Quote:
Originally Posted by OldDartmouthMark
Very interesting post. Some surprising revelations, at least to me.
My thoughts/questions:
- Why are they only able to do one hot lap, as the battery doesn't deplete until 6-8 laps? My impression was that you'd be able to drive them flat out until there was no charge remaining.
- Though I realize that a road course is hard on brakes, especially when you don't have a manual transmission to help engine compression take some load off the brakes to slow the car down (in an IC car), my impression was that an EV would go into regen mode on braking thus taking some load off the brakes. Or are the brakes just not that good on them?
- How long is the track? 2.5 mi? (Sonoma?) If so, does that mean with hard use that some Teslas only get like 20 miles of range? Or maybe I'm misreading it?
It's fascinating to read some of these experiences that you normally don't hear about for one reason or another.  |
Welcome to the maddening world of battery temperature management!
This is what stratifies electric vehicles in real-world terms.
Essentially, charging or discharging a battery creates heat. An EV must manage that heat to protect the batteries. Excessive heat negatively affects the battery's total potential charge capacity, and rapid thermal shock associated with fast-charging cold batteries or repeated/prolonged hard-driving exacerbates this. In all instances, the heat associated with charging and discharging causes something called 'dendrites' to form in the battery cell as infinitesimal areas of the anode and cathode in the battery, for lack of a better descriptor, form crystals(?) that prevent electrons from passing back and forth to one another through the separator. Excessive heat and/thermal shock accelerates their formation.
It's basically inevitable that some dendritic buildup occurs affecting cell charge capacity, but also not insurmountable, and most good EVs are now only likely to lose 15-20% of their total potential charge capacity over the life of the vehicle (multiple hundreds of thousands of kilometers). Furthermore, most of the long-life EVs on the road that may be influencing public opinion about battery longevity (e.g. horror stories of Nissan Leafs with 30km of charge or first-run Model S cars that have ‘lost’ half their battery capacity after a decade of regular Super Charger use) haven't had the benefit of the current generation of battery conditioning and management to moderate dendritic buildup over their lives, so past performance isn’t necessarily indicative of future trends.
This is all not too dissimilar in real-world terms to the mechanical wear endemic to a combustion engine and transmission over hundreds of thousands of kilometers of use. Mechanical parts grinding away at each other slowly saps available horsepower while worsening emissions, lowering fuel efficiency, etc. Plus, nobody who lives in cold weather can deny that the mileage (range) and performance you get in a gas car in city driving when it's well below zero is just appalling compared to fair weather. Eventually the various losses and compromises associated with a high-mileage car of any type may be a deciding factor in a vehicle replacement, not because it doesn't necessarily run or have any usable range, but rather new(er) alternatives are preferable to the owner of a worn-out vehicle.
One future bright spot, though, is that battery recycling promises to be phenomenally effective at recovering the raw anode and cathode material in a lithium ion battery, and it is
better feedstock for the manufacturing of new batteries than even mined raw materials because it is already refined and processed. And those dendrites? Apparently, they are eliminated during reprocessing, meaning the resulting new batteries are brand spanking new and ready for a whole new life. Therefore, expect the end-of-life 'scrap' value of your EV to utterly dwarf that of a used up, gutted old gas car when it's time to call it.
Back to heat management. All EVs at this point have regulators that throttle charging or discharge rates to keep heat within spec and protect the battery. During fast charging, when a lot of electrons are being shifted in the batteries, the first few minutes have a phenomenal charge rate on nearly all cars. EVs with good temperature management can sustain that high charge rate longer, though they all taper off because, remember, we're putting something like two or three weeks of a regular house's total electricity consumption into your car in a few minutes. Likewise, when one floors it, the battery goes from essentially idle to maximum discharge. In all of this, as the kids say, it's gunst to get hot.
The goal of smart EV makers is to design the whole system so that fast charging can be a normal part of EV ownership to facilitate long-distance travel or as-needed to supplement 'normal' L1 and L2 charging at home and longer-term parking situations (like at work). The heat management system should allow routine fast charging that doesn't harm the battery and facilitate a 'fast charging experience' that adds as much useable range as possible during the initial peak and then 'shapes the taper' so that the battery is 'filled' (typically limited to 80% for most EVs) to offer a lot of useable range (400+km) in a reasonable amount of time (~30 minutes), all before the cumulative heat build-up associated with fast charging requires the charge rate to be significantly lowered. All this, with confidence in the required time for a charge and concomitant range, plus a seamless user interface that lets one drive up, plug in, and start charging instantly without messing around with a user interface terminal or point of sale system, and built-in in-car entertainment options like streaming video on a big central screen, video games, wi-fi, etc., results in a total charging experience that at best can be something enjoyable, or at least be sufficiently tolerable. All of this stuff helps further explain why Tesla has made some of the controversial decisions it has made over the years (needing an app to be able to effectively use the car and access Super Chargers, having the dashboard built around a big screen, not having any user interface at the Super Chargers, etc.).
Back to the batteries. Look, I'm not a scientician, so I cannot actually explain all this, but the take-away is that the heat associated with charging and discharging is what affects battery life and this is typically simplified to say that the number of cycles of charge and discharge on a battery is what determines its longevity and charge capacity (or "loss"). The solution to heat-related battery damage is to have a large enough battery and usable range to lower the total number of charge cycles likely to occur over the life of a vehicle and to manage heat properly. The bigger the battery and range (while still being an affordable vehicle) and the more sophisticated the battery heat management, the better.
