Ask an Electrical Engineer

[CHeil402]

Thanks for this! Great thread for us non EE’s and a great read!

So here is something I have been wondering about. The Mach E is said to have a “400 volt” system. Some newer cars like the Porsche Taycan have an ”800 volt system” that articles say can use thinner wiring and can allow for faster charging. Can you explain why and what the difference is?

And a second question . . . Why do these cars need the 12 volt battery anyway with that giant battery under the floor? We have already seen some members have issues with those batteries. Could a system be designed to run all power from the main HVB?

Excellent questions. There's a lot to unpack here, so sorry if the answer runs long.

Part of the reason that a lot of EV suppliers went with 400 VDC is general availability of parts. The motors and power electronics were more readily available (which means cheaper). Secondly, the power you deliver to the motor (measured in Watts or W) is the product of the Voltage (measured in volts or V) and Current (measured in Amperes/Amps or A). That means if you increase the voltage you increase the power... however, doubling the power wasn't the main goal; halving the current was.

Sorry, but we need more technical background to explain why that's a benefit (I promise a less technical summary for those that don't need this level of detail). As I've mentioned before, in an ideal world all the power delivered to the load from the battery would be consumed by the load; however, the battery and wires have some level or resistance (measured in Ohms or Ω). When current flows through a resistance (e.g.: a wire), power is consumed and then burned as our old friend Joule heating that warms the conductors in the wire). Another equation to consider is that Power = Current ^2 * Resistance. That means that as current increases, the power lost in the wire increases exponentially! To reduce the lost heat, you need to increase the diameter of the wire (which reduces the wire's resistance). Current load is the driving factor in sizing your wires. Voltage only dictates the wire insulation thickness... that's why if you ever looked at Romex household wiring it has a 600 V stamp on it; that's the insulation's maximum rated voltage.

Commercial power distribution exploits this same physical property. That's why there are high voltage power lines and low voltage power lines. If a power plant wants to minimize the lost power in long distance delivery wires, they can step up the voltage to thousands of volts and put them up high (equivalent of more insulation), then step them down in transformers near customer houses. That's the glory of AC power and why it won the OG standards war for commercial power distribution over DC as AC power can have its voltage changed very easily using a transformer... but that's a history lesson/topic for another post.

Long story short, increasing the voltage allows you to decrease the current. Decreasing the current allows you to either: use smaller wires, or more importantly, deliver more power using the same wires. Therefore, a higher voltage allows you to charge faster because it exploits the limiting factor which is maximum current.

Now for the second question... and the award for first person to use one of my recommended questions! Why do we have a 12 V battery anyway since we have a big high voltage battery? This is actually a two part question. So first, why do we use 12 V and second, why do we have a low voltage battery?

We use a low voltage battery predominately for safety. Anything below 50 VDC is generally considered "touch safe voltage". Meaning the ideal resistance of your skin will keep the resultant current of you touching that voltage below dangerous levels. That being said... I'm not recommending you go around proving that. Drawing a little information from my industry again, some subway systems operate at about 750 VDC. And on older subway cars in the early 20th century (you know before safety was invented) actually did use that voltage for everything, including passenger lighting. Which would mean that if someone broke the light or a maintainer had to replace that light, they were directly exposed to potentially lethal voltage. Now we use a lower voltage to mitigate that risk and use a DC to DC converter to step down the voltage to a touch safe level. 12 V in particular was just a number that was an easy multiple to get to using lead acid battery chemistry and so that's what the automotive industry picked (although it's actually closer to 13.8 VDC but that certainly doesn't roll off the tongue like 12 V does). For reference, most US trains use 37.5 VDC.

