Is a 12V battery required for a fully electric vehicle (BEV 12v battery)?

[Unsanity]

Does a BEV need a 12V battery in addition to the main battery? That is the question of the day 

Most of the ECUs on the vehicle require 12v to operate, but in theory a DCDC converter can be used to supply 12v power. So why does the 12V battery still exist in a fully electric vehicle? It seems OEMs could save some money if a 12v battery was not required.

Reasons I have seen so far:

• Safety... when the vehicle is off the main battery needs to be disconnected. I am not 100% convinced on this reason because most BEV can charge the 12V battery directly from the main battery when the vehicle is off.
• Legacy... There is a lot less to change if the 12V battery remains. The relay blocks are the same, control logic remains the same, etc.

Reasons I have not seen so far:

• Efficiency... The DCDC converter will have an efficiency range. At lower current output the efficiency might be very low.

What do you think.... Any new reason you have seen? Anything you want to add?

[Gyro]

I don't know about full EV but my Kia Hybrid does not have a separate 12V Lead acid battery. The 12V battery is part of the Lithium battery pack (there's even a handy button to 'bootstrap' it if it gets discharged). Engine start is done from the main HV battery.

If this is possible for a Hybrid, I see no reason why a full EV would require a separate 12V battery.

[Psi]

I think it is mainly for safety, so you have to inject a voltage to close the contactors that provide battery power.
eg, it cannot power itself. Without external power it is totally offline

[Unsanity]

Oh wow, I didn't know it had that function. I will have to check out the bootstrap button. How is the button labeled in the vehicle?

[NiHaoMike]

It could be implemented as DC/DC converters integrated into the main battery casing.

[JPortici]

Current state of things is that the power electronics under the hood is slowly going from 12V to 48V, and the reason is stockpiles.
Not everything is going to change at once because there are still a lot of 12V components already manufactured and sitting there, but you can see on newer models more and more purple wires (48V power), but the infotainment and most of the door electronics is going to stay in a 12V domain for the time being. Less headaches i think.
There are indeed 48-12V DCDC converters already in the system, usually bidirectional because external chargers, in which case a small 12V battery is going to be used, mostly for stabilization of the DCDC converter and because external chargers expect to see a lead acid battery

re: kia i very rarely see kias at work so i can't comment, but for VAG/Mercedes/BMW it's mostly how i described it

[Jeroen3]

I believe the main reason is that you want certain things to have some layer of redundancy, like airbags, you don't want them to rely on some software controller dc/dc.
Many combustion engine today have two, or even three lead-acids. One for cranking, one to keep the radios up during cranking, and one to ensure the parking brake always turns on. Because that also had to become electric for some reason?

There still are many growing pains with ev's, like not being able to roll, onto a towtruck or in a carwash.
Shutting down due to a software glitch, very dangerous because they don't want to roll!
The brake lights are apparently suddenly difficult manufacturers of EV's.
It's ridiculous. Like they don't test drive their own cars.

Just reusing the old trusted 12V battery for all the traditional gear seems like a economical engineering choice to me.

[mikeselectricstuff]

I believe the main reason is that you want certain things to have some layer of redundancy, like airbags, you don't want them to rely on some software controller dc/dc.

Also more mundane things like hazard flashers so people can see your broken-down car at the side of the road at night.
An auxilliary 12V battery is the simplest solution to a number of issues like that, though lead-acid is not really a good choice of battery chemistry.

However the biggest problem in practice is poor software that manages the 12V system, leading to stupidities like many Kia/Hyundais that will drain the 12V battery if the boot ( trunk) is left open for a few hours, as well as excessive power draw for many minutes after turning the car off.

[mikeselectricstuff]

It could be implemented as DC/DC converters integrated into the main battery casing.

Yes in theory, but demonstrating safety in all plausible circumstances can get very difficult in practice. A system that requires an external supply to close contactors is very easy to demonstrate that HV can't be present when the 12V supply isn't there.
And besides safety, there is the challenge of making a  DC/DC with low enough power draw to run just the security and maybe telematics when the car is off, but also provide 100+ amps for power steering etc.   
In practice you'd probably use seperate LV supplies and multiple DC/DC's, but it all adds cost and complication. A seperate 12V battery solves a lot of these problems easily for relatively low cost.

[Gyro]

Oh wow, I didn't know it had that function. I will have to check out the bootstrap button. How is the button labeled in the vehicle?

Photo attached. It uses the high voltage section 12V DC/DC converter to recharge the 12V section of the battery pack. I've never had cause to press it, even after long lockdowns during Covid.

