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Why do we even need the 12 volt battery?

timk225

Active Member
Mar 24, 2016
2,047
1,059
Pittsburgh
What does the 12 volt battery do that the big battery can't? Surely it wouldn't be difficult to tap 12 volts off the main pack.

Lights? Windshield wipers? Horn? Radio and touchscreen? All things the big battery can easily supply power for.
 

jerry33

(S85-3/2/13 traded in) X LR: F2611##-3/27/20
Mar 8, 2012
19,814
23,173
Texas
Should the main battery fail, the 12V can power the power steering and brakes for a short time allowing you to safely bring the vehicle to a stop. No 12V battery was tried in the early Roadsters, but it didn't work out well.
 

Msjulie

Active Member
Jun 26, 2016
2,432
1,745
Monterey Bay Peninsula
My understanding, can't recall where I heard it, was the US gov required it to be able to power emergency flashers and such should the main drive unit battery fail. Personally I liked the idea of never having an extra (motor cycle sized) battery in the car to be replacing fwiw.
 

Alfred

Supporting Member
May 28, 2009
300
259
Zurich
My understanding, can't recall where I heard it, was the US gov required it to be able to power emergency flashers and such should the main drive unit battery fail. Personally I liked the idea of never having an extra (motor cycle sized) battery in the car to be replacing fwiw.
For my roadster - as far as I was told: The main battery is cut off upon impact. Similarly as gas supply in a conventional car. Emergency flashers have then to be able to use another source.
 

Glamisduner

Active Member
Aug 2, 2017
3,582
2,125
Escondido, CA
What does the 12 volt battery do that the big battery can't? Surely it wouldn't be difficult to tap 12 volts off the main pack.

Lights? Windshield wipers? Horn? Radio and touchscreen? All things the big battery can easily supply power for.
Capacitor + an inverter could power most of those things too.
 

McRat

Well-Known Member
Jan 20, 2016
5,771
5,414
LA
I like the idea that I can disconnect the 12v battery, and know for a fact I'll still be alive if I touch an orange wire.
The traction battery has a solenoid that requires 12v to close. No 12v, no 400v.

Hyundai / Kia is going without a separate 12v battery in some hybrids. Not sure if I like that. Interestingly enough, their plug-ins retain the separate 12v battery.
 
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Reactions: PJFW8

ItsNotAboutTheMoney

Well-Known Member
Jul 12, 2012
10,631
7,799
Maine
I like the idea that I can disconnect the 12v battery, and know for a fact I'll still be alive if I touch an orange wire.
The traction battery has a solenoid that requires 12v to close. No 12v, no 400v.

Hyundai / Kia is going without a separate 12v battery in some hybrids. Not sure if I like that. Interestingly enough, their plug-ins retain the separate 12v battery.

Well, it's kind-of-without-a-separate-12V-battery. There is separate 12V battery hardware, but it's inside the same case as the hybrid battery.

Two problems:
- The lifetime warranty on the battery applies only to the original owner.
- Can't jump another car
 

jerry33

(S85-3/2/13 traded in) X LR: F2611##-3/27/20
Mar 8, 2012
19,814
23,173
Texas
- Can't jump another car
Bear in mind that you are not jumping another car in the traditional sense. It is charging the battery of the other car, then disconnecting before any attempt to start the car is made. If an attempt is made to start the other car before disconnecting, it's likely to damage the donor car.
 

ewoodrick

Well-Known Member
Apr 13, 2018
5,285
3,732
Buford, GA
What does the 12 volt battery do that the big battery can't? Surely it wouldn't be difficult to tap 12 volts off the main pack.

Lights? Windshield wipers? Horn? Radio and touchscreen? All things the big battery can easily supply power for.

This is an easy one, the big battery powers 2 things in the car, the motor, the AC. (okay, maybe 2.5, also the 12V charger)
The 12V batter runs everything else, the computers, the lights, wipers, the 12V plug, headlamps and a bunch of others.

You can not get rid of the 12V, it has to exist for at least the 12V plug. And when you look at "low cost" parts, that means mass production and all parts are generally 12V.
Now, I believe that Elon has mentioned that he wants to increase the distribution voltage, to something like 48V, but that tends to suggest that a lot of down-converters will be needed all over the car. LEDs don't run at 48V, but then again, they don't run at 12V, so it is a different converter needed.

48V means that smaller wires will be needed and possibly less wires because larger buss wires can be used.

But just about everything in the car is 12V today, so that 12V battery is pretty crucial to operation. Heck, both the Bluetooth and card key mechanisms are coming off the 12V battery, so it is quite essential.
 

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