Why do we have a 12V battery?

[guinn]

I've been wondering why the MachE has a 12-volt battery. Someone is going to say it's because there are a bunch of things in the car that run off of 12 volts. But you don't need a battery to have a 12-volt source. A DC-to-DC converter could supply 12 volts from the main battery. And, in fact, there is already one in the car that charges the 12-volt battery. Having the smaller battery causes all sorts of problems for the owners, as we all know. Certainly, someone at Ford made an engineering decision to have the battery, but I wonder if anyone knows why.

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[fleeps]

The high voltage circuits are physically isolated from the low-voltage circuits for safety. The 12v battery is needed to power the low-voltage physical relay that connects the two. The two circuits are separate because you don't want the high-voltage circuit running directly to the door, steering wheel, etc., because a tiny electrical leak would be fatal.

 

[E90alex]

All EVs operate this way. It’s not a detriment of Ford engineering or lack thereof.

 

[Kamuelaflyer]

The mach-e HVB operates on (roughly) 400 volts. Powering the 12V items directly creates a rabbit hole no one wants to go down. Here are a few reasons.

Safety Disconnect Failure: The HVB contactors should automatically snap open to prevent electrocution during an airbag deployment accident. If all low-voltage systems (hazard lights, door locks, communication) relied on the HVB, the car would lose all power instantly, potentially trapping occupants and disabling safety features.

"Chicken or the Egg" Startup Issues: The HVB is isolated when the car is off. Turning the car on requires a small amount of power to close the main contactors and engage the high-voltage system. The 12V battery provides this power. If the 12V is removed, the car cannot initiate the connection to the HVB, essentially bricking the vehicle.

Increased Parasitic Drain: Keeping the DC-DC converter (which converts ~400V to 12V) energized 24/7 to power small items such as fob sensors and infotainment items could lead to significant phantom drain on the main battery. This would increase the load on the HVB even when parked, reducing overall range and battery health over time. Then we'd have even more low/lost range threads on the forum. Nobody wants that.

Safety Hazard during Maintenance: Currently, disconnecting the 12V battery makes the low-voltage circuits safe to work on. Without it, technicians would be working on circuits directly connected to a high-voltage, high-capacity system, increasing the risk for arc flashing or shock.

It's Hot: The DC_DC converter would be working overtime. It'd be the equivalent of a 75 watt incadescent lightbulb turned on in a small enclosed space 24/7/365. This would increase the heat on surrounding components and would quite probably reduce the MTBF of the converter and surrounding components. Further, the DC-DC converter is actively cooled because of heat. Making the DC-DC converter work 24/7 would add to the cooling requirements increasing parasitic drain and add additonal cooling requirements.

Money: Most auto accessories are designed to run on 12V or 48V. Redesigning thousands of components to handle 400V-800V directly is cost-prohibitive and technically complex. It's always about the money.

There are actual engineers lurking and not so lurking here. Some are Ford employees. Perhaps one will comment further. It's not a dumb question but the answer is complicated, as usual.

 

[Maquis]

The mach-e HVB operates on (roughly) 400 volts. Powering the 12V items directly creates a rabbit hole no one wants to go down. Here are a few reasons.

Safety Disconnect Failure: The HVB contactors should automatically snap open to prevent electrocution during an airbag deployment accident. If all low-voltage systems (hazard lights, door locks, communication) relied on the HVB, the car would lose all power instantly, trapping occupants and disabling safety features.

"Chicken or the Egg" Startup Issues: The HVB is isolated when the car is off. Turning the car on requires a small amount of power to close the main contactors and engage the high-voltage system. The 12V battery provides this power. If the 12V is removed, the car cannot initiate the connection to the HVB, essentially bricking the vehicle.

Increased Parasitic Drain: Keeping the DC-DC converter (which converts ~400V to 12V) energized 24/7 to power small items such as fob sensors and infotainment items could lead to significant phantom drain on the main battery. This would increase the load on the HVB even when parked, reducing overall range and battery health over time. Then we'd have even more low/lost range threads on the forum. Nobody wants that.

Safety Hazard during Maintenance: Currently, disconnecting the 12V battery makes the low-voltage circuits safe to work on. Without it, technicians would be working on circuits directly connected to a high-voltage, high-capacity system, increasing the risk for arc flashing or shock.

It's Hot: The DC_DC converter would be working overtime. It'd be the equivalent of a 75 watt incadescent lightbulb turned on in a small enclosed space 24/7/365. This would increase the heat on surrounding components and would quite probably reduce the MTBF of the converter and surrounding components. Further, the DC-DC converter is actively cooled because of heat. Making the DC-DC converter work 24/7 would add to the cooling requirements increasing parasitic drain and add additonal cooling requirements.

Money: Most auto accessories are designed to run on 12V or 48V. Redesigning thousands of components to handle 400V-800V directly is cost-prohibitive and technically complex. It's always about the money.

