Other than the 1.5. Roadsters, every other production EV has a 12V battery. Tesla must have run into problems with the 1.5 roadsters, and bagged that idea. The 12V also makes it much safer to work on the car.
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green1
Active Member
It seems to me that the battery itself isn't really the weak link, yes, they can go bad, but it strikes me that the problems are more often in the DC-DC converter, which likely harm the 12V battery. Without the 12V battery I suspect we'd see the same problems, but with even less margin for error, a problem would appear and immediately be crippling, whereas now it's crippling once the 12v is depleted. The main change I would suggest to Tesla to mitigate this would be a warning as soon as the DC-DC converter is having issues combined with a larger 12v battery to give you time to safely get yourself to a repair facility.
Well, safety requirements do push the design in that direction. They want to have a big contactor inside the pack that fails-safe to off, and so you need something to turn that on to get things started. And if that contactor then opens under some fault/disaster scenario you still want to run other critical systems (lights? ABS?) at least until the car cimes to a halt.
Having got that big contactor, you probably can't afford to have it on when the car is sleeping for vampire load reasons, so you end up running sleep mode off the 'starting battery' and topping off occasionally from the DC/DC.
IMO, this is a pretty reasonable architecture - where it's all gone wrong is that the standby power consumption is way too high. Theoretically, it should be possible to get down to smartphone-size standby power, with fast wakeup: that would put much less stress on the 12V battery and we probably wouldn't be having this discussion. But in reality the consumption is much greater and that battery is getting cycled too hard.
It wouldn't surprise me if the 12V system was originally designed on the assumption that the standby power management would be good, but then in the rush to get Model S out of the door power management work got pushed to one side. It is hugely more difficult to retro-fit power management than it is to build it in from the ground up, hence the kludgy sleep mode we have now.
JRP3
Hyperactive Member
I'm not suggesting eliminating the 12V battery, I'm saying they should enable a bypass system so that if the 12V battery fails but the DC/DC is still functional it can power up the system. I'd also like to see the 12V battery be lithium based, which would give it more reserve capacity in a lighter package, with a much higher cycle life. Make it lithium titanate chemistry for ultimate durability and safety.
Apparently it's very hard to get a DC/DC converter to work correctly without an energy sink. Also, as others have said, it keeps your brakes and other safety features alive should the traction battery not be available.
Cottonwood
Roadster#433, Model S#S37
They actually did a nice design for normal safety and safety for first responders. There is a high current contactor for the high voltage, DC connection out of the main battery. That contactor has a 12 Volt coil that closes the contacts when 12 Volts is applied. Without any 12 Volts, no high voltage, high current connection is ever made from the battery to the outside world.
The car bootstraps itself from the 12 Volt battery, getting various controllers up to speed. When everything looks normal and safe, the controllers energize the 12 Volts to that battery contactor and the car now has power from the main battery. There is a DC to DC converter from the high voltage to the 12 Volt system that will run everything and recharge the 12 Volt battery. BTW, that is really more like 14 Volts.
For the emergency first responders, if they disconnect the 12 Volt battery, the battery contactor opens and all the high Voltage issues go away. There is even a place in the Frunk just for them to cut the 12 Volt cable for this purpose.
When the car goes to sleep (a few different depths of sleep, wakeup time and Vampire Energy usage), it uses 12 Volt power and leaves the main battery disconnected. As the State of Charge (SOC) of the 12 Volt battery goes too low, the controller closes the contactor in the main battery and recharges the 12 Volt battery using the DC-DC converter.
The Vampire draw is related to the early death of 12 Volt batteries in the MS. Even deep cycle lead acid batteries have a finite number of charge-discharge cycles before they loose capacity and die. Because the controllers in the MS draw so much power, they cycle the 12 Volt battery a lot, and it just suffers. Hopefully in a future version, Tesla will redesign the controllers needed to keep the MS in a peaceful, healthy sleep to use a lot more power, but for the MS's coming off the line now, they literally suck power. BTW, leaving the HV and DC-DC on would save the life of the battery, but would make the Vampire even thirstier!
An an example of efficient energy usage, the MacBook Air that I am typing on now is using about 10 Watts (7.98V x 1.26A) to keep the screen bright enough for me to easily see it on an outside porch, keep the internet connection happy with WiFi, run a few other applications in the background, etc. That's 10 Watts and it's wide awake with screen on! See below for a power snapshot that I just took.
To sum up my rambling, Tesla did a good job partitioning the 12 Volt and high Voltage systems, but the current MS sleep controller sucks in terms of power efficiency.
PS: A video for first responders from Tesla:
PPS: Wouldn't it be nice if Tesla gave us detailed info on battery state like Apple does?
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EDIT: Yeah, what Cottonwood said. Should read the entire thing before I respond next time.
This is done for safety reasons. In certain circumstances, you want the high voltage to be limited to the battery pack and not available out in some bright orange cables. I wouldn't want to be the first responder using hydraulic equipment on the car and accidentally "find" the high voltage present. Yes, I know they have a guide but in an accident, metal twists and compromises a bunch of stuff you never expect it to.
I can appreciate that, having been in some hairy situations.
I hear the EV1 had a manual contactor override - you could push on the button and it manually closed the contactor to power up the DC-DC converter.
scott jones
Member
I got my car back from Cincy repair shop in a day. They said the 12V, although brand new, was bad. They replaced it, plugged in the ambient drivers side door that wasn't done at the factory, installed the latest software that also wasn't the latest when I got it 2 weeks ago. And fixed my creep mode that was on, when I didn't want it to be. The guys did a great job. They also fixed a few tiny scratches, but now there is actually paint on the car that looks worse than before. I'm guessing that can be buffed flat. Just thought I'd update you all on the fact the bad 12V battery are still a common happening.
scott jones
Member
I did so Mayhemm, thanks! I've pretty much read everything I can on this car before buying it... multiple times. But it's amazing how much stuff there is to forget too!
