Reserve power?

Reserve power?

Does anyone know if the S will have reserve power (after the battery pack is too low to drive the car) to do things like operate windows, emergency brake, touch screen etc. I assume it will. And by the way I don't regularly run my car out of gas...just wondering what would be possible in an emergency.

Volker.Berlin | 13 January 2012

The Roadster has a separate 12V battery for running accessories, in addition to the main battery that provides the juice for driving. We don't know yet the setup in the Model S, but it's not unlikely that it will be similar.

Teoatawki | 13 January 2012

What Volker said.

I can't guarantee the model S behaves this way, but this is how other EVs work.

EVs don't just sputter and die when they run out of juice like ICEs. First off, they are more aggressive about warning you about dwindling range. For the hardcore risk taker who presses on anyway, once they approach the limits of available power, they reduce speed and cap the amount of acceleration energy you can draw on.

If you check out the range graph in the Roadster efficiency and range blog, you'll see range doubles from 65mph to 20mph. This will hopefully let you "limp" to a spot where you can plug in, or at least safely wait for assistance.

Douglas3 | 14 January 2012

I was told that the Model S has a full-size automotive lead-acid battery. Presumably this is for backup purposes, but I'm sure it is also intended to operate 12V DC accessories that users may plug in. They've had a little trouble with that on the Roadster.

stephen.kamichik | 14 January 2012

I think the model S is more advanced than the roadster-no lead-acid battery.

Robert.Boston | 14 January 2012

The Model S will have a single, standard-issue 12V battery to support auxiliary systems.

stephen.kamichik | 14 January 2012

That is too bad. An extra fifty pounds to lug around at all times. I hope that you are wrong.

Douglas3 | 14 January 2012

There are good reasons for it. Early Roadsters drew backup power from a segment of the main pack (ESS), but it caused various problems including unbalancing the pack.

The Roadsters also generate their auxiliary 12V DC using a DC-to-DC converter, and some people plugging in 120V AC inverters have fried their car's electronics.

The small sealed 12V battery in the Roadster eliminated the ESS issues, and is a nice simple solution. Using a full-sized battery in the Model S will allow you to run pretty much any standard automotive 12V accessory from the car without causing problems.

stephen.kamichik | 14 January 2012

Here is what I think is a more elegant solution. The nominal voltage of the 18650 Lithium-ion battery is about 3.75 volts. Three or (four) in series outputs 11.25 volts or (15 volts). Ten banks of these batteries in parallel outputs about 25 amperes. Therefore thirty to forty 18650 batteries could serve as back-up power for the accessories. These batteries would be charged by the same circuit used to charge the lead-acid batteries and the forty 18650 batteries weight a lot less than a lead-acid battery.

Volker.Berlin | 14 January 2012

stephen, as far as I understand it, that's the setup that Tesla came up with for the Roadster at first. It did not prove successful in harsh reality.

stephen.kamichik | 14 January 2012

Perhaps they ironed out the bugs. We shall see.

David70 | 14 January 2012

I can't believe the lead-acid battery will be anything approaching even 25 lbs in the Model S. A motorcycle battery would probably be sufficient.

gjunky | 16 January 2012

The standard Lead Acid battery in our Toyota Highlander died several times which was a pain. The car wouldn't start without the 12v battery having some charge as it power the electronics in the car. The electronics are then powered from the main battery once the computers are booted up and the car is "Ready".

I always thought this was the dumbest design and one of the reasons we got rid of this car.

TM: Please find a better way of dealing with this issue!

stephen.kamichik | 16 January 2012

I second gjunky's motion.

Discoducky | 16 January 2012

I would design in a reserve in the Li-Ion pack that is zoned off for bootstrapping, critical systems and fault tolerance. No seperate energy source or store. Maybe capacitor storage for read/writes from memory to disk in case of catastrophic failure.

And would guess engineers have designed in a variable zone for managing systems when the propulsion zones have been entirely depleted. Similar to the Leaf fail safes of Run, Walk, Crawl speeds.

P.S. If there is lead or acid in Model S I'm out!!! Jk'ing, would suck to explain that to my tech friends who would immediately say "My laptop came with a lead acid back up battery too!"

jbunn | 17 January 2012

As I was starting reading the thread, I was already worried about what gjunky eloquenty said. I've run my 12 volt down many times in my truck and needed to jump it. Usualy, I do this with the radio after the battery gets a few years on it. I can tell when it's time to replace the battery when this happens. But I got gas in the tank. Irony.

Gonna be the worse irony if I have enough power in the main battery pack to run my house for 24 hours, but can't get enough power to boot it's 5 volt embedded circut board, and I get stranded.

It seems really unlikley that the Tesla engineers would have overlooked this part though.

Robert.Boston | 17 January 2012

You could always replace the 12v battery that ships with the Model S with a Li-ion, like this one from A123:

Robert.Boston | 17 January 2012
Brian H | 18 January 2012

Yeah, get one for your truck first, and see how it goes! Looks like a much better product.