Why do we even need a 12V battery?

[Doobster6]

The answer to this question might best be explained by an EV electrical engineer…..please keep it as simple as possible for us non-EE types!
Since the existing 12V battery is anyway being charged by the HVB through some DC-to-DC converter, why not just take the output from that converter directly to provide the 12V power? Including a separate (and tiny) 12V battery seems like an unnecessary/redundant piece of gear….an additional failure point which has been proven a liability now throughout these forums. It ‘looks’ exactly like the sort of thing a Musk-type thinker would want to eliminate.
So even though my recently leased 2024 Premium 4Xt is essentially brand new (<400 miles), I’m already getting anxious about its 12V battery…….I don’t have ‘range anxiety’, I have 12V anxiety’. I only drive an average of 23 miles/day which doesn’t seem like enough miles for the HVB to keep the 12V battery charged. Then I got to thinking…..

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[Snakebitten]

Well as innovative as Tesla is, they haven't eliminated the low voltage battery storage device either. Although they do seem determined to change it to 48V vs 12V

 

[AtomicInternet]

This has been covered extensively: https://www.macheforum.com/site/threads/12v-battery-faq.32744/

To answer your specific question, you can't have 400v running through the standby systems in your car. The 48v standby is WAY more efficient than 12v, but Tesla had to build a ton of proprietary systems to accommodate that.

 

[DYohn]

This has been covered extensively: https://www.macheforum.com/site/threads/12v-battery-faq.32744/

To answer your specific question, you can't have 400v running through the standby systems in your car. The 48v standby is WAY more efficient than 12v, but Tesla had to build a ton of proprietary systems to accommodate that.

Well, you COULD have a 400V aux power system in a car, but you really wouldn't WANT a 400V aux power system since every single item in the electronics package would have to be custom designed.

 

[sukhoi_584th]

You can't reasonably have that much high voltage wiring all over the vehicle. Safety and maintenance chaos.

 

[johnnycombo]

It’s just the way it is, low power always controls the high power in a system, electric or hydraulic, even the ejector seat!

 

[CTZ]

The HVB already charges the LV battery at low voltage, not 350V (obviously). There is a DC/DC converter. There's other reasons but I'm not so sure innovation couldn't manage it by reserving a portion of the HVB.

 

[sean7512]

Why not have a permanent DC/DC converter that is always on, supplying 12V to the car?

 

[dr__bob]

I think that the main reason is probably that you don't want the HV controlling itself. The way it is now, the control systems are all run off of the independent power source called the 12V battery. Sure, the battery gets charged by the HV, but otherwise it is independent. IF the HV goes wonky, the 12V battery is still there, ready to cut off the HV. The DC-to-DC is part of the HV system, not the LV system. But, I'll admit, I am not an EE, just a retired physics prof.

 

[HuntingPudel]

As others have said, mostly for safety. The practical consideration is that legacy automotive electronics function off of a 12VDC input, so convenience from a parts supply view point. ??

 

[bbulkow]

The answer to this question might best be explained by an EV electrical engineer…..please keep it as simple as possible for us non-EE types!
Since the existing 12V battery is anyway being charged by the HVB through some DC-to-DC converter, why not just take the output from that converter directly to provide the 12V power? Including a separate (and tiny) 12V battery seems like an unnecessary/redundant piece of gear….an additional failure point which has been proven a liability now throughout these forums. It ‘looks’ exactly like the sort of thing a Musk-type thinker would want to eliminate.
So even though my recently leased 2024 Premium 4Xt is essentially brand new (<400 miles), I’m already getting anxious about its 12V battery…….I don’t have ‘range anxiety’, I have 12V anxiety’. I only drive an average of 23 miles/day which doesn’t seem like enough miles for the HVB to keep the 12V battery charged. Then I got to thinking…..

First, you can stop being anxious. Ford improved all it's software in this area so dead 12v, other than the 3 year issue of all batteries, is a thing of the past. Unless they write a new bug like the one that has been crippling '22 and '23 models.

Here are some reasons why a 12v is a good idea, although you can imagine moving to a capacitor eventually is a better idea.

The battery is storage between the big battery and the electronics. The problem of a good mission critical, broad voltage, low draw buck converter 400v+ to 12v is not well solved. It is better for the 12v systems to sip, occasionally, from the big battery, and spread that out. Which is exactly what happens, you just need a storage system of the right size, which ford engineers think is best done with a battery.

Walk away lock, remote climate, etc etc needs constant very low draw (milliamps at 12v) is well solved with 12v batteries, which means custom chips that exactly know the curve for 12v lead acid (or agm or whatever). Eventually these systems will get designed (like 48v or lion or capacitors) better, as might the electronics for very efficient very low amp 400v to 3.3v (which is what chips really want) converters. The battery might get removed, but not yet. All the supply chains take years to improve, in automotive. This is where vertical integration helps in early days. It is possible korean manufacturers, with better trust between companies in a group, may move faster than ford.

We'll see.

Today, my ice car stranded me, dead battery (i hope). I usually start it and let it run for 10 minutes once a month, it has been quite a few months since it got out of the driveway. Usually it won't start on the driveway, this time it started 3 times on errands but not a fourth. Now i got to to figure the best way to get a jump.

 

[Doobster6]

This has been covered extensively: https://www.macheforum.com/site/threads/12v-battery-faq.32744/

To answer your specific question, you can't have 400v running through the standby systems in your car. The 48v standby is WAY more efficient than 12v, but Tesla had to build a ton of proprietary systems to accommodate that.

I’m not suggesting that. The 12.85 volts being provided to charge the LVB is coming directly from the HVB via a DC-DC converter. Why can’t it then just provide that 12.85 volts directly to the circuit that needs it?

 

[Doobster6]

I think that the main reason is probably that you don't want the HV controlling itself. The way it is now, the control systems are all run off of the independent power source called the 12V battery. Sure, the battery gets charged by the HV, but otherwise it is independent. IF the HV goes wonky, the 12V battery is still there, ready to cut off the HV. The DC-to-DC is part of the HV system, not the LV system. But, I'll admit, I am not an EE, just a retired physics prof.

Ah, that makes sense. Thanks.

 

[AtomicInternet]

I’m not suggesting that. The 12.85 volts being provided to charge the LVB is coming directly from the HVB via a DC-DC converter. Why can’t it then just provide that 12.85 volts directly to the circuit that needs it?

Among other reasons, in case of crash the car needs to, simultaneously:

Disconnect HV battery
Still display emergency lights
Allow door handles, etc. to operate

It’s disconnected even if it’s intact. This is so you and rescuers are not submitted to the minuscule risk of being electrocuted trying to rescue you or salvage the damaged vehicle.

What you do have in every EV is a DC/DC converter (yes, this also has a transformer, but needs a lot of power electronics to produce a high frequency AC voltage for this transformer). And theoretically, you could use this to directly supply the 12V electronics without a battery. But you need an energy storage in case the HV circuit is switched off. Safety components like brake and steering systems or even simply the lights still have to work.

So the easy answer is ,"Federal safety requirements"

 

[Maquis]

Why not have a permanent DC/DC converter that is always on, supplying 12V to the car?

Continuous power drain.

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