Chuckle...Well, hopefully they will use a lot less power. I think Cotty is making this sound a little worse than it actually is at this point.
Those of us that had early builds suffered huge idle power loads that could literally drain the main battery pack in a week when the car was parked at the airport, for example. Almost all of the computers in the car were awake 24/7 and the 12V battery was discharging and then recharging at such a rate that some became overheated, and most failed very early in their lives. Mine was replaced twice in the first year. The idle power ("vampire") load was so high that it could drain the main pack quickly, particularly when the 12V was near death and required hourly transfusions.
Subsequently, Tesla slowly figured out how to put these little monsters to sleep, and still wakeup relatively quickly. (The first attempts at "sleep mode" were utter failures with the various computers waking up out of proper sequence etc. Screens went blank or wacky, doors wouldn't open etc. I'm sure there was tremendous nail biting going on in Elon-land during those months, along with all the bad press about an 85kW electric car held hostage by a tiny 12V battery.)
Those days are pretty much behind us now. Is the idle power loss as low as a Leaf? Well no, but it really isn't that bad anymore.
Cottonwood
Roadster#433, Model S#S37
I agree that Tesla has greatly reduced the Vampire blood loss since the early days, but it's still pretty bad. The Vampire draw is now down to 1-2 rated miles per day; that't 300-600 Wh/day or 12.5-25.0 Watts average draw. That 12-25 Watts is with the controllers going to sleep a lot. My mac is awake, responsive and doing all its network stuff on 3.6 Watts right now. Your typical iPhone responds to the user, text messages, phone calls, etc and sleeps on power measured in milliWatts. The Prius manages a high Volt battery with DC-DC to 12 Volts, wakes for the user in reasonable time...and sleeps on about half a Watt.
Don't get me wrong; Tesla did a great job on a ground up design of a great luxury EV, but as far as a power efficient sleep design, they need a redo! The vampire does not drink to excess any more, but the vampire still sucks, the power and life out of the 12 Volt battery.
Your Mac or iPhone has one CPU (maybe two in some older Macs). It's a relatively trivial task to get sleep to work. The Tesla has many ECUs and a couple of CPUs that have to have the timing correct. To say it's a non-trivial task to get sleep right is a big understatement.
iPhones have only one (or two actually with the M7) uP, but most cars have a lot of microprocessors. The Volt has 100 uPs, and 12V power drain when off will not drain the 12V battery when sitting idle for 4-8 weeks. All cars from all automakers manage this, except for Teslas. It is not easy, but obviously very doable if you design all of the processors with a proper idle power specification from the start.
GSP
GSP
Some of the relatively recent Audi's and VW's reportedly suck electricity like water as well. It started with remote locking systems and radios with electronic station memory and has mushroomed. Some companies spend the money on CMOS or whatever they need to use to reduce idle power and others don't. There are certainly some that do it much better. All you really want running is the cell signal receiver, fob receiver, security sensors and a critical state of discharge sensor, and then CMOS all the other memory settings and turn off those computers.
Here's an old post showing the Tesla idle power losses vs those of my Leaf (which is for all intents and purposes at 0). If we didn't have 85 kW of backup, our tiny 12V's would still probably be dead overnight. When these graphs were made, the 12V battery probably wouldn't last an hour without a rescue charge from the main pack. Even with the recent improvements in sleep, there is still quite a bit of room for improvement from the world's "premier" green car company.
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Having a 12V battery is also what lets you keep listening to the radio while you are doing THIS.
And lets the car alert you to issues should the main pack fail.
And allow the brakes and power steering to work should the main pack fail.
And let the car keep logging data should the main pack fail.
And allow you to talk to the car via your remote app while the car is parked (and therefore the contactors not engaged)
Etc...
And lets the car alert you to issues should the main pack fail.
And allow the brakes and power steering to work should the main pack fail.
And let the car keep logging data should the main pack fail.
And allow you to talk to the car via your remote app while the car is parked (and therefore the contactors not engaged)
Etc...
Exactly. It's impractical to run all that stuff off the high and variable voltage of the main pack. The 12V can be kept within a tighter voltage range, and this arrangement uses far less power than would be necessary if the DC to DC converter ran all the time. My only complaint is that it appears that the 12V battery is too small for what is asked of it, and Tesla needs to further reduce idle power draws. My Leaf proves that an extremely low idle power state is possible.
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JRP3
Hyperactive Member
The DC/DC is designed to run from a range of pack voltages and still put out the proper voltage to the 12V battery, so that's not really an issue.
I agree this is a large part of the problem, which is also related to the higher than expected vampire loads causing the 12V battery to by cycled more often. I hope that in future vehicles Tesla switches to a more durable lithium solution instead of lead acid.
artsci
Sponsor
It's impossible to know from this thread and other posts if indeed there is a problem with the 12v battery. This site is largely andecdotal and hardly provides a sound empirical basis for making any reliable judgments about whether the 12v battery a systemtic Tesla issue. Elon's comments suggest that the American supplier compromised standards but we really don't know what percentage of cars had 12v failures that required replacement.
One might think that with all of the devices on my car the 12v battery would be a problem. It isn't even though the drain on it is probably several times higher than the typical Model S.
One might think that with all of the devices on my car the 12v battery would be a problem. It isn't even though the drain on it is probably several times higher than the typical Model S.
