that you can't get to. If we get to that point, there's nothing preventing a complete switchover to EVs, at least for cars.

Hi Karen,

The point I really want to make is - current cells can already be charged quickly. The limit isn't on battery capability, it's on the size of the wire connecting the charger to the grid. The older tech LiFePO4 cylindrical cells I work with don't gain appreciable heat during a 4C charge, and the pouch cells run cooler still and are capable of higher charge rates

Jimmy and Mitch - the battery folks (manufacturers, test labs) seem to define 'passive cooling' to include no fans, fans that push ambient air, or fans that pull heated/cooled air from the cabin thru the pack.

Active thermal management gives the pack it's own heater and/or AC unit independent of the cabin heat/air system.

I would expect that going up one wire size if necessary will be be much less expensive than adding active cooling to chargers - the bean-counters will win against us geeks in this, I suspect...

AndyH said:

The point I really want to make is - current cells can already be charged quickly. The limit isn't on battery capability, it's on the size of the wire connecting the charger to the grid.

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There are several limits.

1) There are limits for individual cells. For example, cobalt cells shouldn't be charged in under 45 minutes. A123 phosphates shouldn't be charged in under 15 minutes (last I checked). And so forth.

2) There is pack degradation with temperature. The hotter a pack runs, the shorter it's lifespan. This is dramatic even with phosphates and manganates. For example:



If you're familiar with heat flow issues, it's about volume to surface area ratios. The more cells you cram together, the worse that gets. And you really do want to cram them together; most li-ion cells are best kept under pressure in the pack. Overheating is seldom an issue for lone cells or small packs, but for EV-sized packs, it absolutely is something that you have to deal with, and is the main impediment to rapid charging offerings from most manufacturers.

3) There are connector heating issues. This is more of an issue for inductive than conductive, but it still exists for conductive. For inductive, even at 50kW, Magna had to cool the paddles. Anywhere that you have a disconnectable interface between two wires, you're going to have excess heat generation.

4) There are charging cable mass issues. Ever lifted the cable on a high power Tesla Roadster charger? They're *not* light! That's designed for under 20kw. We're talking currents 7 times higher. Cable mass becomes a serious problem for rapid charging, one that cooling significantly alleviates.

5) There's power feed issues. These can be resolved even with a small feed by use of a battery buffer.

Unfortunately, you keep talking about lone cells. For example:

The older tech LiFePO4 cylindrical cells I work with don't gain appreciable heat during a 4C charge, and the pouch cells run cooler still and are capable of higher charge rates

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Lone cells are not a pack. They don't have the sort of heating issues you get in packs. You're probably also not trying to get as much as a decade of daily service out of them, too.

Karen,

(For just a moment, please join me at the 'single cell' end of the table. )

-Older cells generate more heat than newer cells.
-Cylindrical cells generate more heat than 'laminate' cells.

This suggests that a current-tech laminate cell will generate less heat than an older tech cylindrical cell.

(Moving to meet you at the 'pack' end of the table

I would be very surprised to find that a large pack made from current-tech laminate cells would generate nearly as much heat as a pack made from 2-year-old tech cylindrical cells.

Especially when charged and discharged at or below 200A - about 3C if the LEAF pack is made of two parallel 33Ah strings.

(sorry...part 2 next)

Cells have evolved a great deal in the past couple of years. One example from the LiFePO4 world. Exhibit 1: A 10Ah cylindrical cell that is capable of 10C (100A) continuous discharge and 4C (40A) continuous charge while staying at or below a 60°C max cell temperature. Cell is 1 year old and using chemistry and production methods likely to be 2 years old. Exhibit 2: a 20Ah pouch-type cell made by a tech leader that's capable of more than 30C. (I work with the former and know folks working with the latter - this isn't 'internet knowledge'.)

The strongest LiPo that I'm aware of is capable of 60C discharge and 10C charge. Compare that with LiPo cells from 2003 that were only capable of 8C discharge and 2C charge.

A 10C charge is SIX MINUTES. A 30C charge is faster.

I completely agree that any bunch of cells stuck together will have to be managed to keep the cells cool or warm. One could use a bunch of old-tech cells and a bit of passive management. It's proven to work keeping the Gen 1 Insight's cylindrical NiMH cells relatively happy - in spite of the inefficient NiMH generating 8-10 times as much heat as liMn.

This paper from AeroVironment suggests that for most users, Level I and Level II will be the most used. We know the LEAF is going to be Level III capable, though, and that the pack needs to be kept within temperature limits. I strongly suspect that we'll find, once the cars are in our hands, that Nissan did their homework.

Andy

(sorry for the sections...apparently we have a two-link limit...)

If a single object (a cell) in free 20º air only heats to 55º in a given (charging) test, that would be 35º rise above ambient.

The same cell packed in the center of 6 others might experience 50º ambient, and thus it might heat to 85º.

So, with the heat generated inside of "packs" (no matter how little) still needs to be removed from the pack, or the temperature of the pack rises unbounded.

The single-cell tests provide only part of the data. The mechanism for extraction of heat from the pack is even more important in evaluating the temperature of any cell in the pack.

Yes.

Now take a cell that gains 40°C over ambient at 100A in free air, install it into the middle of a pack, and run it at 30A.
Compare that with a cell that gains 40°C at 30A.

Which cell will run cooler at 30A even buried inside a pack?

You don't have to remove it if you don't make it!

Look at the difference between the Gen 1 Honda Insight pack and a PHEV Prius pack.

