kwh required to charge 1kwh at each charge level?

[user20]

I know that charging speed is faster at lower soc than at higher soc, but is there a difference in the amount of energy required to charge each % of battery at different soc -- that is, does it require about the same amount of kwh to charge each additional 1kwh or 1% at 10% capacity compared to at 90%? And is it close to 1:1 inpututput, or is it significantly less efficient than that?

 

[ucmndd]

Assume losses of about 15% from your electric meter to what actually ends up in your battery due to resistance, losses during AC to DC rectification, and powering the car’s charging electronics.

SoC does not impact this directly, but charge speed does. Heat loss increases with charge speed due to resistance. Also there is some overhead to powering the car’s electronics and charging circuitry, and that overhead is the same whether you’re charging at 100kw or 1kw. So practically speaking, you’ll use more energy charging from 99 to 100% as the car trickles that juice in at ~1kw, which takes more time.

 

[user20]

I see. Can somone point me to some measurements? I'm also curious about the total kwh required to charge from 0-100%, compared to the battery capacity.

 

[jjrandorin]

You should likely ask your question in one of these places, as this is not a model 3 specific discussion:

Battery Discussion
Technical

 

[camalaio]

@jjrandorin a precise answer will be specific to Model 3; general discussion isn't really helpful for answering OP accurately.

Charging efficiency doesn't really change with SoC, not at the rates used for AC charging anyways.

This is the formula I've seen hold true for many Model 3s:

Net Power = ( Volts x Amps x 0.94 ) - 300

or, rearranged for efficiency:

Efficiency = 0.94 - ( 300 / ( Volts x Amps ))​

Broken down:

• 0.94 represents the roughly 94% efficiency of the AC-to-DC conversion.
• 300 represents about 300W of overhead while charging for things like running the coolant pumps and computers.
• Volts x Amps are the AC numbers, to calculate the power.

Therefore, some common North American charging numbers are:

• 73% efficiency for a typical 120V outlet (12A)
• 84% efficiency for a 240V 12A setup
• 89% for 240V 24A
• 90% for 240V 32A (e.g. NEMA 14-50 with included wall charger plus adapter)
• 91.4% for 240V 48A (e.g. installed Tesla Wall Charger or public/private J-1772 unit)

As you can see, 120V charging is pretty inefficient, but there are diminishing returns on increased efficiency at the top end. Doubling the power on a 240V setup from 24A to 48A only nets you about 1.5% more efficiency. The reason is basically that the 300W overhead becomes a smaller portion of the incoming power, but it's a decent chunk at 120V.

Charging outside in the cold will change this quite a bit, but it changes everything quite a bit.

Now, that's just AC charging. DC charging (e.g. Supercharging) is completely different, and ultimately more wasteful due to the battery heating behaviour if nothing else.

 

[camalaio]

I see. Can somone point me to some measurements? I'm also curious about the total kwh required to charge from 0-100%, compared to the battery capacity.

I don't have a reference to the thread, sorry, but somewhere on these forums someone graphed the data. It pretty tightly aligns with my formula above (which was nice to see, phew).

So for total kWh from 0-100%, it will depend on your charging setup. The last bit of charging also takes roughly an hour more than if it could keep charging at full speed, which means an extra 0.3kWh is wasted charging to 100% (an hour of that 300W overhead).

But 0-100% isn't really realistic, yet makes the calculation much more... difficult? So let's take 10-90% of my 2019 Model 3 LR AWD. It's degraded a bit, but my "usable" (0-100%) capacity is about 69.0kWh.

10-90% is 80%, and 80% of 69.0kWh is 55.2kWh. I can redo the above formula for energy (kWh) instead of power (kW) - the ratio of net/gross power will be the same as the ratio for net/gross energy.

Gross Energy (AC) = Net_Energy_DC / Efficiency
= 55.2 / ( 0.94 - ( 300 / (Volts x Amps) )​

So various values that come with that, to charge 55.2kWh:

• Standard 120V: 75.4kWh
• 240V@12A: 66.0kWh
• 240V@24A: 62.2kWh
• 240V@32A: 61.3kWh
• 240V@48A: 60.4kWh

What's interesting to note is that you divide by the efficiency, which multiplies by a "worse" amount. 73% efficiency means 37% more energy required (not 27%). 90% efficiency means 11% more energy required (not 10%).

Takeaway: avoid very low power charging like a standard 120V outlet.

