Ask an Electrical Engineer

[CHeil402]

I have seen more than one online write up claiming Tesla is better vs MME while citing the heat pump. Do you think having a heat pump would have made the vehicle better in a significant way? In other words, are we missing out on something big without it?

thanks, additionally... your thread and time/attention to it is greatly appreciated

Yes, this is certainly a topic of much discussion and debate and without knowing the specifics of the Tesla heat pump, I'll give you some general information and my personal opinion on the matter...

First, what is a heat pump in a few words. Basically it's only job is to move heat from one place to another. For example, you're refrigerator does this as does your air conditioner. Both are examples of heat pumps... neither a fridge or A/C unit technically generate cold, they just remove/move heat. A fun thought experiment puzzle to consider this is if you put a fridge in a closed off room (no outside influence) and left the door open and let it run, what would happen to the ambient temperature in the room? The answer is that it would get hotter. Why? Because the fridge is simply moving heat not generating cold... and since it's not 100% efficient, it will generate some waste heat thus adding to the overall temperature.

A heat pump does this through a refrigeration cycle. If you want more details on how it does that, see here: But basically it uses electricity to exploit physical properties of a refrigerant to do this at greater than 100% efficiency, even up to around 300%+. So for every one unit of electricity you can move about three units of heat! Great!

Thought experiment aside... in a gas car, the heat in the air vents is siphoned off the engine since it generates a ton of waste heat. So what do you do in an EV? The motors aren't conveniently placed and don't get nearly as hot as your controlled explosion gas engine. Your battery does get hot, but it also works better not being too cold, and cold times are the only time we can take advantage of what most people are referring to in the Tesla's heat pump, since as previously mentioned, the MME does have a heat pump to cool the car off (the air conditioner).

To compare, an electric resistive heater (what Ford calls the "E-Heat" in the MME) burns electricity for heat like your oven and toaster do. That is 100% efficient.

So after that long-winded answer, you're probably thinking, clearly the heat pump is better because 300% > 100%! However, a heat pump loses efficiency based on the ambient temperature its scavenging and moving heat into. So the colder it gets out, the less heat there is to scavenge from the air. This is the exact reason that houses that use heat pumps tend to have electric resistive backups for very cold days where an air-sourced heat pump stops being great.

Now you can move heat from the battery, but again, in most really cold days your car is already using electricity to warm the battery anyway! So there goes the benefit.

Long story short, in my opinion, the benefit of the heat pump only makes sense for mildly cold climate and not very cold climate. So this isn't a Tesla slam dunk in my book. Plus the complexity of the heat pump that Tesla uses to find as many sources of heat as possible isn't without its flaws or maintenance headaches when it breaks.

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[ab13]

How to keep the rats away? Tell me no soy was used on the wires please.

There is actually rodent repellant tape for wires, not sure how well it works.

 

[ab13]

Maybe the cooling is not ideal, it does get hot.

It's necessary that two objects or surfaces will have a temperature difference in order to transfer heat. The heat flow is directly proportional to the temperature difference, so if there is no difference then there is no heat flow. Whether it is "too hot" is another question, that depends on what range it was designed for.

A simple example, touch your hand to a cold or hot surface, such as a window. You will feel the heat transferring to or from your hand if there is a temperature difference between the two. If they are the same temperature you won't notice anything. It's also why touching different materials have different heat flow, due to the thermal conductivity of the material. Plastic and wood are insulating, so they have lower conductivity and will not feel so "hot," but touch most metals and you will definitely feel if it is very hot or cold.

 

[CHeil402]

NEMA 14-50 or 6-50 plug/socket preference?

I currently have an old Nissan home charger that is on a 40 amp circuit that has a NEMA 6-50 plug attached. I know that I can use this old charger to charge my upcoming MME but I am concerned about its age (8yrs). I also know that I can purchase another home charger that uses a 6-50 plug, but I cannot use the Ford MME provided charger/cord to plug directly into the wall socket as I believe the MME charger uses a 14-50 plug. I assume manufacturers are using this type plug for many folks who may already have existing 240 circuits in their garages for dryers, etc.

1) What is the purpose of the neutral wire used with the 14-50 plug/socket when used to charge EV at 240 watts? It is certainly not needed if the vehicle can also use a 6-50 plug that has no neutral wire.

2) Would the Ford provided cable/charger (or any other home charger) work if plugged into a 14-50 socket that has no neutral wire attached to the wall socket? If it did and was not a code violation, I could just change the wall socket in the garage when I upgrade my home charger to use a NEMA 14-50 plug, which would also allow me to use the included Ford MME charger. That way I would not need to run a new neutral wire.

Thanks for your time with this subject matter.

