What happens in the edge case of losing HV during heavy braking on the freeway? Does the brake booster still function independently of the HV system, or is the driver going to have to push the pedal a lot harder?
Nothing immediately - Teslas have a both a High Voltage system for the traction battery and a Low Voltage system powered by a separate 12-15V battery. The HV system keeps the LV system charged and most critical safety related functions run on the 12V system. The booster, ABS/ESP, airbags, and steering assist are all designed to remain functional long enough for a controlled stop after an HV disconnection.
You obviously wouldn't be able to speed up again, which depending on the situation, would be where the danger lies.
Not only is the majority of an EV the exact same components as an ICE car, but the electric car industry has been using off the shelf components for decades.
Tesla buys plenty of products from them, including things like electric steering assist.
Bosch wants to stay relevant for longer than ICE cars after all, and a lot of these components were developed for ICE cars anyway.
Tesla have done a lot of vertical integration, but for other manufacturers there's a lot of common electronic components. Stuff like headlamps (even if it's a different plastic housing the board will be the same basic design), door locks, infotainment, dashboard displays where there's little reason to significantly reengineer them for an EV.
I don't think it would have to be only electric cars, if you're building a hybrid where the 12V battery is kept charged by the high voltage battery, you've got basically the same situation.
Availability of accessories seems like it would be inconvenient for any early adopters, e.g. you can readily get USB chargers, portable generators, coolers, tire inflators, battery boosters, etc. that run off 12V... if you get a 48V vehicle today, you'd either need a 110V->12V adapter to run accessories, or you'd be limited to 48V RV accessories.
Virtually all electronics need a step down (buck) converters as they run at lower voltages 5, 3.3, 1.8. 12V > 3.3- 1V would a single step. 48V ones would likely require an intermediate step. The only exception would be running some power systems where it'd require less current.
You're going to need the expensive bits of a power supply anyway to meet transient requirements, so it's not much of a savings to run at native voltage and it gives a lot of design freedom/reusability to have one voltage for everything.
The main savings is current though, because the wiring harness is one of the most expensive parts of a car.
The move to 48v is very much about efficiency within the harness backbone. For the same wattage, less amperage is needed in a higher voltage system, meaning the wires can be smaller and they produce less waste heat.
There are a few different topologies for a 48v harness, but somewhere in the line there's a 12V DC/DC converter in there somewhere.
The wiring for 48V can be a lot thinner than it is for 12V. As there is a square law involved for resistive heating it turns out that wiring for 48V can use 1/16th of the weight of copper as that for 12V.
A switchmode converter can be designed for 48V just as easily as 12V.
It's far cheaper and easier to just pluck a readily available 12V power supply off the shelf than it is to design one that will have limited applications outside of a single manufacturer.
Because all of the IC's that are attached to the battery are designed for 12V. Things like solid state relays (BTS7008 for exammple) and the 5/3.3 volt regulators.
The vacuum reservoir of the brake booster in cars with vacuum servo brakes (whether vacuum is generated by the engine or an electric pump is irrelevant) stores enough energy for 3-4 full applications of the brakes.
EV don’t use vacuum break booster systems anymore. There are much better and more efficient fully electric break booster systems out there which make a lot more sense.
Vacuum break boosters only make sense for ICE vehicles where you already have an existing air pump (the actual engine) providing free “vacuum”, they don’t make sense in EVs where you need to an extra dedicated motor to produce vacuum, to power a vacuum booster system, to boost the breaks. Much better off just using the extra electric motor to directly boost breaks, without the whole vacuum system as a middle man.
Early EVs use vacuum break boosters, but only because they were the only economical solution, given there was little demand for electric break booster systems. After all a vacuum system is cheaper, if you have a free “vacuum source”. But for last decade or so there’s been enough EVs manufactured that electric break booster systems are now more economical for EVs.
To answer GP question, the an electric break booster system is almost certainly powered off the low voltage (12V) accessory system, not the high voltage system. So a high voltage disconnect won’t prevent the break booster from working, assuming the LV battery is working correctly.
That's not reassuring. This recall is the exception. The low voltage 12V battery has been far more unreliable in EV's from all brands than the high voltage battery has been.
Is an EV like an ICE in that the 12V bus has power while the car is running even if the 12V battery is dead? In an ICE the alternator puts 13.5 volts onto the 12V bus so a dead battery will prevent a car from starting but it will stay running on a dead battery if boosted to start. I imagine an EV does something similar but I don't know.
If the car is on (high voltage battery pack energized) then there is 12V supplied from the high voltage pack through a DC to DC converter.
The 12V battery dying is only an issue if the car is parked and the high voltage battery is disconnected. Then there may not be enough power to 'wake' the car up again.
In a lot of EVs, when the car is "on" there is a DC/DC converter powering the 12V system from the HV battery. So if the car was "on" and experienced a loss of the 12V battery it could continue operating for some period of time.
I think the implication is that people could be driving with a dead 12V as if nothing is wrong. Then when they experience the HV failure in the recall, they would have no power whatsoever for safety systems.
