Ask an Electrical Engineer

[Jimmy2]

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.

If the 12 v battery dies for whatever reason, say in your garage. Can you plug into your 240 or 120 outlet and save the day?

Sponsored

 

[CHeil402]

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.

I can second that they suck... but having something like it is a necessary evil. Ford could have kept the DC/DC converter active all the time, but I think the thought process there by Ford was to reduce the "phantom drain" on the battery when the car is off that most people don't understand the nuance of hate.

They could also use a 12 V lithium battery, but they have their own issues as they need a much more intelligent battery management system. The ideal option would be to use a super-capacitor for when the HV battery is off which would never need replacing; however, it's more expensive and based on the mentality of "nobody will miss the second key fob and second row ambient lighting" I think it's fair to say that "advanced 12 V battery replacement" was high on the cost-cutting chopping block.

 

[CHeil402]

If the 12 v battery dies for whatever reason, say in your garage. Can you plug into your 240 or 120 outlet and save the day?

If the battery is dead because it got fully depleted for whatever reason, in theory, yes. Plugging in the EVSE should 'wake up' the HV stuff which should include the DC/DC converter which should then start charging the 12 V battery.

If the battery is dead because it's actually dead, then probably not. Although you might be able to get it to a point where you could start the car and drive to buy a replacement.

Note that this is all speculation based on Ford's programming.

 

[Jimmy2]

If the 12 v battery dies for whatever reason, say in your garage. Can you plug into your 240 or 120 outlet and save the day?

Never mind, should have better read your "dead batter start" comment. Perhaps in 2022, right?

 

[CHeil402]

Never mind, should have better read your "dead batter start" comment. Perhaps in 2022, right?

Yes... hopefully they learned that their design wasn't as bullet-proof as they thought and they add a "dead battery start" function similar to what passenger trains have.

 

[breeves002]

I can second that they suck... but having something like it is a necessary evil.

I think the reason you don't just leave a DC DC converter active is safety. If your DC DC converter fails while driving on the interstate the car will just turn off. That is seriously dangerous.

I had a friends fusion hybrid that had the DC DC fail. Battery light came on and said check charging system. Battery finally died (it was weak too) at my house. Had to leave it on the jumper cables to keep the engine running. 92k miles but covered under warranty with Ford.

 

[breeves002]

If the 12 v battery dies for whatever reason, say in your garage. Can you plug into your 240 or 120 outlet and save the day?

No because the car can't communicate with the charger then. The charger will plug in and won't hear a response from the car since the modules are all offline. The EVSE doesn't provide power until the car tells it to so it won't wake up to tell the charger to provide power!

 

[CHeil402]

No because the car can't communicate with the charger then. The charger will plug in and won't hear a response from the car since the modules are all offline. The EVSE doesn't provide power until the car tells it to so it won't wake up to tell the charger to provide power!

I guess that falls under the category of "dead battery start". Oh well. There's always next model year.

 

[breeves002]

I guess that falls under the category of "dead battery start". Oh well. There's always next model year.

It is a tough problem to solve. J1772 could update the standard (or create a new standard) to implement two low voltage pins. 12v applied to wake up the modules and allow the DC-DC converter to enable.

Otherwise you're stuck jumping the 12v to get it to kick on.

 

[Space Pony]

My wife and I picked up our MME last Saturday(yay). Right now we're stuck with slow charging on 110v in our garage. I was hoping we'd just be able to get the 14-50 outlet installed in our garage and use the mobile EVSE that came with. It appears however that we only have 40amps in the garage.

I've been finding conflicting information on how many amps the mobile EVSE draws. Can anyone confirm the actual number for me? Sources appreciated!

 

[CHeil402]

My wife and I picked up our MME last Saturday(yay). Right now we're stuck with slow charging on 110v in our garage. I was hoping we'd just be able to get the 14-50 outlet installed in our garage and use the mobile EVSE that came with. It appears however that we only have 40amps in the garage.

I've been finding conflicting information on how many amps the mobile EVSE draws. Can anyone confirm the actual number for me? Sources appreciated!

First off, congrats on the car! I hope you're enjoying it.

Charging on 120 V (L1 charging) obviously isn't ideal, but for some it may suffice depending on how far you travel on average and how long you leave it plugged in overnight. Assuming (since you asked) that it isn't sufficient for your needs, L2 charging is what you need.

