Why the 12 volt battery?

[arg]

Although arg, I'm under the impression that the battery monitoring is 24/7, so it never "powers down". Maybe not?

Well, this is all Tesla's secret sauce and we're speculating as to the details - likely there is some monitoring that's 24/7 (or at least, polling at regular intervals throughout 24/7), but it will want to be more intensive when actually drawing a load. And the coolant circulation doesn't generally run during 'sleep' while it does (to equalize cell temperatures) during charge/discharge. Maybe you could argue that you could draw just a little bit of current through the DC/DC without doing coolant circulation, but then you need some way of monitoring that - maybe the DC/DC converter can tell you from its duty cycle, but then you've got a chunky DC/DC with intelligence in it, which has probably stuffed its efficiency at very low load......

It would be fascinating to know the full set of trade-offs that led to this design, but I doubt they'll ever tell.

 

[William3]

I thought I heard (back in the Roadster days) that they wanted a 12V battery for crash situations. During a crash the car will disconnect the traction battery for safety, but you'd still want your power windows and door locks and door handles and hazard lights to work. So there you go... 12V battery.

 

[TEG]

Yeah, I think that is basically it.
Also, if your main "traction pack" fails, it would be nice if headlights and ABS brakes keep working until you are able to coast safely to the side of the road.

Note, Nissan LEAF also has a conventional 12V "car battery" in it as well.
So, does my old Ford RangerEV. It seems to be "conventional wisdom" to design EVs with an "old-school" Lead-Acid 12V aux battery.
(It almost seems like there is a regulation mandating it, but I am not aware of any such regulation.)

Oh, another benefit - you can help jump start an ICE vehicle from your EV, although the EV manufacturers tend to recommend against doing this.

 

[jerry33]

Or for a blender for tailgating! :biggrin: We have a 1500W inverter that I used with my old car for such purposes, but that requires direct connection to the 12V car battery, which looks to be a pain to access on the S.

That should probably be "a pain in the S to access".

Every car that has a big battery also has problems with it's 12V battery. I think the problem isn't that there's a 12V battery, but that the 12V battery is just way too small. This causes the battery to drain very quickly and have a very short life. Even though a full size battery would add 10-15 kg, it would be worth it (and you could always lose the extra 10-15 kg with exercise

 

[tezco]

Note, Nissan LEAF also has a conventional 12V "car battery" in it as well.
So, does my old Ford RangerEV. It seems to be "conventional wisdom" to design EVs with an "old-school" Lead-Acid 12V aux battery.

There are so many existing componets designed for 12V that it makes sense. The vampire load in the Leaf is so low that it only has to recharge once every five days if the car is parked.

 

[TikiMan]

Replace 12 Volt Battery service notice...

Every time there is a new software update, right before I get the update notice I also get a "Replace 12 Volt Battery" (maintenance notice). Then when I try and do the update, it tells me that it can't do it because the 12 volt battery needs replacement.

The last software update, I took it into my local Tesla service center, and they said nothing was wrong with my 12 volt battery, but replaced it anyway, and then did the 4.0 upgrade.

Thus, this is the 2nd time this has happened, and I am curious if anyone else is getting this error also?

 

[Ingineer]

The main reason for the 12v battery is for standby loads. The traction pack is not connected when the car is at rest, so something must provide power to start up everything. This includes detecting the key, unlocking the doors, booting the computers, etc, and finally closing the traction pack's main contactor so it can "take over" and then recharge the 12v.

It's basically similar to a normal ICE car; When the engine is running the alternator provides power for the 12v electrical loads, but to get that going you need to somehow start the ICE, which is where the 12v battery comes in. In an EV, the DC-DC converter takes the place of the alternator and provides power for all the 12V loads, and is also responsible for maintaining the charge in the 12v. The DC-DC converter takes the ~400 volt DC traction pack voltage and converts it to an isolated 12-14v.

Why not just install a small DC-DC that's always on? For one, safety dictates that there be a main battery contactor (relay) that completely disconnects the traction pack. This also prevents your expensive traction pack from being "bricked" in the event the car is left unattended. All converters have losses and fixed overheads, so this load would drastically increase the possibility of a "bricking". A small lead acid battery is cheap and easily replaceable, so it is tasked with running the standby systems and booting the car up, all without putting stress on the traction pack.

It also solves several other problems, as many of you have concluded; it provides a safety backup for critical systems in the event of a sudden traction pack failure, or in the event of a collision where the traction pack is automatically disconnected for safety. It also handles occasional surge loads so the DC-DC converter doesn't have to be sized to handle them (thus making it even less efficient, more expensive, and heavier).

-Phil