That wasn't intended to be my implication. The replies to the original comment answered my question. Generally if you have a dead 12V you know it because you need a boost to get started, and you don't drive like that for long. I'm not worried about a cascade failure, I was worried about the 12V dying while somebody was driving and then having no brakes. That concern has been addressed.
When the 12v battery is dying, at least in a Tesla, it warns you, and starts disabling certain features, more likely to help preserve the battery. For example, heated seats run off 12v. It makes sense that a lot of commodity auto industry parts would run off 12v because the supply chain is there, and because you want low voltage in the cabin anyway.
Like another poster in this thread, my original model 3 battery went ~5 years (typical 12v failure age in a car), and I bought it for $89(!!) at Tesla. Autozone wanted $125 for the same group battery. I did a DIY replacement. For some reason, that one failed after a year and a half. Just bad luck I guess.
Yes, this drives a requirement for latent fault detection of LV battery faults. In general when a safety function is decomposed to provide statistical safety, there is a requirement to time-bound a single failure, since without that the decomposition doesn’t buy you anything. Latent fault detection is the standard option for time bounding for automotive — for aviation, you have a second escape hatch that issues that are reliably found during annual / 100 hour inspection and can be safely missed for that long can be caught by inspection instead.
> "The low voltage 12V battery has been far more unreliable in EV's from all brands than the high voltage battery has been."
Some EV makers, including Tesla, have switched to Li-ion (often LFP) low-voltage batteries. These tend to be better suited to EV duty cycles than lead-acid, and improve reliability and longevity, as well as saving space and weight.
I can't speak for all EVs but my Ford with a 400v hybrid system (DC-to-DC, no alternator) was able to keep operating perfectly with no 12v battery whatsoever. There was an assembly defect where the positive terminal connecting the battery to the fuse box eventually came partially loose and would disconnect as the engine bay warmed up. It would start up fine and drive with zero issues but then completely black out as soon as the car was turned off.
There's speculation that move to unboxed manufacturing process is going to ditch hydraulics altogether (can't easily connect hydraulics in such process) and use electric actuators for brake pads.
> EV don’t use vacuum break booster systems anymore. There are much better and more efficient fully electric break booster systems out there which make a lot more sense.
To lecture us on EV brake systems while repeatedly misspelling the word is making me twitch far too early this otherwise fine Wednesday morning.
Tesla brakes are conventional. Yes, the driver has to push harder if the hydraulics fail, same as any car. There's nothing unique to the failure here, normal cars run their boosters off of the power train too (via belts or vacuum usually, also some have 12V pumps and would be subject to exactly this kind failure).
As to whether the booster is run off of the 400V or 12V bus I don't actually know. My guess would be the latter, honestly, since the parts would be more generic. But in any case it probably doesn't matter if the main battery fails as the 12V battery is tiny and would probably not provide enough power to run the hydraulics without the DC/DC converter.
>12V battery is tiny and would probably not provide enough power
LiFePO4 is capable of providing massive amounts of current for its size (way higher than a conventional acid one). 100A is not that high amount of current to run even with 4s4p setup. A 10kg battery would be beyond sufficient (should be able to fully power the brake system for 1h use).
Note: jump on the brakes is expected to consume around 1200W
Because it fundamentally is just a laptop battery. The job of the 12V bootstrap battery is to bring the main control hardware (not even the MCU, I don't think) online so that the DC/DC converter can power the 12V bus from the main battery. There are a few other systems that expect to be powered in the event of a main battery failure, like door locks and charging.
But I really don't think hydraulics like power steering/braking are on that list. The fallback for hydraulic failure is manual pressure, just like it is on any car. It's a naturally redundant system.
2kg would be a monster of a laptop battery of course, esp since it's regular Li-Ion (not the one w/ Fe/P). LiFePO4 should be ~150Wh/kg. So indeed, the battery is 'tiny', like 1kg only. Yet, it'd have no issue supplying the brakes - it's less than 10C discharge rate. LiFePO4 tends to have much higher discharge rate which is the important part not the total capacity.
I don't doubt you could run into problems in extreme cold, but our Model Y SR (LFP pack) has been fine charging in -5C or so. The car is pretty good at managing thermals and making sure the battery doesn't get too cold.
Right, you need a heater to keep things happy! I added a little heater to my LiFePO4 battery packs that live outside. It seems fine like that to take the edge off. Way better than exploding!
Seems really odd to me to have 400V brake regeneration and run the brake booster off of 12V, but quick searches don't reveal the answer for a Tesla or even a Prius.
The most recent Teslas (Model Y Juniper) have gotten a bit less conventional as they now have blended braking. ie: the brake pedal controls both regenerative and friction brakes, with the brake pedal "feel" effectively implemented by software.
The brake booster should have enough pressure for a few seconds of hard braking. They commonly fail on my vehicle (2003 LX470) and most people get a warning during failure and are able to come to a stop during the short few second window.