The second part to the second part of this long post, is why do we need a low voltage battery at all, why not just use a DC/DC converter? And the answer to that is that the MME actually does this already. The 12 V battery is (supposed to) be recharged by the high voltage battery using the onboard DC/DC converter since the capacity of the 12 V battery is pretty low and would be consumed very quickly (which is what you're seeing in the other posts when it isn't properly recharged). However, for the above-mentioned safety reasons when the car is off we want to isolate that pesky dangerous high voltage by opening contactors... but when we do that, how do we close them again? Plus, when the car is off we still want some systems on the car to remain awake and phone home for those sweet sweet OTA updates. Therefore, we put in a low voltage battery to act essentially as a buffer for the DC/DC converter to store some power for when the car is off and to act as a backup if there are any hiccups with the DC/DC converter. Pulling more train analogies (not to get this already long answer "off the rails"... sorry), when there is a gap in the third rail that delivers that high voltage, we don't want the lights to turn off, so we use the low voltage battery to fill in the gap (even though we have a large DC/DC converter) that under normal conditions is what powers the low voltage side of things. Now one feature that trains have that I think Ford wishes the MME had, is what's called a "dead battery start" where you effectively can manually close those contacts that isolated the high voltage battery from the DC/DC converter to kick it back on and start recharging the battery when the battery is dead... maybe next model year.

In summary, the 12 V battery acts as a buffer for the high voltage battery DC/DC converter and is used to keep systems alive after the car is turned off and to turn the high voltage systems back on. We use a 12 V battery simply because they're readily available and a lot of existing car electronics expect this voltage to operate.

That's a lot to digest, so feel free to reach out with follow up questions.

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[CHeil402]

Good summary...

The neutral is needed by code not for when things go right, but for when things go wrong.

Let's say something starts failing in your EVSE, and instead of the normal zero current on the "no neutral 14-50" ground wire, it's carrying some current. You can't easily touch an exposed part of a neutral wire in your house, but you can for a ground wire. Likely just touching the screw holding a plate on a light switch in your house makes contact with ground. If ground is now "energized", there is some slight chance you could be energized, which is not good. I'm not saying this is a likely event (it's not), but electrical code is designed so the chance of this is 0%.

That's why you wouldn't want to tie neutral and ground together (except where they are bonded in the circuit breaker). I was referring to leaving the neutral pin disconnected (or floating) which is not up to code and non-ideal. I think you're referring to what would happen if the ground and neutral pins were tied together in the outlet to get the 120 V... which is also not up to code, but also and much more importantly, is a safety hazard as you pointed out.

 

[jhalkias]

Chris, do you have an answer to the question in this thread?
https://www.macheforum.com/site/threads/battery-for-frunk-release-to-store-in-bumper.4443/

 

[CHeil402]

Chris, do you have an answer to the question in this thread?
https://www.macheforum.com/site/threads/battery-for-frunk-release-to-store-in-bumper.4443/

I coincidentally was just looking at that thread. There have been other posts about this topic, but as far as I know, nobody has tried using a 9V battery to open the frunk. Although, even if it does work, getting the frunk open with a dead 12 V battery only gets you so far if the 12 V battery is actually dead (assuming the software fix addressed the DC/DC converter issues). Although theoretically, if the 12 V battery is dead, you should be able to 'turn on' the car using any (relatively low power) 12 V power source and the DC/DC converter should be able to power the low voltage stuff from the high voltage battery until the car 'turns off' again... hopefully at an auto store where you can get a replacement. Much like the 'dead battery start' methodology of passenger trains I mentioned above.

The reason we use lead acid car batteries is because lead acid batteries are good at providing high amperage which was needed to start an ICE engine. That doesn't hold true in an EV as all you're really doing is powering up the car computers. Something the size of a cell phone or laptop battery should be sufficiently powered to 'boot it up' and have the HV battery take over. You might be able to rig up a USB-C laptop battery pack to do this as it would allow for 12 V. The frunk release is likely less sensitive to the 9 vs 12 V; however, the computers are probably less tolerant to that low of a voltage... and this is all speculation without knowing the control logic of the MME's DC/DC converter, etc.

I'll look into some details for similar solenoids now, but if anyone has schematics or datasheets or part numbers on the frunk emergency release solenoid I'd be happy to dig into the details.