The Insight pack is NiMH, and made from 80 cylindrical 'D' size 6.5Ah cells in three layers. It's 'passively' cooled by pulling cabin air thru the pack. Pack temperature is also managed by reducing the electric demand as necessary. In spite of both measures, areas of the pack reach 60°C - the upper limit for the cells. (Source)

The PHEV pack with which I'm familiar is LiFePO4, and made from 160 40138 size cylindrical 10Ah cells. The cells are arranged in four layers. The pack is in a metal box and installed under the carpet in the back of the car. The charger is installed under the battery box and heats the pack during charging. (Source) (This is for a prismatic pack. Watch the video and note the lack of cooling. For images of the cylindrical cells, Google AutoBeYours High Speed Stealth and roll half-way down the page. I've supplied parts and assistance to both of these businesses.)

A 100Ah LiFePO4 pack made from 100 3C charge capable cells stuffed inside a steel box with ZERO ventilation will not gain significant heat when charged at 1C - 100A at 365V - 36.5KW. Getting into the 50-70KW range with 100Ah prismatic cells (Thunder Sky, Sky Energy/CALB, HiPower) is only 1.5C (150A @365V for 54.7KW). At 1.5C charging, a 10Ah 4C capable PSI cylindrical cell cools during charging!

I don't know the LEAF pack voltage. Nissan says it's made from 48 modules. Each module is filled with 4x33Ah cells for 14.4V. If the pack is arranged into two independent parallel strings the way other OEMs built their packs, that gives us two strings of 24 modules for 345V at 66Ah. Ignoring charging losses, Level 1 charging at 1400W is only pushing 4A into the pack. Level II low @ 3.3KW takes us to 9.5A. Level II high (6.6KW) takes us to 19A. Level III at 50KW is 145A (2.2C) and Level III at 70KW is 203A for 3C.

Sorry...I just don't see a monster waiting to jump out of the battery box.
Andy

Karen - for super-fast charging where pack heat is an issue - how much heat can one pull from the cells with conduction via aluminum plates between the cells? Transfer the heat from cell to heat-sink plates to the bottom of the wide, shallow battery box. Use a cold plate in the ground that pops up to contact the bottom of the battery box during charging. Add a pre-cool phase if necessary to bring the temp down a bit if necessary, then use a combination of flowing coolant and charge rate throttling to keep things under control Might need to add pack temp status signaling from the vehicle to the charger to close the loop.

Take care of the heavy cable load with a rope and recoil unit like a fuel pump with a heavy hose. Use low resistance connectors, maybe with a lever to engage for the 98 lb driver. It doesn't completely eliminate any of the challenges, but might reduce them enough to work.

Something like this would more likely be necessary for fleet operators that would really need quick-turns - and this would mean the equipment wouldn't be generally used by the public.

Going back to whether really-really quick charging is really necessary - you've already pointed out that more storage changes the picture. What if the 2012 Leaf has a 200 mile pack?

The Aerovironment paper referenced earlier had a very interesting observation. In TEPCO fleet testing, service vehicle drivers often brought trucks back for a recharge with 'half a tank'. Once a second level III charger was installed, drivers drove farther without recharging. Their range anxiety was appearantly cured with only the AVAILABILITY of a quick charge - most of the time they never actually used it.

Why go waaaay overboard on a technological fix for a largly psychological problem when one can get great 'bang for the buck' with a cardboard cutout of a charger and a sentence in the company newsletter about the 'new charger'? Keep a tow-truck on the ready for the driver or two that might actually have used the charger if it was real. :lol:

Re, aluminum plates between cells: yes, that's passive cooling, and it helps. But not *that* much. Remember that heat flow through a wire (yeah, this is an irregular shape, but the same thing applies) is proportional to its diameter. So a thin plate doesn't help much.

As for older cells versus newer cells: yes, you can get cells that heat less than others. But you have to pay for that. For example, the new A123 prismatic cells are a piece of beauty -- barely any heating at incredible currents -- able to tolerate up to, what, 30C? Something insane like that. But they're, what, $1.50/Wh, something along those lines? You get what you pay for.

Re, your sample Thundersky pack at 1.5C: that's a 40 minute charge. Hardly something that'll replace gasoline cars. Do you think the average American will wait 40 minutes for a recharge? 8 times faster, and you might have something. And as for your "cools during charging" comment, you're still talking about lone cells.

Garygid made some good comments above. Cells on the inside have to have their heat flow out through either whatever cooling system you provide or out *through other cells* which are also heating. So you're continually compounding the heating problem the bigger you make your pack, and the way to counter this is with cooling.

A couple final comments: one, you can build a pack that lets the inner cells overheat during a rapid charge just fine. What you won't get is *longevity* out of such a pack. And lastly, a number of manufacturers have already discussed these problems. It's a known issue. It's preventing some manufacturers from even hitting the bare minimum of "rapid charging" (~40-50kW), let alone something that could actually compete with gasoline (~250-400kW).

Why this almost maniacal focus on replacing "gas cars" with friendly greenish EVs if we only fast charge here and there, 300-1000mi range? The last thing this world needs is a fleet of BEVs/PHEVs/EREVs fast charging during peak loads, and this will happen unless it's made prohibitevely expensive as gov. policy design to push people out into nightly charging only.
If you need 1000mi range, take a train, if you don't have one, kick some political butts to build it pronto.. If the Tesla drivers fund dedicated peak load capacity fine, lets have it, but not piggybacking it on the overall infrustructure.

I usually disagree when J.H. Kunstler is making fun of the EV scene and its hopes and aspirations, but this is clearly an example of technomaniacs not thinking through the systemic effects of their plans.

Mesuge said:

I usually disagree when J.H. Kunstler is making fun of the EV scene and its hopes and aspirations, but this is clearly an example of technomaniacs not thinking through the systemic effects of their plans.

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More PV = more power during day time.

So, what were you saying ?