The previous revision (it's a Niro by the way) had a full sized 12V Lead acid battery in a compartment at the rear. I think they changed over as a weight saving measure, and one less thing to need replacing.

[mtwieg]

I believe all types of EVs have an auxiliary battery (whether it's 12V, 48V, or whatever), completely independent from the drive train battery. It seems like common sense to me. At the very least, you should have a means of powering basic safety (emergency blinkers, power locks, air bags, main battery management, etc) and diagnostic (ODB-II) systems available in the case of a main battery failure. I'd be surprised if this isn't codified in laws somewhat...

[JPortici]

However the biggest problem in practice is poor software that manages the 12V system, leading to stupidities like many Kia/Hyundais that will drain the 12V battery if the boot ( trunk) is left open for a few hours, as well as excessive power draw for many minutes after turning the car off.

tell me about it. Now it takes less than 15 minutes to drain a battery to the point the car won't be able to start (mercedes. others take one hour or so). We also do OBD tools, in one we regularly ping the bus to check if the ignition is on to perform some stuff. Up until a couple of years ago if the car was off, it was off. No answer, great, car is off. Now instead the whole car (minus the giant screen that is better than the needles for some reason) comes to life and all the electric pumps start rotating and actuators start moving.
I assume it's because the car is getting ready for diagnostic operations, but it's bad news to us.
Or you have a BMW and if you are communicating via OBD and the car is locked and the key is not inside, if you don't stop in less than 30 seconds the car alarm starts. That's because you don't even need a key sto start a BMW if you have access to the OBD port (a very well known fact) handy for maintenance, handy for burglars. But at least they have the courtesy to also send a frame that indicates the state of the ignition.

[NiHaoMike]

Yes in theory, but demonstrating safety in all plausible circumstances can get very difficult in practice. A system that requires an external supply to close contactors is very easy to demonstrate that HV can't be present when the 12V supply isn't there.
And besides safety, there is the challenge of making a  DC/DC with low enough power draw to run just the security and maybe telematics when the car is off, but also provide 100+ amps for power steering etc.   

Have it wired so that while the main contactors do not disconnect the DC/DC converter, the service disconnect still does.
The efficiency at low currents is easily solved by operating in burst mode. Just needs a bit more output capacitance.

I believe all types of EVs have an auxiliary battery (whether it's 12V, 48V, or whatever), completely independent from the drive train battery. It seems like common sense to me. At the very least, you should have a means of powering basic safety (emergency blinkers, power locks, air bags, main battery management, etc) and diagnostic (ODB-II) systems available in the case of a main battery failure. I'd be surprised if this isn't codified in laws somewhat...

Or it could be implemented as multiple DC/DC converters running from separate sections of the main battery pack.

[mikeselectricstuff]

Or it could be implemented as multiple DC/DC converters running from separate sections of the main battery pack.

You can't really split the pack as that would result in imbalance. I believe this is what killed a lot of original Tesla Roadster batteries.
Multiple DC/DCs running off  the full pack voltage aren't a major problem though, just gets tricky to design a provably bulletproof way to ensure safe isolation in all plausible accident/damage scenarios.
Once you look at the cost of the DC/DCs, connectors & control systems, especially when you consider high peak loads like power steering, you'll struggle to get anything cheaper than an external 12V lead-acid battery.
AFAIK Tesla is the only mainstream maker to replace the lead-acid with lithium at around 16V, but they have the advantage of design control over pretty much all the electronics in the car. for teh same reason, I have little doubt they will be the first to go to 48V.
The end of incandescent lighting has removed one of the remaining reasons to use 12V, and pretty much everything else is either motors or something with a DC-DC converter inside, so I expect 48V to become more common in the not-too-distant future.

[Marco]

What's the point of going 48V? Wouldn't it make more sense to just create an extra standardized voltage for the AC and leave the rest alone? Even for the high beam the copper is hardly relevant.

[Siwastaja]

This is one of those cases with no absolute right way to do it. There are good engineering arguments for either way. When coming from the ICE legacy, and doing things like mechanical isolation of HVDC battery when car is off (requiring power to turn it on), it's quite obvious it's by far easiest to put the classic 12V battery in, and use a small DC/DC from the HVDC battery to keep it topped while car is on, but it doesn't need to be this way.

[f4eru]

Main purpose for the 12V battery is:
1) act as a big buffer for any current peaks
2) start the HV relays and everything else in the car when waking up
3) provide power to all things needed during sleep (receivers, clocks, sensors)
4) provide emergency power when HV fails (broken safety loop, etc)
5) provide an excuse for auto dealers to sell you services.