There are actual engineers lurking and not so lurking here. Some are Ford employees. Perhaps one will comment further. It's not a dumb question but the answer is complicated, as usual.

The “chicken vs egg” scenario is the key.

Even battery storage backup power systems normally have a small “cold start” battery.

 

[Peugfan]

Question for the electrical experts: Why do North America households operate mostly on 120 volts when much of the world is 230 volts? Safety? Would not the higher voltage reduce the conductor wire size needed thereby saving money on copper wire. When we need the power, like EVSE chargers or electrical motors we we use 240 volts or higher. Once standards are established we are stuck with them.

 

[Teslaeata]

The mach-e HVB operates on (roughly) 400 volts. Powering the 12V items directly creates a rabbit hole no one wants to go down. Here are a few reasons.

Safety Disconnect Failure: The HVB contactors should automatically snap open to prevent electrocution during an airbag deployment accident. If all low-voltage systems (hazard lights, door locks, communication) relied on the HVB, the car would lose all power instantly, potentially trapping occupants and disabling safety features.

"Chicken or the Egg" Startup Issues: The HVB is isolated when the car is off. Turning the car on requires a small amount of power to close the main contactors and engage the high-voltage system. The 12V battery provides this power. If the 12V is removed, the car cannot initiate the connection to the HVB, essentially bricking the vehicle.

Increased Parasitic Drain: Keeping the DC-DC converter (which converts ~400V to 12V) energized 24/7 to power small items such as fob sensors and infotainment items could lead to significant phantom drain on the main battery. This would increase the load on the HVB even when parked, reducing overall range and battery health over time. Then we'd have even more low/lost range threads on the forum. Nobody wants that.

Safety Hazard during Maintenance: Currently, disconnecting the 12V battery makes the low-voltage circuits safe to work on. Without it, technicians would be working on circuits directly connected to a high-voltage, high-capacity system, increasing the risk for arc flashing or shock.

It's Hot: The DC_DC converter would be working overtime. It'd be the equivalent of a 75 watt incadescent lightbulb turned on in a small enclosed space 24/7/365. This would increase the heat on surrounding components and would quite probably reduce the MTBF of the converter and surrounding components. Further, the DC-DC converter is actively cooled because of heat. Making the DC-DC converter work 24/7 would add to the cooling requirements increasing parasitic drain and add additonal cooling requirements.

Money: Most auto accessories are designed to run on 12V or 48V. Redesigning thousands of components to handle 400V-800V directly is cost-prohibitive and technically complex. It's always about the money.

There are actual engineers lurking and not so lurking here. Some are Ford employees. Perhaps one will comment further. It's not a dumb question but the answer is complicated, as usual.

Remembering also that 12V DC wouldn’t even cause a tingle, 400V DC would fry you from the inside and kill you instantly with no warning hence the 12V DC powers all the low voltage systems and the control systems including the safety interlock system for the HV system.

Almost any body can work on 12V systems reasonably safely, only those fully trained and equipped with all the HV safety equipment & insulated tools should go anywhere near the HV system and its component parts.

 

[Shayne]

Safety Disconnect Failure: Solid state

"Chicken or the Egg" Startup Issues: You do not need to boot everything just close the contacts. Actually there is 2 sets of contacts one for high voltage and one for the 12V computers. I do not think the CMOS battery in a PC boots the GPU.

Increased Parasitic Drain: You mean on a 100 KWh battery

Safety Hazard during Maintenance: Pulling the 120V out of the power supply does the same. Need a switch.

It's Hot: It is running the whole time you drive they may need to look into that.

Money: 12V's is good just like a PC mother board just need a better power supply to run on the big battery.

This car is only 365V but it would need to work with higher voltage packs like new 800V vehicles. I know I plug my computer into 120V and the power supply boots/runs most the components. The MME charges the 12V and runs the cars 12V components after booted. Just need to cut down on what Fords booting and running off the 12V and use a power supply like a PC does to do it. Often wondered why the CMOS battery on the PC motherboard was not rechargeable but it runs for years on the same one. A PC will boot now when the CMOS battery is dead just looses time etc. Need better motherboards and logic for these computers on wheels. Once solid stage batteries and electrical fires are eliminated/minimized we will see change. There are many young companies now with skin in the game so innovation is not yet dead.

 

[SonicBlue]

Would be pretty cool if there was a big physical lever to close the contacts and turn on the car. “KaChunk!” and “Let’s Drive!”

Also, I hate everything about the 12v in this car. How hard it is to reach the terminals. How hard to swap it out. The lack of SOC monitoring. The short lifespan. The death without warning. The insufficient battery management system. The specialized size and cost. And especially the Rube Goldberg means of opening the frunk to access it when it dies without warning. Everything.