UPDATE 3/25: Thanks to @louibluey for taking the plunge in testing this. Confirmed that 9V battery doesn't work. https://www.macheforum.com/site/thr...ease-9v-battery-does-not-work-12v-works.4774/ And 8 AA batteries do work! https://www.macheforum.com/site/thr...tery-does-not-work-12v-works.4774/post-141023

 

[jhalkias]

I coincidentally was just looking at that thread. There have been other posts about this topic, but as far as I know, nobody has tried using a 9V battery to open the frunk. Although, even if it does work, getting the frunk open with a dead 12 V battery only gets you so far if the 12 V battery is actually dead (assuming the software fix addressed the DC/DC converter issues). Although theoretically, if the 12 V battery is dead, you should be able to 'turn on' the car using any (relatively low power) 12 V power source and the DC/DC converter should be able to power the low voltage stuff from the high voltage battery until the car 'turns off' again... hopefully at an auto store where you can get a replacement. Much like the 'dead battery start' methodology of passenger trains I mentioned above.

The reason we use lead acid car batteries is because lead acid batteries are good at providing high amperage which was needed to start an ICE engine. That doesn't hold true in an EV as all you're really doing is powering up the car computers. Something the size of a cell phone or laptop battery should be sufficiently powered to 'boot it up' and have the HV battery take over. You might be able to rig up a USB-C laptop battery pack to do this as it would allow for 12 V. The frunk release is likely less sensitive to the 9 vs 12 V; however, the computers are probably less tolerant to that low of a voltage... and this is all speculation without knowing the control logic of the MME's DC/DC converter, etc.

I'll look into some details for similar solenoids now, but if anyone has schematics or datasheets or part numbers on the frunk emergency release solenoid I'd be happy to dig into the details.

Some of us thought about putting a battery jumper like an NGCO in the Frunk for emergency jumps. But it does no good if you can’t get into the frunk, thus the idea of the hidden small batteries. Ford didn’t give us a key entry into the car, and if the LVB is dead we may be SOL to get to our jumper spare battery.

 

[breeves002]

At OPs request I did some quick research into the frunk. They don't provide specifications on minimum voltage to operate the latch. I don't believe a 9v battery would work though as they have pretty low current output.

Ford says this about the hood latch: "The external battery jump port can provide external 12-volt power, through front trunk release relay module, to the hood release latch actuator when the 12-volt battery State-of-Charge (SOC) is less than the allowable threshold."

So the good news is at least it checks battery SOC before just opening. You can't just randomly open someones hood.

 

[CHeil402]

Some of us thought about putting a battery jumper like an NGCO in the Frunk for emergency jumps. But it does no good if you can’t get into the frunk, thus the idea of the hidden small batteries. Ford didn’t give us a key entry into the car, and if the LVB is dead we may be SOL to get to our jumper spare battery.

At OPs request I did some quick research into the frunk. They don't provide specifications on minimum voltage to operate the latch. I don't believe a 9v battery would work though as they have pretty low current output.

Ford says this about the hood latch: "The external battery jump port can provide external 12-volt power, through front trunk release relay module, to the hood release latch actuator when the 12-volt battery State-of-Charge (SOC) is less than the allowable threshold."

So the good news is at least it checks battery SOC before just opening. You can't just randomly open someones hood.

See my thoughts on this here: https://www.macheforum.com/site/thr...k-release-to-store-in-bumper.4443/post-132773

 

[jhalkias]

At OPs request I did some quick research into the frunk. They don't provide specifications on minimum voltage to operate the latch. I don't believe a 9v battery would work though as they have pretty low current output.

Ford says this about the hood latch: "The external battery jump port can provide external 12-volt power, through front trunk release relay module, to the hood release latch actuator when the 12-volt battery State-of-Charge (SOC) is less than the allowable threshold."

So the good news is at least it checks battery SOC before just opening. You can't just randomly open someones hood.

Correct. That release will only work when the 12V LVB is "flat"

 

[Jimmy2]

In summary, the 12 V battery acts as a buffer for the high voltage battery DC/DC converter and is used to keep systems alive after the car is turned off and to turn the high voltage systems back on. We use a 12 V battery simply because they're readily available and a lot of existing car electronics expect this voltage to operate.

That's a lot to digest, so feel free to reach out with follow up questions.