I'll break this down in a few parts. First, my personal preference (and what I installed in my garage) is the NEMA 14-50 outlet. Mostly because that 'seems' to be the de facto EV plug most auto manufacturers are supporting. For example, the Ford Mobile Charger the car comes with only comes with the NEMA 14-50 option for 240 V charging. Also, theoretically, the NEMA 14-50 outlet gives you more future-proof options as you can also pull 120 V from it (more on that later).

Next point I feel obligated to at least mention is that a NEMA 14-50 or NEMA 6-50 outlet are rated for 50 A (hence the "-50" in the name) and therefore should have wiring going to it rated for 50 A and be protected by a circuit breaker rated for 50 A. You didn't mention anything about the wiring that runs to it, but assuming it is rated for 50 A, you could replace the circuit breaker with a 50 A one. Technically, it isn't unsafe to have a smaller CB, but it could result in nuisance trips for an appliance pulling close to the trip point, especially since that trip point isn't 100% precise (there's some +/- to the rating). To continue this tangent, you have to de-rate the outlet by 80%, so any conforming appliance should never pull more than (50 A * 80% = 40 A) which is why this has probably worked without issue in the past... but if you were pulling close to 40 A the 40 A CB could trip.

I wouldn't be too concerned about using an 8 year old EVSE on the MME if there are no obvious signs of wear/damage. That being said, there has been a lot of discussion on using an adapter to convert between NEMA 6-50 and NEMA 14-50. While this is technically possible for an appliance that doesn't use 120 v (again, it's coming I promise) while I believe (haven't personally checked) the Ford Mobile Charger doesn't use, you will not likely find a UL compliant adapter that does this and if anything bad happened, that would certainly be something your insurance company could point to. So in summary, I don't think using an adapter would be an issue, but I also wouldn't recommend it.

So to answer your first real question after all that... what is the purpose of the neutral wire? The plug that has 4 conductors (NEMA 14-50) has the neutral and the plug that has 3 conductors (NEMA 6-50) does not. Typical power provided to residential houses in the US are split-phase 240 V. I wrote another post with more detail on this here https://www.macheforum.com/site/thr...aration-and-installing-charger.610/post-56290 if you are interested, but to simplify, the two hots are -120 V and +120 V, so the difference between them is 240 V, so the NEMA 6-50 can ONLY provide 240 V. When you add the neutral at 0 V to the mix, you can now pull both 240 V and 120 V (see, I told you it was coming) from the plug. Some appliances take advantage of this... for example an electric dryer that might use the 240 V for the heating element but 120 V for everything else (controls, lights, etc.).

To summarize, you need the neutral wire to get 120 V. For an appliance that doesn't need the 120 V, you don't need the neutral. That's why you can buy an EVSE with a NEMA 6-50 plug... but it limits what you can use the plug for in the future. If you don't care about that, then stick with NEMA 6-50. If you already have a plug installed, it's certainly not worth the cost of upgrading to a NEMA 14-50 in my opinion, but for a new install, I would go NEMA 14-50.

To address your second question, assuming the Ford Mobile Charger doesn't utilize 120 V for anything which I haven't verified, yes... technically it would work if you changed the plug to a NEMA 14-50 and left the neutral pin floating. BUT as you astutely pointed out, this is indeed a code violation.

To summarize that, if you don't want to use your existing EVSE you could technically get an adapter for the Ford Mobile Charger to convert the outlet from NEMA 6-50 to NEMA 14-50 which I would recommend before changing the plug and leaving the neutral pin floating... BUT I wouldn't recommend either of those solutions as permanent solutions (maybe for occasional usage only). But my personal recommendation would be to leave the Ford Mobile Chagrer in the trunk for emergencies while out and about and either keep using your exisitng EVSE, or replace it with a new EVSE with a NEMA 6-50 plug. Personally I use the Grizzl-E which has said option and is pretty inexpensive ($399 minus the 30% tax credit): https://grizzl-e.com/product/usa/chargers-usa/grizzl-e-classic-nema-6-50-2/

I hope this helps, feel free to ask a follow up question, or message me.

 

[HRT_Florida913]

Lets be real though - is the lack of a heat pump a deal breaker? All of the 2019 Tesla lineup doesn't have heat pumps. Sure you're going to use more energy in cooler weather but for 99% of people it doesn't matter.

Especially if you love in South Florida, like I do. I rather would have had ventilated seats! LOL

 

[JamieGeek]

*All* of my EVSE's (except the one that came with the car) are 2013 vintage and they've all worked just fine with the car.

 

[RonTCat]

To address your second question, assuming the Ford Mobile Charger doesn't utilize 120 V for anything which I haven't verified, yes... technically it would work if you changed the plug to a NEMA 14-50 and left the neutral pin floating. BUT as you astutely pointed out, this is indeed a code violation.

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%.

 

[jhalkias]

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?

