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Does lowering the charging rate save electricity?
- Thread starter mercer2000
- Start date
And, speaking as an aficionado of DC-DC switching power modules: These things have efficiency curves that are low at low amperage, peak somewhere around 75%-90% of full load, and a slow drop-off as one approaches 100% load. That wide, "75%-90%" bit depends upon the vendor, and some of these parts hit 95% on a good day, more if one pays Extra for the Good Stuff. But they all drop off a bit at max power. Resistive losses, don't-ya-know.
Where, in the universe of things, the rectifiers/DC-converter assemblies in a Tesla lie, I have no clue. But it's a reasonable guess that doing Level 1 charging at 120 VAC and 12-16A (depending upon the adapter) is probably less efficient than chugging along at 48A and 16A per rectifier/switching.
In addition: If one goes out and buys a DC-DC converter, it turns out that the ones with high input and high output voltages tend to be more efficient than the ones with either low input or low output voltage; and if both the input and output are low voltage, then 80% max efficiency is where one lives.
It's a gimmie that the Tesla engineers who designed all this were doing clean paper designs with an eye to wasting as little energy as possible. So, for example, it would by likely to my eye (that's a guess, sir) that there's a broad efficiency curve on these 16A modules (there's two in a Standard Tesla; three in Long Range or Performance) with a peak efficiency somewhere between 12A and 16A, with a 0.5% variation in efficiency between those extremes. Again, guessing, but that's where the technology sits.
Whatever it is, my guess is we're probably talking 3% or less differences in efficiencies. So, not first order effects, or even second, but likely third.
Hm. the M3 and MY in the garage are fed 240VAC at 48A. As they come up to speed, one observes the current rising from 0A to 16A; a brief drop, then the current rises to 32A; another brief drop, then it rises up to 48A. It's pretty clear that that's one 16A after another coming on line. With all three chugging along, that'd presumably be 16A per module.
Note that the Tesla Mobile Connector, when hooked up to a Sufficiently Large Current socket (40A or greater circuit) will do a maximum of 32A. Which, and I'm guessing here, means that two of those 16A converters would be running at max current and the third would be Off.
Suppose that one had a Tesla Wall Connector but, instead of running it at 48A, ran it at, say, 35A. First converter: On, and running at (peak efficiency?) of 16A; second converter, On, and also running at 16A; and the third converter, On, and running at 2A. Stare at that a bit and one might come to the idea that the third converter might only be running at, say, 70% or 80% efficiency, with the other two running at 90%.
Or Tesla's hardware/software engineers might look at this, divide the current evenly between the three modules, and run each at 11 2/3A at perhaps a slightly higher overall efficiency. Or something like that.
Conclusion: Tesla has probably optimized the individual converters for running at max efficiency between 12A and 16A. Reducing the current so that, theoretically, some converter or other might run at 20% of its full load, might reduce the overall efficiency, depending whether or not if Tesla Thought About That (likely) and Took Steps. If Tesla Took Steps, then a slight (less than 2%?) drop in efficiency might occur; otherwise, it might be larger, on the order of 3% or 4%. Therefore: Don't drop the charging current. It probably (but unknowably) could make things worse.
Where, in the universe of things, the rectifiers/DC-converter assemblies in a Tesla lie, I have no clue. But it's a reasonable guess that doing Level 1 charging at 120 VAC and 12-16A (depending upon the adapter) is probably less efficient than chugging along at 48A and 16A per rectifier/switching.
In addition: If one goes out and buys a DC-DC converter, it turns out that the ones with high input and high output voltages tend to be more efficient than the ones with either low input or low output voltage; and if both the input and output are low voltage, then 80% max efficiency is where one lives.
It's a gimmie that the Tesla engineers who designed all this were doing clean paper designs with an eye to wasting as little energy as possible. So, for example, it would by likely to my eye (that's a guess, sir) that there's a broad efficiency curve on these 16A modules (there's two in a Standard Tesla; three in Long Range or Performance) with a peak efficiency somewhere between 12A and 16A, with a 0.5% variation in efficiency between those extremes. Again, guessing, but that's where the technology sits.
Whatever it is, my guess is we're probably talking 3% or less differences in efficiencies. So, not first order effects, or even second, but likely third.
Hm. the M3 and MY in the garage are fed 240VAC at 48A. As they come up to speed, one observes the current rising from 0A to 16A; a brief drop, then the current rises to 32A; another brief drop, then it rises up to 48A. It's pretty clear that that's one 16A after another coming on line. With all three chugging along, that'd presumably be 16A per module.
Note that the Tesla Mobile Connector, when hooked up to a Sufficiently Large Current socket (40A or greater circuit) will do a maximum of 32A. Which, and I'm guessing here, means that two of those 16A converters would be running at max current and the third would be Off.
Suppose that one had a Tesla Wall Connector but, instead of running it at 48A, ran it at, say, 35A. First converter: On, and running at (peak efficiency?) of 16A; second converter, On, and also running at 16A; and the third converter, On, and running at 2A. Stare at that a bit and one might come to the idea that the third converter might only be running at, say, 70% or 80% efficiency, with the other two running at 90%.
Or Tesla's hardware/software engineers might look at this, divide the current evenly between the three modules, and run each at 11 2/3A at perhaps a slightly higher overall efficiency. Or something like that.
Conclusion: Tesla has probably optimized the individual converters for running at max efficiency between 12A and 16A. Reducing the current so that, theoretically, some converter or other might run at 20% of its full load, might reduce the overall efficiency, depending whether or not if Tesla Thought About That (likely) and Took Steps. If Tesla Took Steps, then a slight (less than 2%?) drop in efficiency might occur; otherwise, it might be larger, on the order of 3% or 4%. Therefore: Don't drop the charging current. It probably (but unknowably) could make things worse.
If you are not using sentry mode, a faster charge time may allow your car to spend more time in the low power sleep mode. At ~7% per day, sentry mode is pretty costly.
Actually, it would cost more electricity, since the car is "turned on" while charging, consuming about 0.4 kW. So AC charging losses will be higher at lower AC charging rates.
The cost of the car being "turned on" while charging is 5% of the 7.68 kW from 32A 240V charging, but 10% of 3.84 kW from 16A 240V charging and 28% of 1.44 kW from 12A 120V charging.
Faster AC charging could also help in being able to squeeze the entire charging session into the time frame of the cheapest electricity rates if you have time-of-use billing.
Is it also important for those in colder areas where battery heating is needed that charging at a lower rate puts less into the battery since part of the power is used for heating, so if you have the ability to charge at 48 amps it 'should' be more efficient, since the charge limit will be achieved in a shorter time.?.
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