Onboard Charger Efficiency Measured

[phidauex]

Nice work, appreciate the confirmation! I agree that the difference is due to the fixed overhead of the charging operations plus the charger itself. Efficiency graphs that show a rise in efficiency, a flat zone, then a gradual drop at the far upper end can usually be represented as the sum of two losses, a fixed loss and an I^2*R loss, where the fixed loss dominates at low power, and the I^2*R loss dominate at high power.

But one thing I've reminded people of a lot in terms of losses is that a 40A 240V charger, while powerful for a home appliance, is still quite wimpy compared to the size of the battery. That is only 9.6kW, or less than 0.1C charge rate on the battery, which is extremely gentle. Intentionally going down to a lower charge current than that doesn't get you anything in terms of battery longevity. If you had a 300A charger at home I might recommend you dial it back for daily use, but 40A is still a very easy charge.

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[Just Lurking]

Efficiency graphs that show a rise in efficiency, a flat zone, then a gradual drop at the far upper end can usually be represented as the sum of two losses, a fixed loss and an I^2*R loss, where the fixed loss dominates at low power, and the I^2*R loss dominate at high power.

That's insightful and makes sense to me.

Would you say the only benefit of charging at a lower Level 2 amperage (or even Level 1) is giving the Mach-E a better opportunity to keep the 12V battery topped up for low mileage / infrequent drivers?

I read on these forums that there may be some OTAs rolling out that improve the 12V charging logic so perhaps even that benefit will be no more once those OTAs roll out broadly?

 

[Maquis]

That's insightful and makes sense to me.

Would you say the only benefit of charging at a lower Level 2 amperage (or even Level 1) is giving the Mach-E a better opportunity to keep the 12V battery topped up for low mileage / infrequent drivers?

I read on these forums that there may be some OTAs rolling out that improve the 12V charging logic so perhaps even that benefit will be no more once those OTAs roll out broadly?

Yes, the longer you are charging the car, the more charge the 12VB receives. It’s not worth it unless you are having 12VB issues.

 

[azerik]

Perfect. I've been planning on a 240v/24amp circuit at my cabin as I don't need a whole lot of charing and I don't need it fast. For the price I pay up there getting around 95%+ is a nice to have.

 

[emichnov]

I did a test last night. I set my ChargePoint Charger to 17 amps, and let it charge essentially all night up to 80% - long sessions help keep the 12V battery charged. FordPass says "Energy Added" was 22.1 kWh, and my ChargePoint app says I used 23.5 kWh to do this. I therefore calculate my charging efficiency at 22.1/23.5 = 94%, which is right in line with the OP's chart.

Is this an reasonable / accurate method of calculating charger efficiency?

 

[fcubi]

I regulary charge my MME at two different places. Both places have utility grade meters. Comparing meter readings vs fordpass charging reports these are my numbers:
87% efifciency @ 16A, 230Vac. Mild weather.
92% efficiency @ 32A, 230Vac Mild weather.

Seems there is some power consuption whenever charging happends no matter how fast or slow. Battery heating? Inverter cooling? Who knows...

These are my two cents on the matter. (shoud I say two percentages)


Paco

 

[Sam2084]

Another thing to consider is that even though it appears that the efficiency is slightly lower at 40A vs say 20A, you are charging for only half the time to achieve the same SoC at 40, so the total energy “lost” is actually less at 40A.

This doesn’t make sense. You’re losing more energy at 40A than 20A (if this graph is accurate) no matter how short or long you charge.

 

[BadgerGreg]

The 120v charging rate for the Ford cable is 12 amps, which appears to be around 85% efficiency. Not too bad for occasional use. Thanks for this info.

I've tested Level 1 charging on my Mach E and BMW i4 over a couple dozen sessions. I've found the Level 1 losses to be closer to 25% (75% efficiency). This makes sense, as there is a lot of BMS "overhead" during charging, which gobbles up a lot of the measly 1.0-1.2 kW that the car sips when plugged into a 120V outlet. Due to this, Level 1 is a terrible idea for owners who want to plug in to a 120V outlet every night...you're giving away a quarter of the juice.

 

[AZBill]

How exactly was the method for calculation done?
For instance, were you using a current transformer monitoring AC amps ahead of the EVSE? Were you looking at actual power in the battery? For lower charging rates, given that the DC/DC comes on as do pumps, it's a given that there will be more losses given that the vehicle's LV side is using about 500 watts depending on variables.

The only logical way to gauge the efficiency is to compare the watts that came out of the wall vs the watts that are in the battery.

I have done this comparison of wall versus car on three EVs and measured 93% efficiency.

 

[sebloos]

Thank you for that.

I have a Wallbox coupled with Solar panel, as i have setup the WallBox to only charge on green energy(which is a options which set to starts charging only if when surplus is equal to or greater than 6A ), when green energy is low at this level(6A), i have also realized this huge loss at low level 25/30% between what my Wallbox charging session display and what the fordpass app display.

 

[Mach-Lee]

My measured efficiency is 93.8% at 40A.

 

[ClarkT]

Thanks for posting your measurements of AC charging efficiency as a function of current! I have seen only a few others in the blogosphere, and none on the Mach-E... and I cross-posted to the FB group "NZ EV Owners" with the following commentary:

--

The drop in charging efficiency can be very substantial when you charge very slowly. If you have a vehicle with a 6.6kW onboard charger, and you're running it at only 6A 230V ( = 1.4kW) I think it likely you'll have a charging efficiency of about 80%. By contrast, you can get more than 90% efficiency from an AC-DC converter, if it's running at half or more of its rated power. And if you have a vehicle (such as a Renault Zoe) that can AC-charge at 22kW, its on-board charger may "refuse" to charge on a measly 10A supply of single-phase 230VAC ( = 2.3kW, nominal) -- perhaps to avoid overheating the AC-DC converter, or perhaps to comply with regulatory restrictions on its injections of harmonic distortion into its AC supply.

Almost 10 years ago, a team at the USA's INL carefully measured the 2015 Nissan Leaf's charging efficiency. "As the vehicle charge rate decreases, efficiency decreases, power factor decreases, and total harmonic distortion increases. The decrease in power factor is negligible, but the decrease in efficiency and increase in total harmonic distortion is not negligible and should be avoided when possible."
See https://avt.inl.gov/.../SteadyStateLoadCharacterization20...

Similar efficiency dropoffs with slow charging were measured recently -- albeit *much* less carefully -- for a Mach-E, by a blogger at https://www.macheforum.com/.../onboard-charger.../.

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