Sounds like the MMe 12 v battery is asked to to a lot. Would you predict it has a shorter life span as a consequence? Will need replacement sooner than in an ICE?

 

[breeves002]

Sounds like the MMe 12 v battery is asked to to a lot. Would you predict it has a shorter life span as a consequence? Will need replacement sooner than in an ICE?

In theory no. It should last the same amount of time. Honestly it should last longer. It isn't subjected to the heat of the engine bay which can lower battery longevity.

The MME 12v battery is only a 300 CCA 35AH battery. It is tiny. It doesn't have to turn an engine over so they decided to go cheap on it. Which I guess I understand. It has fairly low capacity, though even sitting in accessory mode the DCDC converter is supposed to kick on to keep it going.

 

[mr_raider]

In theory no. It should last the same amount of time. Honestly it should last longer. It isn't subjected to the heat of the engine bay which can lower battery longevity.

The MME 12v battery is only a 300 CCA 35AH battery. It is tiny. It doesn't have to turn an engine over so they decided to go cheap on it. Which I guess I understand. It has fairly low capacity, though even sitting in accessory mode the DCDC converter is supposed to kick on to keep it going.

300CCA? Yeesh...

The Canadians are already getting nervous.

 

[CHeil402]

Sounds like the MMe 12 v battery is asked to to a lot. Would you predict it has a shorter life span as a consequence? Will need replacement sooner than in an ICE?

The answer to that would depend on the proper operation of the MME DC/DC converter. In an ideal operation, the 12 V battery is only needed to power a few essential things when the car is off (the same as it does in an ICE car) and the high voltage (HV) battery is isolated.

In an ICE car, by far the biggest load is starting the engine which puts a lot of stress on the battery (large starting current demand). This is evidenced even more so in cold weather when you typically see a dead ICE 12 V battery as to start moving the engine when the oil is thicker takes even more current. Once an ICE car starts, the alternator (generator) takes over and should be what both powers the low voltage stuff in the car and recharges the battery. That's why it's bad to run for an ICE 12 V battery for several back-to-back short trips, it takes a large effort to start the car and you're not giving the alternator a chance to catch up by recharging the battery.

In an EV, the 12 V battery's job in starting the car is to keep the 'turn on' button functional by powering the module that starts the car. This in turn should un-isolate the HV battery and start the DC/DC converter which mimics the job of the ICE alternator... it provides low voltage power to the car modules and recharges the 12 V battery. The big difference is it takes significantly less effort to close a contactor to un-isolate the HV battery compared to turning an engine in an ICE car.

The more times that you cycle the 12 V battery from a low state of charge (SOC) to a high SOC the more life you ultimately take out of the battery and eventually it cannot hold a sufficient charge. Technically a near end of life 12 V battery is also more likely to be able to start an EV car where it would otherwise not be able to provide the current needed to start an ICE engine.

So to answer the original question, assuming the vehicle logic is done correctly, you can think of the 12 V battery as just a small reserve battery that the HV battery tops off when needed. If this is happening correctly as there is less of a need in an EV to deep discharge the 12 V battery, in theory, it should last much longer as is evidenced by Ford (arrogantly) burying the battery thinking that it is unlikely to need to replace it.

 

[Jimmy2]

So to answer the original question, assuming the vehicle logic is done correctly, you can think of the 12 V battery as just a small reserve battery that the HV battery tops off when needed. If this is happening correctly as there is less of a need in an EV to deep discharge the 12 V battery, in theory, it should last much longer as is evidenced by Ford (arrogantly) burying the battery thinking that it is unlikely to need to replace it.
[/QUOTE]

Makes good sense to me, very well explained. Thanks.

 

[CHeil402]

Makes good sense to me, very well explained. Thanks.

Happy to help! One more point that I also added to my answer is that technically a near end of life 12 V battery is also more likely to be able to start an EV car where it would otherwise not be able to provide the current needed to start an ICE engine.

 

[breeves002]

The only other thing I want to add about the 12v battery is that they suck. 12v batteries just fail without reason. Maybe it'll last 7 years, maybe it'll last 1 year. I still think it is really dumb where Ford put that battery.

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