 

[Maquis]

To summarize that, if you don't want to use your existing EVSE you could technically get an adapter for the Ford Mobile Charger to convert the outlet from NEMA 6-50 to NEMA 14-50 which I would recommend before changing the plug and leaving the neutral pin floating... BUT I wouldn't recommend either of those solutions as permanent solutions (maybe for occasional usage only).

I would definitely use an adapter before I installed a 14-50 without a neutral.

 

[RonTCat]

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?

Wiring is sized for current. Power = Voltage x Current. Double the voltage and half the current gives the same power. Half the current = smaller gage wire needed.

 

[Maquis]

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%.

Not exactly.

There is nothing in the EVSE that connects to the neutral so it cannot be used "when things go wrong".

The neutral connection is required only if the load utilizes 120V in addition to 240V.

It is the ground wire that is provided for safety, not the neutral.

Installing a 14-50 receptacle without a neutral will have no consequence for loads that do not utilize 120V. The problem occurs down the road when something that does require a neutral is plugged in - best case it it fails to function, worst case is that the equipment is damaged.

 

[breeves002]

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?

Part one. Voltage. Ohms law is what you need to know. V=IR or voltage = current*resistance. You can solve for any of those variables to get one or the other. Voltage*current = power (watts).

Let us assume the Mach E battery pack is at 400v for simplicity. In reality it will be lower most of the time, but just for the math we'll assume 400v. At 400v and 150kW DC fast charging you are putting 375A of current into the battery. We find this by taking 150,000 watts and dividing by 400 volts. If you have an 800v battery pack you now are only putting in 187.5A of current. Battery cells are current limited for charging, not power limited. Therefore the same cells arranged in an 800v configuration can take double the charging current the 400v system can. Now the 800v system can take 300kW input power.

The other thing to think about is heat. The cells have to be able to stay cool enough. The higher the current the higher the heat. Heat is caused by resistance. Even though the resistance is nearly 0 there is still some and that generates heat. The batteries themselves has resistance that gets worse the older they get.

The last thing to think about is the charger. A 350kW DC fast charger is probably getting 480v 3 phase power. It is possible it is getting 1kV+ though considering the current requirements for 350kW. At 480VAC you'd need 729A of current to feed to the car. That's an incredibly large amount of power you can't just easily tap into a Walmarts mains and get.

As to part 2. 12v battery works the exact same way it does in a normal car. The electronics related to the car are all powered by it. There are a few major reasons to keep it.

First of all HV is dangerous. You can't send 400v to every module in the car and in all of the car's wiring. That's a great way to kill someone.

Now you ask - but that's why we have the DC DC converter. Precisely. However that has its inefficiencies. In the buck conversion you lose some power as heat. So if you have to constantly drain the HVB a little bit to keep everything running it is a waste of power when you could just use a standard 12v battery.

The largest safety reason though is in case of DC DC converter failure. You'd have the car instantly turn off and all modules power down. This would be extremely dangerous and possibly cause an accident. Imagine middle of a turn on the interstate trying to merge or something and you suddenly lose power steering and your brake booster. Not fun.

 

[breeves002]

Wiring is sized for current. Power = Voltage x Current. Double the voltage and half the current gives the same power. Half the current = smaller gage wire needed.

You're right, but I believe this is more about the battery cell max charge current than it is about the wires. Of course the CCS plug has a max current rating for the standard. Therefore you have to increase the voltage to get more power.

Not exactly.

There is nothing in the EVSE that connects to the neutral so it cannot be used "when things go wrong".

The neutral connection is required only if the load utilizes 120V in addition to 240V.

It is the ground wire that is provided for safety, not the neutral.

Installing a 14-50 receptacle without a neutral will have no consequence for loads that do not utilize 120V. The problem occurs down the road when something that does require a neutral is plugged in - best case it it fails to function, worst case is that the equipment is damaged.

You beat me to it. The reason it is there is for down the road when you move houses and someone else is trying to use that outlet. If the neutral has no connection it isn't going to carry any current.

 

[RonTCat]

Not exactly.

There is nothing in the EVSE that connects to the neutral so it cannot be used "when things go wrong".

The neutral connection is required only if the load utilizes 120V in addition to 240V.

It is the ground wire that is provided for safety, not the neutral.

Installing a 14-50 receptacle without a neutral will have no consequence for loads that do not utilize 120V. The problem occurs down the road when something that does require a neutral is plugged in - best case it it fails to function, worst case is that the equipment is damaged.

Sorry, I was unclear, I am assuming an EVSE that needs 120v.

 

[mr_raider]

Long story short, in my opinion, the benefit of the heat pump only makes sense for mildly cold climate and not very cold climate. So this isn't a Tesla slam dunk in my book. Plus the complexity of the heat pump that Tesla uses to find as many sources of heat as possible isn't without its flaws or maintenance headaches when it breaks.

great explanation We heat our house with an HSPF 9.5 heat pump. But below -10C, it starts to struggle big time, and at -14C I just lock it out and use only strip heat.

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