Home Charging Efficiency

[SDRick]

What seems to be the charging efficiency consensus with the wall charger or other home charging methods? My preliminary data is showing something less than 80% of the kilowatts that I am paying for is reaching the battery. Does that seem to jive with everyone else's experience?

 

[ran349]

I have been tracking charging efficiency my car. It is between 81-82 % on average. I found you have to collect data for a significant period of time to get a good average because from day to day you get quite a variation in the number. I do know that my measureable efficiency losses are in the car charger and battery, not from the wall to the car because power from the wall meter matches the power reading in my car during charging. My numbers are consistent with others I have seen reported, so yours are in the ballpark if by something less than 80% you mean close to 80%. I use a NEMA 14-50 at 40 amps.

 

[SDRick]

I have been charging at 48 A. For example, my last charge showed 15.1 A going into the charger and the Tesla showed exactly 12 A to the battery, which is an efficiency of 79.47%.

 

[ran349]

I have been charging at 48 A. For example, my last charge showed 15.1 A going into the charger and the Tesla showed exactly 12 A to the battery, which is an efficiency of 79.47%.

I am a little confused by what you are saying. Where are you getting your readings from? To calculate efficiency, you need to know how many kWh your car used for the day, and how many kWh it took from the wall to recharge your car to get the kWh back. When I charge at 40 A, the reading I get from my car is 40 A and 240 Volts. But I don't use that for my calculation of efficiency. I calculate efficiency by first knowing how many kWh I used for the day from my car trip meter display. I then look at the Edison usage in kWh the next day to see how many kWh I used for the charge the previous day. The car kWh used divided by the Edison meter used gives the efficiency.

 

[Neil_S]

80% efficiency is not unusual. In the past year of monitoring (energy meter @ panel + car readings) I've ranged 51-97% efficiency.
My efficiency will vary based on the ambient temperature at time of charging, the temperature of the battery pack when charging starts and the charge time / amount.
In the dead of winter I have observed 6A @ 240V going into my car for 10 mins with little/nothing going to the battery. The charger allows 40A, but the car only "requests" 6A since the battery is too cold to charge and the power is going to the inductive heater to warm the battery. When this happens I assume that little is going into the battery since the app / car predicts +48 hours to charge, but once the car starts pulling 40A the estimate drops to 1 hour. Obviously for small top ups (<5KWh charge) at cold temps inefficiency will be horrible i.e. 50%.
Likewise in higher temps, power has to be diverted to cool the battery, especially if charging at higher amps. (charge at SuC on hot day and listen to the AC scream!)
I've found that if I'm concerned about home charging efficiency: in winter don't charge if <10KWh will be added, or charge immediately after driving; in summer charge at coolest time of day and not immediately after driving. Spring and fall (live in Canada) are my best efficiencies.
The AC/DC converter also has an efficiency rating which I assume is pretty constant and my guess very efficient. (+98%)
I also observe that my HPWC consumes 0.1KWh / 6 hours (0.4 kWh / day) regardless of whether car is plugged in. I don't bother to include this in my observations unless +4 hour charge.

 

[scaesare]

I have been charging at 48 A. For example, my last charge showed 15.1 A going into the charger and the Tesla showed exactly 12 A to the battery, which is an efficiency of 79.47%.

I'm not entirely sure where you are taking the 15.1A reading as you mention charging at 48A, but...

The current the battery is drawing will be different than the current the charger is drawing. The charger is sourced with 120-240V on it's input, but then converts that up to ~400V to supply to the battery.

That having been said... your ratios seem odd. 240V @15A is 3.6KW, yet 400V @ 12A is 4.8KW. If you were only charging at ~300V (very low end of the battery), that's right at about 3.6KW as well, but then I'd expect you to be drawing a lot more than 15A from your 48A source... unless it was very cold or something...