Installing a NEMA 14-50 outlet is the best route in my opinion for L2 charging... but based on your question, I have a few questions. You mentioned that you were hoping to get a NEMA 14-50 outlet installed (meaning you don't have one yet) but then you say it appears that you only have 40 A in the garage. I'm not sure what you mean by that. Do you already have a 240 V outlet in your garage but it's rated for 40 A?

The current that the EVSE draws is an independent issue from what the outlet supports. But since you asked, the Ford Mobile Charger supports a current of 30 A.

(Source: https://www.ford.com/buy-site-wide-content/overlays/mach-e-overlays/ford-connected-charge-station/)

But again, that's a separate issue. A NEMA 14-50 outlet is rated for 50 A, but if you plug in the Ford Mobile Charger, it will only pull 30 A which is fine. If you plugged it into an outlet that didn't support that 30 A (like if it were in a 15 A circuit breaker) then it would still try to pull 30 A but it would trip the circuit breaker.

If, for example, you already had a circuit breaker on your main panel going to your garage rated at 40 A that goes to a sub-panel that distributes that as two 20 A breakers, then you have a few choices. If the conductor that runs to the sub-panel is actually rated for larger than 40 A, then you can up-size the 40 A breaker you reference. If not, you're going to either need to run new cable, or get an EVSE that can use something like a NEMA 14-30 plug (capped at 30 A) and sacrifice that current load from any other circuits in your garage (e.g.: 30 A going to NEMA 14-30 and 10 A for other stuff).

Note also that you have to de-rate the load of the outlet rating by 20% per electrical code. So a NEMA 14-50 outlet, even though being rated for 50 A, can actually only draw 40A (or 80%). Which is why other plug-in EVSE's that use a NEMA 14-50 outlet (e.g.: Grizzl-e) can only draw a max of 40 A. If you want more than that, you'll need a hardwired install to an EVSE with a 60 A circuit breaker (which is how the Ford Connected Charge Station gets 48 A (or 80% of 60 A)).

If that doesn't answer your question, let me know and provide more details on what you mean by the statement that you "only have 40 amps in the garage".

 

[JimCapri]

At OPs demand I did some speedy investigation into the frunk. They don't give determinations on least voltage to work the hook. I don't really accept that a 9v battery would work however as they have pretty low current yield.

 

[CHeil402]

At OPs demand I did some speedy investigation into the frunk. They don't give determinations on least voltage to work the hook. I don't really accept that a 9v battery would work however as they have pretty low current yield.

Thanks. This was confirmed that a 9V battery will not work, but 8 AA batteries will, see the update to this post...

https://www.macheforum.com/site/threads/ask-an-electrical-engineer.4307/post-132392

Particularly because the old assumption that it was a simple relay control was wrong

https://www.macheforum.com/site/thr...k-release-to-store-in-bumper.4443/post-132773

 

[Cherie]

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.

Hi. Calif here, so a few cold mornings.... If I turn off e-heat, how does that conserve electricity, and will the car warm up eventually? What is used for warm up while still plugged in?

 

[CHeil402]

Hi. Calif here, so a few cold mornings.... If I turn off e-heat, how does that conserve electricity, and will the car warm up eventually? What is used for warm up while still plugged in?

My understanding of how Ford implements this, is that they refer to their resistive heating as "E-Heat". It's slightly odd that they even give you the option to toggle it in my opinion because it leads to confusion. I think it's really there so you can definitely turn off that load when you're battery is low and you're trying to stretch your remaining distance to your next charger.

When turned off, there is no true source of heat and you really just have a fan blowing ambient temperature air through the ducting in the car that may get slightly warmer as a result of the electronics that generate some heat near the duct work, but nothing significant. So it really won't ever get much warmer than ambient.

When you turn on E-Heat, the fan is now blowing the ambient air over a resistive heating element, much like how home ceramic heat fans work...

Your best bet in either scenario is to use recirculated air since you're not bringing in fresh cold air from outside and trying to reheat it.

When you remote start your car or if you schedule departure times where the car heats itself up when plugged in, it's doing the same thing except it's pulling the energy from the plug and not from the car's high voltage battery.

Sponsored