Why do they make EV batteries round instead of flat sheets? - Page 1
Forum section: electronics / general

author: Beamin
author_rank: Super Contributor
timestamp: January 06, 2021, 02:32:17 am
content: I understand using 18650;s intially they are already being made cheaply but now they are going to a new battery 45% bigger but still round. The obvious fact is that you have alot of empty space. Why not make huge 1 sq meter X 1 cm flat sheets with metal plates in between where you can run coolant to cool or heat the batteries. No wasted space, minimal bus wires to link them together and could be cut into weird shapes to fit things like drive shafts and other things . Same with house power banks flat sheets win.

author: wraper
author_rank: Unknown
timestamp: January 06, 2021, 02:45:55 am
content: You would need to wind internal layers with extremely sharp angles to keep the square shape, and it's a big no-no. Also I guess it becomes way more difficult to produce airtight shell of such shape. IMHO you would end up with a lot of empty internal volume instead of empty external volume.EDIT: did not notice 1m square. That will not fly. You simply will not get required voltage as you need like 100 cells in series. And such large cell will be extremely unsafe besides other issues.

author: SVFeingold
author_rank: Frequent Contributor
timestamp: January 06, 2021, 03:06:31 am
content: A few reasons spring to mind...Round, steel can cells still have superior energy density to pouch-style cells. Even accounting for packing inefficiency.These batteries get tested many, many times before making their way into a car. As individual cells after formation, then during incoming quality control at the OEM, then again if they've been sitting for awhile, then again after they're assembled into modules, then again after they're assembled into packs. Sometimes multiple times at each stage. Round cells are far, far easier to handle in mass automation/testing compared to big, thin, fragile 1x1m sheets.If one of your 1x1m cells fails a test, that is a whole lot of material that now has to be scrapped and recycled. And the bigger you make it, the harder it will be to achieve the same yield.I imagine that with cells so large you might run into weird issues given that some areas of the cell will charge/discharge faster than others. Some areas may be significantly hotter or cooler than others. One such area getting overdischarged or overcharged due to chemistry imbalances, temperature imbalances, etc. is not really much different to an individual cell in a big parallel pack suffering the same. Only now this virtual "cell" is buried in some arbitrary portion of the huge single-cell pouch and there is no easy way to monitor or correct for such things. And this is a runaway process. Hotspot forms -> supplies/draws more current -> gets hotter -> repeat. "Thermal runaway" is industry-speak for "big metal fire."On the same note, current density variations throughout the cell can become a problem for the same reason.If you look at a teardown of a Tesla pack, you'll notice that all of the cells are connected with wire bonds. These serve as handy little fusable links in the event that any particular cell is subject to excessive current draw. With just a few giant cells? This sort of "last line of defense" safety doesn't work so well.It's much easier to deal with a high pack voltage (~200-400V) for such applications. With giant flat cells you either need to connect a lot of them to get this kind of voltage (again, very thin, fragile cells to make such a stack), or you need to step up a very low voltage which is much less efficient. P=VI being what it is, for a given power output your current draw goes up in inverse proportion to your voltage. Packs in cars like Teslas can easily source a couple kiloAmps in short bursts. The more you drop the voltage the more current you need to draw to achieve the same power output. More current = more heat = more cooling required + much larger harnesses, bus bars, more demand on power conversion components, etc... It's a compounding problem. There's a reason some EV makers are going to 800V packs.Definitely not saying something like this could never work. But like most things, it's more complicated then it initially seems.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: January 06, 2021, 04:32:53 am
content: They just went and bought standard items from a nearby store for the first car. And then everyone else said, " Hey, guys, this is working! Let's all make battery-powered cars now."

author: amyk
author_rank: Super Contributor
timestamp: January 06, 2021, 06:35:34 am
content: Quote from: Beamin on January 06, 2021, 02:32:17 amQuoteWhat do you think the empty spaces between 18650s are being used for...?

author: Berni
author_rank: Super Contributor
timestamp: January 06, 2021, 06:48:31 am
content: The Nissan Leaf already used flat battery cells way back. Here is a photo of its battery pack. But as you notice its small rectangular cells similar to the ones used in mobile phones as that is easier to manufacture.But yes what do you think is in the empty spots between the cells of a tesla? Its coolant. Rechanging a Tesla in half an hour on a Tesla Supercharger is no small feat. If you ever charged your phone from empty to full using the newfangled USB fast charge tech you will notice that the phone gets darn hot during it. This is fine for a phone with 1 cell, but when many cells are packed together inside a metal box with no ventilation things would get really toasty really fast. To make things worse lithium cells degrade a lot faster when cycled at high temperatures. So for Tesla to be able to have this fast charge functionality the battery is liquid cooled.Electric cars typically already have liquid cooling for the power electronics and motor. Tesla just extended that to the battery.Also EVs also have to charge in -20°C weather while lithium really hates being charged frozen, so also heaters are used to warm up the battery before charging.

author: Beamin
author_rank: Super Contributor
timestamp: January 08, 2021, 12:12:38 am
content: Just seems weird to me that its like running a car on a bunch of AA's.I didnt realize they pumped coolant inbetween they empty spaces. Now I see why they do it.