Tesla has had active temperature management in its battery packs since the Model S launched in 2012. Specifically, for cooling during charging and discharging, a fluid (glycol? oil?) is pumped through channels between all of the cells in the module and the heat is dumped overboard through a radiator. The batteries are also heated (I'm not sure if it was originally a direct heating unit or a heater in the fluid cooling loop) during cold weather operation and especially in advance of charging so that the thermal shock is minimized.
Likewise, all those sometimes goofy names for performance modes on a Tesla (e.g. "launch mode", "Ludicrous speed", "Cheetah stance", or "Plaid mode") are in part actually astonishingly sophisticated battery conditioning programs that begin to pre-heat the battery and maximize the efficacy of the heat management system in advance of heavy driving to avoid thermal shock and achieve the best possible performance with the least wear and tear on the batteries. Furthermore, when a driver puts in a Super Charger location as the destination in the navigation system, the car begins pre-conditioning the batteries an appropriate distance from the Super Charger to be able to optimally accept Level 3 fast charging with minimal battery wear and tear from the moment they plug in.
Until recently, Tesla's competitors have generally gone the route of limited or unsophisticated battery conditioning/heat management. This has generally taken the form of heatsinks built into their battery modules to passively move heat away from the cells (cells are placed into modules that are arranged to form a pack). The module heatsinks may directly contact, or just sit in proximity to, some form of pack-scale heat exchanger that moves and discharges heat. Most now use a liquid cooling loop leading to a radiator to dump heat overboard while some just have larger heat spreaders that dissipate the heat over a larger area, potentially with air channels over which blowers move air to dump heat overboard. The Nissan Leaf has been the hilariously bad outlier in that it has no battery conditioning system at all. Zero. No heatsinks, no cooling loop, no radiators, no fans. Just the battery cells cooking their lives away sealed up in their modules and further insulated inside their packs. No wonder their real world performance is so bad (terrible range loss in cold weather because of no battery heating, slow charging even on L1 and L2 chargers like most owners use at home, miserably slow fast charging because the batteries get hot so fast and cannot dump heat, poor highway performance with diminishing throttle response over time because the batteries just can't sustain the heat that's built up, etc.). Don't buy a Leaf, or judge all EVs based on them.
While most manufactures have come around to active battery conditioning, their overall heat management approach is still a carry-forward of combustion engine cars (i.e. heat is managed separately for each source, which is why there are so many radiators in most legacy OEM EVs). The heat management approach of the newer Teslas have just started lapping their competitors because in addition to active battery conditioning on all models, a few years ago they added heat pumps to harvest surplus heat from various parts of car operation to move it to where it is needed, all in one unified system. Excess battery heat is used for cabin heating before a dedicated heating coil is used. All electronics produce waste heat and this is harvested to help battery conditioning or cabin temperature. The HVAC system is used to dump car heat overboard or harvest it from the outside air and distribute it as needed. In winter, when everything needs heating, the system makes the most efficient use of what is created to minimize waste and the amount of battery capacity used in its creation. Most EVs have separate heating and cooling systems for the cabin, the electronics, the electric motors, and the batteries, but Tesla has a single unit (initially the "super bottle" and more recently the "octo-valve") that does all of this in a preposterously small form-factor unit, thus maximizing energy management while reducing weight, cost, manufacturing complexity, and most importantly, protecting the batteries.
Back to the batteries, again. Varying types of battery chemistry raise or lower the rate of formation of dendrites, in exchange for other trade-offs. Thus, a battery chemistry like LFP (Lithium Iron Phosphate) is generally considered "worse" than a NCA (Nickel-Cobalt-Aluminum Oxide) because the former has a lower energy density than the latter and charges and discharges slightly slower, meaning one needs physically more of them to hold the same total charge as a NCA battery system, and their slower discharge rates mean they aren't as suitable for high-performance vehicles. On the other hand, LFP is
far less susceptible to dendrite formation (thus longer battery life with less range loss) and is much cheaper on a raw materials basis (we have lots of cheap lithium and iron on this planet, nickel, not so much, and cobalt is problematic, to say the least, which is why Tesla is [and hopefully others follow] ditching cobalt altogether in their next-gen 4680 battery form factor and silicon-based anode battery chemistry). So if one can engineer a crazy efficient EV drivetrain, lightweight/low-drab body, and the best thermal management system in the business, like the Tesla Model 3, using LFP for the low-end models with smaller battery packs and less performance simultaneously keeps the price-point down, matches battery chemistry to use case, reserves the high-performance cells for high-margin high-performance vehicles, and actually promises a longer-life battery and
better range retention.
Finally, for high-performance EVs, a high-performance battery conditioning system is necessary. Early Model S performance versions like the P100D and those with Raven drivetrain refinements and Ludicrous Mode could humiliate even supercars in drag races but their performance would fall off on subsequent runs because despite the phenomenal power output of the motors and batteries, the heat just couldn't be sufficiently dissipated. With incremental updates to software, chassis, drivetrain, motor design, and shift to 21700 battery form factor from 18650, that hard driving-performance endurance problem began to abate. And with the Model S Plaid it is apparently a solved problem based on the numbers put down at the Nürburgring, Laguna Seca, Pike's Peak, etc. The solution was the Octo-valve, bigger radiators, smarter software, more powerful motors, lighter vehicle weight, more pre-conditioning, lower coefficient of drag, active aero, etc., etc.
Anyway, I tend to ramble in these things. I hope all this stream of consciousness was of some interest.
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