Higher amp charging is more efficient??

As I said in another thread (110 vs 240 charging) since the energy loss is given by the integral of the resistive loss (RI*I) over the charging time I think that the losses are higher for smaller currents because the charging time is higher (also if the current is lower). The fact that the charging time is much higher can be easily seen in the extreme case of the 110V outlet (more than 3 times with respect to the 240V outlet).

Tom Saxton did a study on the roadster charging at various levels and he found that charging was more efficient as current was increased up to a point. 240V @ 32A and above all had a similar efficiency. It was theorized that this was the case for the Model S as well.

Tesla Roadster Charging Rates and Efficiency - Tom Saxton's Blog

Some of it can be attributed to charging losses/efficiency. If you have an %85 efficient charger, the less time it's "on", the lower the losses... At some point during higher power charging, cooling could become a factor, but since you are charging for much less time overall, there is less "waste" over a longer time period. Really, anything over 24A-32A seems to be a few pennies difference, and not significant.

mitch672 said:

Some of it can be attributed to charging losses/efficiency. If you have an %85 efficient charger, the less time it's "on", the lower the losses... At some point during higher power charging, cooling could become a factor, but since you are charging for much less time overall, there is less "waste" over a longer time period. Really, anything over 24A-32A seems to be a few pennies difference, and not significant.

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You've written that before but it makes no sense. It doesn't matter if you put 15kWh into the car in 15 minutes or in 15 hours. If the charger is 85% efficient then you've lost 2.25kWh either way.

The loss of efficiency at lower charge rates comes from overhead. Things like running the pumps to circulate coolant (I believe they run all the time while the car is charging?).
Also now that we have sleep mode, the car does not sleep while charging, so there are some losses there too.

mnx said:

You've written that before but it makes no sense. It doesn't matter if you put 15kWh into the car in 15 minutes or in 15 hours. If the charger is 85% efficient then you've lost 2.25kWh either way.

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Sorry but I don't agree with you. In the case that you are mentioning (energy storage of 15kWh) the charger gives you a certain amount of energy/hour (power=P). If you want to get the energy of 15kWh you have to integrate this power P for all the time that is needed and the loss due to the efficiency of the charger will be as great as the time that is needed for the charge because also such a loss is the result of an integration over time. That's why in the table given by yobigd20 you can see that the loss is much bigger for bigger time charge.
Then in this reasonment you should also add the resistive losses due to the internal circuitry of the power module of the Model S that are also time dependent as I said in my previous post in this thread.

Sorry, but this explanation is too simple. Things are a bit more complicated.

Power loss to resistive load as in I²R cannot explain the facts. Look at a supercharging screen shot, you can see 330Amps at 360V or so. That would mean the pack has around 1Ω when being supercharged. Since 90% of DC power is stored in the pack, loss to resistive loads would mean R=0.1Ω.

Power loss to charger efficiency (85%) cannot explain because a constant loss of 15% is time independent no matter how you integrate it.

I think the following factors are relevant:
- reduced efficiency of AC-DC power conversion in the charger at lower loads. We don't know the curve but it is pretty common to have efficiency drop when not running at nominal load.
- constant power usage by the car (pumps, BMS, touchscreen computer) that add up during charging power. The longer you charge, the more energy is consumed there. This decreases the ratio of energy stored in the battery pack to used energy from the wall plug.

A part of this energy is consumed anyway if you charge for a shorter period (aka vampire load) because it discharges the battery after charging completes, instead of spinning the meter while charging. So this is not energy lost to charging slower, but charging longer. :wink:

Agree that vampire loss is a factor. If you're talking about a vampire loss of 100W just from the car being powered on, that works out to 7.7 kWh over the additional 77 hours charging at 110V 12A rather than 240V 80A. If you're talking about 200W, that's 15.4 kWh.

Another factor is the voltage. 110V is less efficient than 220-240V. If you're charging at 110V 12A, you're receiving just as much power as when charging at 220V 6A, but the resistive losses will be 4 times as high.

Yggdrasill said:

Agree that vampire loss is a factor. If you're talking about a vampire loss of 100W just from the car being powered on, that works out to 7.7 kWh over the additional 77 hours charging at 110V 12A rather than 240V 80A. If you're talking about 200W, that's 15.4 kWh.

Another factor is the voltage. 110V is less efficient than 220-240V. If you're charging at 110V 12A, you're receiving just as much power as when charging at 220V 6A, but the resistive losses will be 4 times as high.

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+1

The Leaf has between 200 and 300W in overhead and it overall is a much more efficient beast than the Model S. Overhead explains nearly all the difference in efficiency between charging rates. This has been studied significantly in the Leaf and there is widespread agreement that overhead is the primary issue. I haven't seen anyone quantify the voltage part but my guess is that it is less than 10% of the overall efficiency difference.