author: SVFeingold
author_rank: Frequent Contributor
timestamp: January 08, 2021, 01:53:04 am
content: They don’t pump coolant between the empty spaces per se. The batteries are attached to cooling tubes running lengthwise down the pack. Each tube has a line of cells adhered to either side. Then these assemblies are stacked up to make modules and all of the remaining space is filled with a fire retardant foam that provides some shock absorption. The potted modules are then assembled into packs. This is how the Tesla 3 and Y do it. The S and X had a different method that used only a cooling plate on the bottom (and a lesser cooling plate on top) of the cells. Much worse cooling performance hence why the S and X have abysmal thermal performance compared to the 3/Y, nevermind something like the Taycan.Even with that the 3 and Y have a lot of room for improvement from a thermal POV.

author: wraper
author_rank: Unknown
timestamp: January 08, 2021, 02:23:55 am
content: Quote from: SVFeingold on January 08, 2021, 01:53:04 amYou are completely wrong. S/X uses the same principle. But there is no flame extinguishing compound. Nor your claim about battery cooling performance bears any truth. If anything, motor temperature is limiting sustained speed. Taycans' better performance at high speeds comes from 2 gears and the fact it's not a family car.

author: Red Squirrel
author_rank: Super Contributor
timestamp: January 08, 2021, 03:42:06 am
content: I think they are also easier to manufacture, as the flat ones are basically sheets laid on top like "plates" and then need to be glued/welded etc there's just more steps than just making a few really long sheets then rolling them.Speaking of coolant, do they also pump heat through the cells for charging? Ex: can you charge an EV in -40 outside? Not everyone has a usable garage to park a car in so ideally they need to be designed so you can charge them while outside, I would hope at least.

author: Berni
author_rank: Super Contributor
timestamp: January 08, 2021, 06:07:22 am
content: Quote from: Red Squirrel on January 08, 2021, 03:42:06 amYep they do have heating elements for the battery for that reason.Only problem is that when changing from a regular extension cord the heaters take so much power that barely any power is left for actually changing the batteries. So the car becomes a glorified outdoor heater.

author: floobydust
author_rank: Super Contributor
timestamp: January 08, 2021, 06:57:25 am
content: Spiral plate technology has been around for decades in lead-acid gel cells. Quoted advantages are higher surface area, closer plate spacing, lower ESR due to the plate's interconnects, better quality control of plate thickness (pasting made as a continuous cast strip), better vibration resistance.Hawker Gates/EnerSys Cyclon and Optima are examples.US Patent 4,606,982

author: S. Petrukhin
author_rank: Super Contributor
timestamp: January 08, 2021, 11:01:57 am
content: Quote from: Berni on January 08, 2021, 06:07:22 amAnd how much energy will then be consumed from the battery to heat the cabin? And in the summer, the air conditioner is battery-powered...

author: SVFeingold
author_rank: Frequent Contributor
timestamp: January 08, 2021, 05:09:24 pm
content: Quote from: wraper on January 08, 2021, 02:23:55 amYes I got the wires crossed a bit. The powerwall uses the flat plate cooling system, not the S/X. 3/Y modules are potted. S/X are not. S and X both use similar bandolier styles to the 3/Y, except that each module in the former has a single cooling loop that spirals its way through the module. The 3/Y use multiple bandoliers per module so the coolant only has to pass through the module once, end-to-end, instead of multiple times as with the S/X.Which yes, is a much more effective cooling system. There's really no argument on which module system has better cooling. As far as the Taycan is concerned, there is likewise no argument about which cools better, and not because of the transmission. The Taycan can do multiple full-power hard launches, back-to-back, without any thermal throttling of the battery/motor. The 3 and Y cannot. As for the S it's pretty well known how rapidly the performance is impacted due to overheating. Far as family cars go, whatever floats your boat. They're both 4 seaters with a trunk.

author: wraper
author_rank: Unknown
timestamp: January 08, 2021, 06:40:51 pm
content: Quote from: SVFeingold on January 08, 2021, 05:09:24 pmDon't forget that 18650 cells are thinner = easier to cool.QuoteModel S is 5 seater. And not for gnomes on the rear seats.

author: SVFeingold
author_rank: Frequent Contributor
timestamp: January 08, 2021, 06:56:07 pm
content: Quote from: wraper on January 08, 2021, 06:40:51 pmBut kids are gnome-sized.The 18650s would be slightly easier to cool but it still doesn't make up for the inferior cooling system of the S vs. the newer modules in the 3/Y. I have no doubt the new Roadrunner cell packs will cool even better than the 2170s despite being much larger. Time will tell on that one.

author: Red Squirrel
author_rank: Super Contributor
timestamp: January 08, 2021, 06:58:29 pm
content: Quote from: Berni on January 08, 2021, 06:07:22 amThat's good to know. I'd hate to find out that you can't charge at all in cold weather, or worse, damage the battery. But yeah I imagine it will be less efficient due to needing to keep the battery heated. You'd probably want to be on a 240v outlet for charging outside. A faster charge rate may also be better since the time spent keeping the battery warm is shorter so you use less power overall. As long as the BMS is smart enough to still regulate charge based on temperature during the initial charge as it probably takes a while for the entire cells to come up to temp even if temp sensor says it's safe to charge. I'd like to think the engineers have all of this figured out.