Don't underestimate the buffer function.
the DC/DC converter cannot power everything 12V directly.
Especially motors ! a small motor from a seat adjustment or other gimmick will take immense power when starting up.

[mikeselectricstuff]

What's the point of going 48V? Wouldn't it make more sense to just create an extra standardized voltage for the AC and leave the rest alone? Even for the high beam the copper is hardly relevant.

The question is more why not - once you no longer have incandescent bulbs, where higher voltage filaments are be fragile, the sole reason for 12v is legacy hardware.
The intrinsic cost of most 12V devices is little, if any, more in a 48V version, and you save weight and copper everywhere.

[uer166]

1) act as a big buffer for any current peaks

I come from automotive (EVs), do not underestimate this^. The EPS/EPAS and iBooster (steering and brakes) can have hundreds of amps of consumption peaks. Good luck doing that with only a DCDC cost-effectively and safely.

[uer166]

I have little doubt they will be the first to go to 48V.

48V does incur serious costs when you combine all the tiny DC/DC converters in the car. You can't really do 48V DCDC monolithically and cheaply, the supply of such devices is maybe 1/10th of what's available at lower voltages, and you're now going to require DCDCs in places where 12V wouldn't need it (e.g. interior and exterior lighting, sensors running direct off of 12V, seat controls, encoders, etc etc). Efficiency of large computer loads (and autopilot and car computer are large loads) will drop since the conversion ratio to 0.8V core voltages is insane. HVAC, both compressor for AC/heat pump, and the PTC heaters in Teslas runs off of pack voltage (390V or what-have-you), so that is a non-issue.

I do expect a hybrid 12/48V system though, 48V would be handy for the real high loads such as brakes, steering, rear defrost, etc. 12V for the tiny stuff. Maybe the move is a 48V battery with a 12V bus converter.

[mikeselectricstuff]

I have little doubt they will be the first to go to 48V.

48V does incur serious costs when you combine all the tiny DC/DC converters in the car. You can't really do 48V DCDC monolithically and cheaply, the supply of such devices is maybe 1/10th of what's available at lower voltages,

Number of devices available does not correlate with cost. There are plenty of 48v-in DC/DC chips in a range of power levels.

and you're now going to require DCDCs in places where 12V wouldn't need it (e.g. interior and exterior lighting,

Why would you need them for lighting? - high power LEDs will already have buck (or occasionally boost) drivers, so a 48V solution would be little different, and for very low powers like interior, you can use more, smaller LEDs in series

Efficiency of large computer loads (and autopilot and car computer are large loads) will drop since the conversion ratio to 0.8V core voltages is insane.

I'm sure a lot of devices like these use intermediate voltages already - I;d be surprised to see a raw 12V car supply directly feeding a 0.8v POL regulator.
48V is a common supply in stuff like telecoms & networking.

[f4eru]

Any automotive LDO or buck is specced to 45-80V anyway, due to old school load dump transients, so you can probably use them 1:1 in an application that is meant for 48V without those transients.

[uer166]

I'm sure a lot of devices like these use intermediate voltages already - I;d be surprised to see a raw 12V car supply directly feeding a 0.8v POL regulator.

Why would that be surprising? A one-stage 12V to core is probably the cheapest and most efficient. Looking at AP PCBAs it looks like that's exactly what's happening (not 100% clear on that though). Regardless, an intermediate voltage doesn't really help the total step-down ratio if it's non-isolated. Every cent is fought hard in cars, so "just throw in some more DCDCs and increase the cost of others" doesn't pass the sniff test on that, unless like you said the copper offsets are worth it.. I'm sure whatever Tesla comes up with in the future would be reasonably optimal, but I don't think it'll be 48V on everything, it's an awkward middle of the road voltage that kind of sucks for both low power and high power. It's more optimal to have the really high power loads on the traction voltage, and low power on sub-20V.

[uer166]

Any automotive LDO or buck is specced to 45-80V anyway, due to old school load dump transients, so you can probably use them 1:1 in an application that is meant for 48V without those transients.

That's kind of true, but EVs don't have load dump, and therefore don't actually need to use those higher Vin parts in general.

[NiHaoMike]

What if, instead of the 48V or whatever intermediate voltage being DC, have it be high frequency AC on the order of kHz? Then it can be further stepped down with small transformers. Similar to how it's done on aircraft.

If reducing input current wasn't a concern, the ideal input voltage for a high current buck converter would be about half the target output voltage. So have a transformer and synchronous rectifier supplying the input voltage to the buck converter, while the power distribution would be done at a relatively high voltage and low current.

