Why do we need 12V batteries in EVs?

[CTZ]

With all the problems that short-lived or dead 12V batteries are causing, I have to wonder why we even need a 12V battery in an EV. Why can't manufacturers just partition and reserve a portion of the HV battery unusable by the motor and use a voltage transformer to power the necessary components?

You would get the benefit of better battery technology, it wouldn't create any more weight since you would shed the old lead acid battery, and it would never go dead in normal circumstances. They already reserve a portion of the battery for longevity purposes.

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[Vulnox]

I believe it's a bit of a chicken/egg situation. What powers the modules that manage your approach/keyless entry? If you use the transformer, does it just stay on full time powering both the module that manages the transformer as well as the keyless entry sensors? So you are draining your HVB, even if small in the scheme of the battery, to run those things, and it's made worse as there are usually losses even in transformers, so you could have drain simply by the system existing.

Then even if you are fine with that, there is the safety aspect. In the case of a crash where airbags deploy, the HVB disconnects, or emergency services pull the disconnect. What then powers your windows, hazard lights, etc?

I think the last thing is it simplifies engineering and support. Mechanics understand 12v architecture supported by a 12v battery and how to work safely around it. Unplug the battery and then you can work on almost anything just like an ICE vehicle. Maybe lower risk, but still a consideration.

Those are items as I understand it anyway, with varied levels of severity and I suspect we will eventually solve for it in a way that allows it to be removed completely.

 

[CTZ]

I believe it's a bit of a chicken/egg situation. What powers the modules that manage your approach/keyless entry? If you use the transformer, does it just stay on full time powering both the module that manages the transformer as well as the keyless entry sensors? So you are draining your HVB, even if small in the scheme of the battery, to run those things, and it's made worse as there are usually losses even in transformers, so you could have drain simply by the system existing.

Then even if you are fine with that, there is the safety aspect. In the case of a crash where airbags deploy, the HVB disconnects, or emergency services pull the disconnect. What then powers your windows, hazard lights, etc?

I think the last thing is it simplifies engineering and support. Mechanics understand 12v architecture supported by a 12v battery and how to work safely around it. Unplug the battery and then you can work on almost anything just like an ICE vehicle. Maybe lower risk, but still a consideration.

Those are items as I understand it anyway, with varied levels of severity and I suspect we will eventually solve for it in a way that allows it to be removed completely.

I appreciate that there would be changes to the battery infrastructure without a 12V battery, but none of those seem that difficult. Having the HV battery powering the low voltage all the time isn't like leaving an ICE motor running. Your cell phone battery is powering something all the time and it only consumes as much as it needs.

A lot of the hardware is already there as the 12V battery already gets a low voltage feed to keep it charged. It seems like something they could have done if the engineers were directed to do it from the start and given all the problems that people are having with the 12V battery, maybe they should have? Of course, I'm not a battery engineer, so I'm just speculating here. ?

 

[Vulnox]

I appreciate that there would be changes to the battery infrastructure without a 12V battery, but none of those seem that difficult. Having the HV battery powering the low voltage all the time isn't like leaving an ICE motor running. Your cell phone battery is powering something all the time and it only consumes as much as it needs.

A lot of the hardware is already there as the 12V battery already gets a low voltage feed to keep it charged. It seems like something they could have done if the engineers were directed to do it from the start and given all the problems that people are having with the 12V battery, maybe they should have? Of course, I'm not a battery engineer, so I'm just speculating here. ?

How are you envisioning they solve the safety issue if the vehicle is in an accident and activating lights and allowing you to exit the vehicle (between power locks, power doors, power windows) if the HV battery is disconnected? That would have to disconnect the transformer as well.

I realize you aren't a battery engineer, either am I, but you did say none of these seem that difficult and I suspect the battery engineers would love to know the answer. ?

 

[burritoman]

I think the 12v battery is here to stay for safety reasons, but there could be changes on the way:

While traditional 12V lead-acid batteries have powered vehicles for a long time, we're starting to see a shift in the landscape. For example, newer Tesla Model S and Model X vehicles don't have regular old battery systems. Instead, the company opted to use a new 12-volt lithium-ion auxiliary battery system.

And while a Li-ion 12V battery costs more to replace than the older style, they're lighter, smaller, and last longer. We could see more auto manufacturers switch to this system or even a bigger 48V auxiliary battery. Why? Because they'll run longer, they don't need the cranking apps to turn over a gas engine, yet deliver more than enough juice to keep up with the growing demand for infotainment systems, self-driving, sensors, cameras, and more.

From https://www.howtogeek.com/146674/why-evs-still-have-a-12v-battery/

 

[Rt1AWD]

How are you envisioning they solve the safety issue if the vehicle is in an accident and activating lights and allowing you to exit the vehicle (between power locks, power doors, power windows) if the HV battery is disconnected? That would have to disconnect the transformer as well.

I realize you aren't a battery engineer, either am I, but you did say none of these seem that difficult and I suspect the battery engineers would love to know the answer. ?