 

[jjrandorin]

I feel like these formulas and your conclusion should be saved somewhere, because the question on charging efficiency of 120v comes up fairly frequently. With that being said, we already have a lot of sticky threads in this section, and I dont know "where to put it" per se. Your formulas and results are model 3 specific, so they obviously belong here, in the model 3 section, in the charging section.

Looking for recommendations on where we should save this, or if its just me who feels like this is information that should be saved and easily accessible vs buried in the forums.

 

[camalaio]

I feel like these formulas and your conclusion should be saved somewhere, because the question on charging efficiency of 120v comes up fairly frequently. With that being said, we already have a lot of sticky threads in this section, and I dont know "where to put it" per se. Your formulas and results are model 3 specific, so they obviously belong here, in the model 3 section, in the charging section.

Looking for recommendations on where we should save this, or if its just me who feels like this is information that should be saved and easily accessible vs buried in the forums.

If there's some sort of master post for this one day, I'd definitely want to find the graphs that someone did on the subject. I'll try to find those, I'm sure I have a bookmark somewhere.

Definitely high on the sticky count though.

 

[Schulmann]

This is a good question that I am very interested in. The answer is easily available with a good logging software like Teslafi or Scan My Tesla. The efficiency of the charge depends on one sole factor: Battery temperature and 12v system’s state. (or something like that). I am still in search for the 100% efficiency but I think it doesn't exist. Actually the right question is how fast can I reach my destination then the best solution will maximize efficiency. I was stunned to see 85% efficiency at -0C on 120v after a supercharger session vs 65% the day before on cold battery. Today was a cold day -8c , level 2 efficiency 77% on the road vs 97% at 6c two days ago.

 

[AlanSubie4Life]

If there's some sort of master post for this one day, I'd definitely want to find the graphs that someone did on the subject. I'll try to find those, I'm sure I have a bookmark somewhere.

Definitely high on the sticky count though.

Well, if @jjrandorin wants to piece a few things together, here is the plot some may be remembering:Charging 120v vs 240v efficiency

I agree that the best possible data on this should be stickied.

FWIW the average efficiency with the setup Tesla uses for EPA testing (I assume set to 32A with the UMC, but they do not specify (???)) is 88%. It is remarkably consistent on every vehicle (can see it in the constant tables in that thread). The AC-DC average efficiency is 90% or so. But these numbers include the slower final topping charge to get to 100% as well, though for a full charge I doubt that would affect efficiency overall very much.

 

[Gasaraki]

I am still in search for the 100% efficiency

That is physically not possible. That's like saying that no components will generate heat, the cable has 100% efficiency moving the electrons to the car, etc, etc.

 

[michaelnorbert]

Only charge at home on 14-50, 32 amp. Teslafi shows 92% efficiency since I got the Y, 2600kms.

 

[Shadowfox]

This is a good question and there is actually a relationship between SOC and charging efficiency. Lithium ion batteries typically have higher DCR (internal resistance) at high SOC and low SOC. It is pretty flat in the middle. Charging at low SOC will often be the most inefficient point due to the combo of high DCR with low battery voltage.

Another potential influence is the optimal operating point of the charger (onboard or off board) Most power converters will be most efficient at a particular voltage ratio, so with battery voltage varying 25% between full and empty one would anticipate it is more efficient at some SOCs than others.

 

[AAKEE]

I just got my WC installed. 3 phase 400V. Also bought a meter for used energy installed before the WC (good enough to legally professionall debit per the meters data).

First charge as I checked was from 47 to 71% on the screen battery meter, put the carging to 6kW.
Scan my tesla said SOC 47,92% before charging and 71.04% after. Thats 23.12%.
SMT values = 40,5kwh till 58,30 kwh = 17.8 kwh
The electric meter went from 14.98kwh to 34.89kwh = 19.91kwh bought.

In total 89.4% charge efficiency.

I started the charge immediately after I came home from work. -10 degrees outside and the car was outside whole day but parked in my garage which keeps +10C after the ar gets heated upp after some hours so I guess the battery was kept warm during charging.

 

[Candleflame]

I know that charging speed is faster at lower soc than at higher soc, but is there a difference in the amount of energy required to charge each % of battery at different soc -- that is, does it require about the same amount of kwh to charge each additional 1kwh or 1% at 10% capacity compared to at 90%? And is it close to 1:1 inpututput, or is it significantly less efficient than that?

if you have degradation the scale is different i.e. 0 -10% holds more energy than 80-90%. At 1% SOC left the car behaves like it has no degradation but your range/% decreases to display your degradation until maximum at 100%.