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2019-03-07 11:27 by Karl Denninger
in Stupidity , 249 references Ignore this thread
Physics Does Not Care
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The "latest" wild-eyed crazy announcement from Musk is that he's installing "new" Superchargers with an even higher rate of charge capability.

The bad news is that for the real world it matters not one bit.

Lithium chemistry batteries can accept charges at very high rates up to a certain point of charge.  The limit is caused by how charging works chemically; you are transporting ions, basically, and at a certain point the exchange stops being efficient.  The faster your charge the lower the state-of-charge is where that transition comes into play.

When it does you must slow down.

Take a single-cell lithium battery such as is in a cellphone.  You can "fast charge" it at a nearly unlimited rate until it starts to heat up.  If that battery has a 3,000 mah capacity you can actually stuff current into it at 3 amps or more if it's significantly discharged -- say, if it starts at 20% of capacity.

But at about 80% capacity the cell starts to heat rapidly because the efficiency of ion transport slows down.  If you were to force-cool the cell you could keep stuffing energy in at that same rate but an ever-increasing amount of the energy would go to heat instead of increasing the state-of-charge, so it's pointless to do so.

If you increase the charge rate more the point where the heating takes over goes down in state-of-charge!

The amount of time to completely charge the battery doesn't change much.  In other words while you can definitely get to 60% charge faster by increasing the rate if you charge at an excessive rate the heating starts between 60-70% instead of at 80% or so and either way a complete charge, to 100%, requires about the same amount of time!

And by the way, all of the modern cell phones intentionally run their chargers "hot" because this speeds up the rate of charge for you but at the very-significant price of battery lifetime.  The manufacturers don't care; they literally design these chargers to abuse the cells to the point that roughly half of their cycle life is impaired should you actually "fully charge" the battery!  They set those chargers so a person who charges daily will get out of the 12 month warranty but they do not care beyond that point.  Note that most lithium-chemistry batteries have a design cycle life of ~500 cycles yet most people start having capacity problems with their phones well before 2 years pass; this is intentional as the maker doesn't give a damn once the 12 month warranty period has expired even if the battery is sealed in the phone and can't be easily replaced by the user.

If you never charge your cellphone battery beyond 80% odds are good it'll last for 3-5 years instead of 1-1/2.  My PH-1, on which I've practiced this since its first day, still has 99% of its original capacity remaining despite being nearly 8 months into my ownership.  It would be down close to 10% at this point had I charged it like most people do.

The so-called "faster" Tesla charging stations are misleading because while they can likely get your car from 20% to 60% charge materially faster than the lower-rate stations they can't charge from 60% to 80% at the faster rate and they also can't get the vehicle to 100% much faster at all.

Whether this matters depends on what sort of range expectation you have.  Remember that most gasoline cars light the "low fuel" light when you have about 50 miles of range remaining.  If you have the same expectation on an EV and subtract off 50 miles for "reserve" then taking the vehicle to a 60% state of charge quickly only gets you about 100 miles of usable driving range; that next 20% state-of-charge, which is good for about another 50 miles does not come at the accelerated rate.

If the car's charger logic allows what amounts to "forced charging" (using active cooling to prevent the pack from catching on fire) much like a cellphone often does then permanent damage is done to the pack instead.  That abuse is something the logic can easily track and if you think Tesla won't use that logic and tracking of it to allow you to do that right up until it might cost them a warranty replacement on the pack you're dead flat wrong.  Of course that sort of "management" of the damage is insanely harmful to you as an owner just as it is with a cellphone -- the incentive for Tesla to have the vehicle's logic manage the pack so that it it will not fail in-warranty, but likely will fail as soon as the warranty expires is exactly the same as it is for cellphone manufacturers!  Neither has any economic reason to give a damn one day (or mile) beyond the warranty period and in fact both have incentives to try to force you to spend more money with them as soon as the warranty has expired!

Since the software in said vehicle is closed source and there is no mandatory disclosure of exactly how such management takes place anyone care to bet against Musk having done exactly that?  After all both Apple and Samsung have gotten away with this for years and nobody has managed to tag them with it -- so why would anyone believe Musk wouldn't do the exact same thing?