Explain the Math of AC vs DC Charging

When Ford unveiled the new F150 Lightning, with an extended range battery pack net capacity nearly twice as big as the MME 4X, but that supposedly fast charges back to 80% in roughly the same amount of time, it got me wondering how that is possible. Which got me thinking about how much I really know about charging math.

Here is what I think I know. And I’m asking you guys to correct me and fill in the gaps.

First, up to a limit set by Ford, charging speed is determined by the kW that can be supplied by a given source. kW = Amps X Volts / 1000, so my 32amp mobile charger on 240v wiring is supplying 7.68kW. In an hour, that charger will give me 7.68kWh. which means I’ll need 11.5hrs to fully recharge my 88kWh of useable capacity from 0-100%.

The actual AC/DC conversion occurs inside the car’s on-board charger, which is capped at 11.3kW. But this doesn’t really matter since I don’t have an AC source that can supply that much kW. My circuit is 7.68kW.

Next, DCFC stations bypass the on-board charger because they are directly supplying the DC current. So I don’t need to worry about the OBC’s 11.3kW cap there, either. So a 50kW charger will give me 50kW in an hour (50kWh) - at least until I hit 80% SOC at which point Ford dramatically tapers off the charge speed down to _____.

A 150kW should give me 150kWh, but it doesn’t because, again, Ford tapers the speed. The exact charging curve varies, but you are basically supposed to get from 10-80% SOC in _____ minutes.

So did I get all that right?

So this brings me the F150. How does it charge at roughly double the speed of the MME? It sounds like Ford is allowing it to charge twice as fast. But if Ford can allow that, why can’t it allow the same for the MME? Is this a function of basic math that I’m not aware of, or is this more a function of complex engineering which allows the F150’s pack to charge faster, safely?

A lot of it will be in the charging curve. With the F150 having a much larger battery, It will likely hit the full 150kw and stay there longer than the MME can. Currently the MME can only charge at 150kw for like 3 minutes before they slow it down to close to 100kw.

As you've laid out in the original post, there can be many limiting factors. However, when it comes to the limitation of the battery itself, the safe charging speed is generally proportional to the capacity of the battery.

For a big battery pack, you can think of it as two smaller battery packs. Say the smaller battery pack when packed into a car (Mach E) alone can charge at speed X. Now pack two of those battery packs into a car (F150). Each of them can charge at speed X. So the overall charging speed is 2X. Now, of course, this is assuming you don't run into other limiting factors.

Speaking of other limiting factors. The charging curve of Mach E is mostly a relatively flat line near 90kW to 100kW. Twice that would be 200kW. A 350kW Electrify America charger can do 200kW max at 400V. The car would need to be 800V to take more than 200kW. Therefore, to charge F150 at twice the speed of Mach E doesn't require any special charger equipment, and doesn't require 800V architecture. It sounds like a totally tractable engineering problem (use thicker wires in the car, etc).

According to pre-release specs, the F150 Lightning has two on-board chargers. Assuming they can work in parallel, that *might* be where they are getting nearly double the throughput (it’s not going to be exactly double due to inefficiencies). Changes to the charge curve could account for the rest.

So essentially you guys are saying that none of my rudimentary math set out above really comes into play. Because the F150’s pack is bigger, or possibly also because Ford engineered that pack to safely accept faster charging, it can therefore charge twice as fast. Ford has throttled the MME’s DCFC charging speed to what it believes the pack can safely accept without significantly diminishing battery life. And that speed generally tops out at around 100kW. Is that about right?

I heard a report today that says some of the new trucks may have TWO charging ports.....There goes all of our public charging spots....

HuntingPudel said:

According to pre-release specs, the F150 Lightning has two on-board chargers. Assuming they can work in parallel, that *might* be where they are getting nearly double the throughput (it’s not going to be exactly double due to inefficiencies). Changes to the charge curve could account for the rest.

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That’s what I’ve heard, but I’m not sure how that is relevant because I’m pretty sure DCFC bypasses the OBC.

Mirak said:

Because the F150’s pack is bigger,

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Mirak said:

Ford has throttled the MME’s DCFC charging speed to what it believes the pack can safely accept without significantly diminishing battery life. And that speed generally tops out at around 100kW. Is that about right?

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Mach E peaks to slightly above 150kW for a minute or two, at the beginning of the charge. This doesn't matter/help much in reality.

That aside, what you said is correct.

Another thing that I haven't seen anyone point out.

The Mach-E is marketed as charging from 10%-80% in 43 minutes
The F150 from what I've seen shows charging from 15%-80% in 44 minutes.

benk016 said:

Another thing that I haven't seen anyone point out.

The Mach-E is marketed as charging from 10%-80% in 43 minutes
The F150 from what I've seen shows charging from 15%-80% in 44 minutes.

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That’s a good observation. But it’s still impressive (to me as an MME owner) seeing the F150 charging speed almost double that of the MME. I guess it all comes down to pack size and maybe some other (probably heavier) enhancements they engineered to get the speed up. Otherwise a pack that size would just take too long to charge.

Nklem said:

I heard a report today that says some of the new trucks may have TWO charging ports.....There goes all of our public charging spots....

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That seems highly unlikely unless they have two separate battery packs. Especially moving forward, one DCFC plug should provide plenty of current.

Nklem said:

I heard a report today that says some of the new trucks may have TWO charging ports.....There goes all of our public charging spots....

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It does have dual on board chargers. But they will only be used if you have the 80amp Ford charger. Guessing the one charge port will deliver the full 80 amp, and each charger will pull 40amp from the one plug.

I don't think this could work with DCFC since the station connects straight to the battery, unless they like split the battery in half and let half of it charge with one port and the other charge with a 2nd port.

I have a feeling if you could plug in 2 DCFC cables, they would have advertised it as "Up to 300kw charging*"

My understanding is that the f150 extended range battery is, when it comes to charging, basically 2 batteries with their own charging pathways. This means from the plug, the electricity flows down 2 separate paths, through the separate converters (or skips the converters if DC is what is being supplied), through the charger, and into the battery. Where this helps is that on a typical 150kW ChargePoint, you only see peak current for a short amount of time. The dual charger setup allows the truck to draw the higher current for longer, because the chargers are sharing the available current. So, if my Mach-e pulls 150 for 3 minutes then drops to 62kW, and the F150 exhibits the same behavior, it’s going to feed both chargers about 75kW for that first 3 minutes, then when the chargers drop to that 68kW range (EACH) that going to be 136kW the truck is still feeding into the pack.

before you come for me over the numbers, please realize that I am using fictitious numbers just to explain what is happening according to the vehicle experts I spoke with in very general terms. The actual observed charging rates may be different, but the concept is still the same.

Mirak said:

That’s what I’ve heard, but I’m not sure how that is relevant because I’m pretty sure DCFC bypasses the OBC.

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Duh, I'm an idiot. You're right. I had my mind buried in L2 and wasn't thinking of DCFC.

In a different thread, someone posted that the F150 Lightning more than likely had a more robust cooling system, which would likely allow higher current charging for longer periods of time (changing the curve on which the charge rate diminishes). This seems likely. As @JoeDimwit just posted, the separate incoming paths effectively halve the charge rate to each of the battery packs, reducing the amount of heat generated in each pack over time. I wonder if each pack has its own cooling system?

HuntingPudel said:

According to pre-release specs, the F150 Lightning has two on-board chargers. Assuming they can work in parallel, that *might* be where they are getting nearly double the throughput (it’s not going to be exactly double due to inefficiencies). Changes to the charge curve could account for the rest.

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The F-150 likely supports 22kW AC charging, and that's usually provided by dual OBCs.