 

[Maquis]

Question for the electrical experts: Why do North America households operate mostly on 120 volts when much of the world is 230 volts? Safety? Would not the higher voltage reduce the conductor wire size needed thereby saving money on copper wire. When we need the power, like EVSE chargers or electrical motors we we use 240 volts or higher. Once standards are established we are stuck with them.

North American households are also 240V (nominal), it’s just that we split it into 2 - 120V “phases”. That’s why anything that consumes significant power (not just EV charging) uses 240V. Our system is arguably safer in that the voltage to ground is 120 instead of 240.

 

[Space_Pony]

North American households are also 240V (nominal), it’s just that we split it into 2 - 120V “phases”. That’s why anything that consumes significant power (not just EV charging) uses 240V. Our system is arguably safer in that the voltage to ground is 120 instead of 240.

I've been zapped by 120. Glad it wasn't 240.

 

[Kamuelaflyer]

Safety Disconnect Failure: Solid state

"Chicken or the Egg" Startup Issues: You do not need to boot everything just close the contacts. Actually there is 2 sets of contacts one for high voltage and one for the 12V computers. I do not think the CMOS battery in a PC boots the GPU.

Increased Parasitic Drain: You mean on a 100 KWh battery

Safety Hazard during Maintenance: Pulling the 120V out of the power supply does the same. Need a switch.

It's Hot: It is running the whole time you drive they may need to look into that.

Money: 12V's is good just like a PC mother board just need a better power supply to run on the big battery.

This car is only 365V but it would need to work with higher voltage packs like new 800V vehicles. I know I plug my computer into 120V and the power supply boots/runs most the components. The MME charges the 12V and runs the cars 12V components after booted. Just need to cut down on what Fords booting and running off the 12V and use a power supply like a PC does to do it. Often wondered why the CMOS battery on the PC motherboard was not rechargeable but it runs for years on the same one. A PC will boot now when the CMOS battery is dead just looses time etc. Need better motherboards and logic for these computers on wheels. Once solid stage batteries and electrical fires are eliminated/minimized we will see change. There are many young companies now with skin in the game so innovation is not yet dead.

You’re looking at how a 400v system might work, which is fine of course. That, however, is not what the op asked; they wanted to know why its the way it is. I tried to answer that question in a relatively simple manner. Could BEV manufacturers eliminate a 12/24 v battery? Probably, but its not particularly simple or cheap, and would likely cost more. And despite Ford’s losses on BEVs, they’re not a not for profit organization.

 

[Teslaeata]

Even HINOs - hybrids in name only - (hybrids that are not hybrids but are just named as such) which have 48V systems also have the base 12V system for everything else, if they can’t do away with 12V for such vehicles they’ll never do away with 12V for EVs, it just ain’t safe & practical, doubt it has anything to do with cost, profit etc

 

[phidauex]

The issue isn't really the voltage, it is the available fault current, and the fact that it is direct current.

If you solidly short out the wiring in a home, even at 240V, two things will happen. First, fault current coming from the grid will start to flow into the circuit, causing current to rapidly rise. It might hit a few thousand amps max inside the first 60hz cycle, but inductance from the long cables to your utility pole will limit that. The magnetic force from that inrushing current will overload the spring in your circuit breaker, and it will start to open. There will be an arc inside the breaker from the high current passing through the air between the contacts.

Next, as the voltage drops naturally in the 60hz cycle, the current will reduce as well, until it reaches 0 volts and 0 amps at the crossing point, at which point the arc will quench in the breaker, and the circuit is open. In a short circuit, this tends to play out in a handful of cycles, probably less than 50ms.

An EV battery, on the other hand, instead of having 2-4kA available as fault current, has more like 30-50kA of available fault current. Additionally, since it is DC, the fault current ramps up much faster (it doesn't have to wait for the 60hz voltage cycle to increase), and it doesn't have a zero crossing to extinguish the arc. In this case, the fault current could hit maximum levels in 1-2ms, and there is nothing to stop the arc. This is orders of magnitude more dangerous. It is why a residential circuit breaker costs $12, and a HV battery contactor costs hundreds.

The contactors that are in the HV junction box have to be able to open these faults, and quench the arc without a zero crossing. They may only be able to do this once before having to be replaced. This is why the HVJB contactor recalls early in the MachE life were so important - Ford understandably didn't want to mess with a failure in that critical component.

That is all to explain the heart of why you need two electrical systems - one needs to start up, safety check the other systems, hear from the BMS that the battery insulation resistance is good, and then close in the main contactors. If the first system detects a fault, it won't close the contactors.

Now, the MachE 12V system is indeed annoying, it is too complex, hard to access components, etc. which is why their next gen platform is going 48V with a lot of simplification. I'm fully supportive of that. But you'll always need one system to bootstrap the 2nd system.

 

[moritzes]

Would be pretty cool if there was a big physical lever to close the contacts and turn on the car. “KaChunk!” and “Let’s Drive!”
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