AC charging past 80% energy waste ?!

[BigApple]

Wanted to see if anyone out there knows
On DC charging it is straight forward.
The closer you are when charging to 100% SoC the lesser you car will draw from the charging station. But how is it with AC charging ?
I ask to know if you actually waste AC energy when you charge your car past the 80% SoC. On DC it won’t waste as it is same current. How about AC ? Will the 40-50 amps still be drawn from the house power while the car gets less kWh charged ?
It seems there is less energy lost during DC charging while on AC it seems you loose more. Wondering if anyone knows how that behaves ? Thank you.
If there is a bigger AC energy waste by charging past the 80% would be helpful to know. I usually don’t, to prolong the Batterie life. But I think it would be helpful to know in case I needed 100% SoC.
Thank you.

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[SergeyIndy]

Yes, there is more energy loss past 80%. Using parking lot analogy, when parking lot is 80% full, electrons take more time and energy to find a spot. There is no point of charging past 80% unless you really need it for a long trip and you start using up that energy right away as best practice.

 

[craz8]

Technically, correct. However, the ~10kw max from AC doesn't get limited by the charging curve of the Macan until about 97% (it's hard to read on the charge curve charts). Until that point, the car can handle as much as you can send at ~10kw.

So in practice, there is no slowdown or extra losses until you're above 95%. The last few percent may take a little longer as the car backs off the power demand.

 

[Fluff]

At first I thought you meant that charging to 100% leads to energy waste, because of no or limited regen. Which is lost energy. Especially on my trip home from work, with fast highway but with roundabouts, so you go from fast, to fairly heavy braking, very early in my trip.

 

[ColdCase]

Just want to mention that the current isn't constant, just the voltage.

Watts is voltage times current. Since the voltage is constant in either case, as the kW charge indicated drops, the current drops. In other words the current isn't a constant 40-50 amps. It will drop as the power delivered drops.

AC charging will always be less efficient than DC as the battery needs DC to charge and there is some conversion loss in on board car charger when it converts AC to DC.

Otherwise, from a battery point of view, its all DC charging, just at different rates. Dunno off hand if charging the battery at a say 80 kW rate over a short period is more or less efficient than charging at 8 kW over a longer time.

So current is watts divided by voltage.

9 kW from a typical 240V level 2 source is 37.5 amps

80 kW from a typical 300V DC source is 266 amps
When it drops off to 60kW indicated, the current drops to 200 amps

 

[dbsb3233]

This is the first time I've heard a suggestion of extra energy loss as the power draw slows down. The car's BMS simply tells the EVSE to deliver less power. It's not continuing to deliver max power and the car only accepts some of it, if that's what you're thinking.

There's always some power loss from wire and connector resistance (translates to heat), but I don't see how that would scale up as the power goes down?

 

[BigApple]

This is the first time I've heard a suggestion of extra energy loss as the power draw slows down. The car's BMS simply tells the EVSE to deliver less power. It's not continuing to deliver max power and the car only accepts some of it, if that's what you're thinking.

There's always some power loss from wire and connector resistance (translates to heat), but I don't see how that would scale up as the power goes down?

Yes that was my question. If the power draw from the house grid gets lesser as the porsche AC/dc converter draws less is what I was hoping for. Just wasn’t sure if that’s the case. Thank you for yours and everyone’s input.

 

[dbsb3233]

Yes, there is more energy loss past 80%.

Time loss, not energy loss. The car simply tells the EVSE to deliver less power, which means charging slows down and takes longer.

The car's BMS is always telling the charger (whether AC or DC) what the max power level to deliver at any second. Sometimes the car will accept the max power the charger/EVSE can deliver, other times less. For instance, you could have a 40A EVSE that is capable of delivering 9.6 kW, but an older EV that's only capable of accepting 7.6 kW. It has to tell the EVSE to deliver only 7.6, even from the very start.

 

[DHS]

So do I understand that while it will take longer to charge my car from 80% to 100% than from 60% to 80%, it is no more expensive to do so? In other words, are the only disadvantages to charging to 100% that it will take a long time, and the battery will wear out sooner?

 

[ColdCase]

Time loss, not energy loss. The car simply tells the EVSE to deliver less power, which means charging slows down and takes longer.

The EVSE cannot manage the power (current). The EVSE tells the car how much power (current) is available. The car uses what it wants. The EVSE will act like a circuit breaker and cut power when a car draws too much current,

 

[ColdCase]

So do I understand that while it will take longer to charge my car from 80% to 100% than from 60% to 80%, it is no more expensive to do so? In other words, are the only disadvantages to charging to 100% that it will take a long time, and the battery will wear out sooner?

Yes, you are buying electricity by the kWatt.

In round numbers for example, the Macan has a 100 kW battery, more or less. To increase SOC 20% it needs 20 kWatts.

In the US market Macan will draw ~9 kWh AC from a L2 charger at any % SOC because that is the limit of the US on board AC battery charger (the battery can take much more under any everyday condition). So it takes the same time to charge from 80-100% as it does 60-80%. 20/9 is about 2.2 hours regardless

The Macan will take much more DC power from a DC charger, but it will limit the rate to protect the battery based on things like state of charge and battery temperature. Given that he DC charger can deliver what the Macan wants, and the Macan will want less at 80% than 60%, it will take longer to charge the Macan from 80% to 100% than from 60% to 80%.. But you are paying for 20kWatts in either case.

In either case you are paying for kWatts, 20% increase requires the same 20kW regardless of charge level or time.

Technically there are distribution losses that can vary with the amount of current, but this is in the noise in this case.

 

[rcomeau]

Keep in mind that some public L2 chargers may charge by the hour (and not by energy delivered), so in these rare cases, you may pay ~$1 more at the end of the charge cycle, but it is pretty much a round-off error.

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