It would be enough to just provide contacts for the external battery in a few places

 

[Logan Nolag]

I think the best solution is to drop the lead acid 12v and switch to a lithium 12v. I believe this is what Tesla did A few years ago.

 

[AtomicInternet]

Automotive has used 12v power for accessories for almost 100 years now. That's a lot of the industry requiring a 12v power source. Redesigning all that for a different voltage will not be cheap, so for now the EV needs a 12v source (and for the relays / etc mentioned above as well).

Tesla is one of the first automotive companies to switch to 48v from 12v, but the press is already saying it's a stunt and we'll never escape the 12v legacy.
https://www.motor1.com/features/704878/tesla-cybertruck-48-volts/

 

[JohnFoxeSheets]

As @Vulnox has noted, there are good reasons to separate the HVB-powered portion of the vehicle from all other electronics. There are no existing automotive modules that operate at ~400 volts (other than the power train modules that obviously must do that). Changing those modules to run at such a high voltage would make them much more expensive and dangerous since you'd have high voltage running all over the car rather than in just a few isolated places. The isolation requirements for the high voltage system is substantially higher than for the 12V system. An "alternative" would be to continuously run the DC-DC converter to drop the voltage from ~400 V to 12 V. This too has a couple of challenges: 1) the DC-DC converter would need to run at ~400 V (AFAIK today's runs at 12 V), 2) the DC-DC converter parasitic loss is non-trival - note that it doesn't use a transformer since those only work with AC, not DC (and ever transformers are by no means 100% efficient). As Vulnox notes 12 V battery technology is extremely well understood and been around a long time. Yes, Ford could and and should have done a better job of designing the system to keep the 12 V battery charged, but let's not throw out the baby with the bath water.

FWIW, Tesla went to a 48 V system for the modules on the Cybertruck. According to @Mach-Lee, they made some engineering innovations which include the 48 V system. It's an interesting development. But for now we have what we have and Lee (who AFAIK is a battery engineer) has offered Ford a number of ideas for improving their 12 V battery management.

 

[Teslaeata]

With all the problems that short-lived or dead 12V batteries are causing, I have to wonder why we even need a 12V battery in an EV. Why can't manufacturers just partition and reserve a portion of the HV battery unusable by the motor and use a voltage transformer to power the necessary components?

You would get the benefit of better battery technology, it wouldn't create any more weight since you would shed the old lead acid battery, and it would never go dead in normal circumstances. They already reserve a portion of the battery for longevity purposes.

It’s to do with separating HV & LV systems.

There should be no direct connection between the two systems.

Nearest we get to that is the DC/DC converted which charges LV batt from HV batt by means of induction, not direct contact through components which reduce HV to LV.

 

[Mach1E]

The real answer is $$$$.

Cheaper and easier to run everything on 12v rather than redesign all your components.

Plus, what am I going to hook my aftermarket subwoofer to??


I also like the idea of the lvb and HVB being completely separate systems. If you have a power drain, I would much rather my lvb run to zero than my HVB.

 

[Peugfan]

Sorry, but the OP's idea is ridiculous. That would just be make it a more complex, more expensive, less safe system. Ford just did not have the BMS system designed properly. A lead acid battery can last a long time if properly maintained. I have a large capacity battery in my Peugeot diesel that is still fine after 15 years Granted it is lightly used but when I'm not using it in the colder winter months I put the battery on a 0.75 amp charger on a 2 hr per day schedule.
My 3 yr old MME battery is doing ok. I just wish they had made it simpler for the owner to change it.

 

[ChehRob]

It took Tesla a few years to get the LVB to working well, I suspect that Ford's will improve over the next few years. My salespeople told me that they are not seeing problems. Most of us want this to be just one more invisible to the user technology.

 

[dtbaker61]

With all the problems that short-lived or dead 12V batteries are causing, I have to wonder why we even need a 12V battery in an EV. Why can't manufacturers just partition and reserve a portion of the HV battery unusable by the motor and use a voltage transformer to power the necessary components?

You would get the benefit of better battery technology, it wouldn't create any more weight since you would shed the old lead acid battery, and it would never go dead in normal circumstances. They already reserve a portion of the battery for longevity purposes.

the 'parasitic drain' from the dc-dc converter to keep the 12v side powered is fairly significant ... If it were 'always on' it would increase the drain on the HV quite a bit, and there would be no way to turn the vehicle completely off other than pulling the HV disconnect; which would have to be changed to a physical breaker between the HV and the dc-dc rather than a 12v relay powered by the 12v battery.

I went without a 12v in my DIY conversions... and it is certainly possible. But, you have to have a HV breaker in the right place, and a way to get to if the car has NO power. The other concerns are if the dc-dc fails while the vehicle is in motion.... what systems are running on 12v pumps and relays ? Steering, brakes, ABS, airbags....

 

[Fixbear]

Right now the only HV is in the battery pack when the car is off. Using any HV outside of that is only protected areas to the motors and the DC DC converter. To have any connection out to fans, lights, electronics is asking for a problem that if a fault happened, it would be dangerous. Also the 12 volt battery acts as a stabilizer of the low voltage system.

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