[Jeroen3]

I believe the main reason is that you want certain things to have some layer of redundancy, like airbags, you don't want them to rely on some software controller dc/dc.

Also more mundane things like hazard flashers so people can see your broken-down car at the side of the road at night.
An auxilliary 12V battery is the simplest solution to a number of issues like that, though lead-acid is not really a good choice of battery chemistry.

However the biggest problem in practice is poor software that manages the 12V system, leading to stupidities like many Kia/Hyundais that will drain the 12V battery if the boot ( trunk) is left open for a few hours, as well as excessive power draw for many minutes after turning the car off.

Yes, lead acid is not ideal. But it is the most available, and I think also the easiest to recycle. Any lithium or nickel based chemistry would have severe cold problems.

Funny you say that. My 2009 hybrid civic was dead a few weeks ago, couldn't even open the doors anymore. Luckily this car comes with this special vintage feature. A mechanical lock! So I could jump start it with it's old battery I replaced 3 years ago. It could wake up the NiMH powered dc/dc and I could start the engine.
Suspect was a stuck AC-clutch, apparently this happens sometimes.

[...]
Don't underestimate the buffer function.
the DC/DC converter cannot power everything 12V directly.
Especially motors ! a small motor from a seat adjustment or other gimmick will take immense power when starting up.

The DC/DC powers my entire civic, no dynamo. It has a 100A fuse, so it can do some current. I have a small 50 Ah battery for doors when and ignition when off. I think you mean it can't supply the surge loads from window motors or seat adjusters? Maybe. Sure you can engineer around this limitation. But a lead acid is a cheap bypass when you need long standby time as well.
Those batteries are mature technology, reliable and can take a beating.

[Gyro]

What if, instead of the 48V or whatever intermediate voltage being DC, have it be high frequency AC on the order of kHz? Then it can be further stepped down with small transformers. Similar to how it's done on aircraft.
...

I think the original reason for 400Hz AC supplies in aircraft was down to needing a suitable supply for syncro transmitters, resolvers, gyros etc. Higher frequency would certainly make transformers smaller, but I don't know how they would stack up aganst DC-DC converters on cost, size (and habit now). Probably not well - unless the frequency is high enough to use Ferrite cores, but that's probably 50kHz upwards for sensible power transfer.

[Siwastaja]

I think the original reason for 400Hz AC supplies in aircraft was down to needing a suitable supply for syncro transmitters, resolvers, gyros etc. Higher frequency would certainly make transformers smaller, but I don't know how they would stack up aganst DC-DC converters on cost, size (and habit now). Probably not well - unless the frequency is high enough to use Ferrite cores, but that's probably 50kHz upwards for sensible power transfer.

This, and aircraft stuff is not exactly optimized for the lowest price possible.

[mikeselectricstuff]

I think the original reason for 400Hz AC supplies in aircraft was down to needing a suitable supply for syncro transmitters, resolvers, gyros etc. Higher frequency would certainly make transformers smaller, but I don't know how they would stack up aganst DC-DC converters on cost, size (and habit now). Probably not well - unless the frequency is high enough to use Ferrite cores, but that's probably 50kHz upwards for sensible power transfer.

This, and aircraft stuff is not exactly optimized for the lowest price possible.

I imagine 400Hz pre-dated resolvers, I'd guess it's mostly about reducing size and weight of generators,transfomers & smoothing caps while being low enough to not cause audio breakthrough on radios or have too much core loss

[Siwastaja]

400Hz is pretty much sweet spot to leverage the existing simplicity or iron core transformers, motor laminations etc. They need to be made from thinner steel but still nothing special.

[f4eru]

What if, instead of the 48V or whatever intermediate voltage being DC, have it be high frequency AC on the order of kHz? Then it can be further stepped down with small transformers. Similar to how it's done on aircraft.

- EMC nightmare
- transformers are superfluous in automotive LV
- skin effect over long cable runs, coupled with low efficiency of low conduction angles rectifiers
- full of compromises on all sides

Nope. makes no sense.

[mikeselectricstuff]

What if, instead of the 48V or whatever intermediate voltage being DC, have it be high frequency AC on the order of kHz? Then it can be further stepped down with small transformers. Similar to how it's done on aircraft.

- EMC nightmare
- transformers are superfluous in automotive LV
- skin effect over long cable runs, coupled with low efficiency of low conduction angles rectifiers
- full of compromises on all sides

Nope. makes no sense.

It would need to be ultrasonic to avoid audible artefacts, so losses would be pretty terrible.