 

[SDRick]

I must apologize for my post up thread as I meant my ratio was 15.1 kW (not 15.1 A) and 12 kW. during my last charge to 90%
Thank you for the feedback, especially Neil. I also notice consumption of the wall charger whether or not it is being utilized. I have not calculated it yet but 400 W a day sounds high, but I will check. On a yearly basis, that would be like 500 miles of driving?

 

[scaesare]

Ah... makes sense now.

Cool, thanks for the clarification.

 

[Neil_S]

80% efficiency is not unusual. In the past year of monitoring (energy meter @ panel + car readings) I've ranged 51-97% efficiency.
My efficiency will vary based on the ambient temperature at time of charging, the temperature of the battery pack when charging starts and the charge time / amount.
In the dead of winter I have observed 6A @ 240V going into my car for 10 mins with little/nothing going to the battery. The charger allows 40A, but the car only "requests" 6A since the battery is too cold to charge and the power is going to the inductive heater to warm the battery. When this happens I assume that little is going into the battery since the app / car predicts +48 hours to charge, but once the car starts pulling 40A the estimate drops to 1 hour. Obviously for small top ups (<5KWh charge) at cold temps inefficiency will be horrible i.e. 50%.
Likewise in higher temps, power has to be diverted to cool the battery, especially if charging at higher amps. (charge at SuC on hot day and listen to the AC scream!)
I've found that if I'm concerned about home charging efficiency: in winter don't charge if <10KWh will be added, or charge immediately after driving; in summer charge at coolest time of day and not immediately after driving. Spring and fall (live in Canada) are my best efficiencies.
The AC/DC converter also has an efficiency rating which I assume is pretty constant and my guess very efficient. (+98%)
I also observe that my HPWC consumes 0.01KWh / 6 hours (0.04 kWh / day) regardless of whether car is plugged in. I don't bother to include this in my observations unless +4 hour charge.

My bad! Slipped a decimal point on HPWC power consumption.
Should be 0.04 kWh / day. i.e. 40 watts / days

 

[Neil_S]

I must apologize for my post up thread as I meant my ratio was 15.1 kW (not 15.1 A) and 12 kW. during my last charge to 90%
Thank you for the feedback, especially Neil. I also notice consumption of the wall charger whether or not it is being utilized. I have not calculated it yet but 400 W a day sounds high, but I will check. On a yearly basis, that would be like 500 miles of driving?

Thanks for calling me out! Yes I slipped a decimal.
Should be 0.04kWh / day -> 40 watts per day, 14.6kWh per year
Phantom loads add up!

 

[SDRick]

Thanks for calling me out! Yes I slipped a decimal.
Should be 0.04kWh / day -> 40 watts per day, 14.6kWh per year
Phantom loads add up!

Bart

Well I guess we are even. I am having trouble with calling kilowatts amps and you are having trouble doing some math. Thanks for the clarification. 40 W a day sounds a lot better than 400!

 

[SDRick]

Neil, no idea how "Bart" got to the beginning of my post. By the time I saw it I was unable to edit. One of the downsides to voice recognition.

 

[David99]

There are two losses. The charger itself is about 90% efficient. Maybe a little better. It's just the nature of converting the power from 240 Volt AC to 300-400 Volt DC. The battery is about also about 90% efficient when charged. In other words, when you put 10 kWh in, you can take about 9 kWh out. Both of these together roughly add up to 80% charging efficiency. These number depend a little on power level and temperature. For example the charger is more efficient at it's maximum power. It becomes less efficient when running at a lower level. People think charging slower reduced losses. It might reduce losses in cables a little but the charger itself is less efficient.

 

[SDRick]

Thanks for calling me out! Yes I slipped a decimal.
Should be 0.04kWh / day -> 40 watts per day, 14.6kWh per year
Phantom loads add up!

If you are reading .04kWh x 24 hours wouldn't that be 96 watts per day or 35 KW per year? Isn't that equivalent to about 100 miles of driving? That is a bit of a phantom. (My meter bounces around between 2 and 6 watts per hour while the wall charger is not charging).

 

[Neil_S]

If you are reading .04kWh x 24 hours wouldn't that be 96 watts per day or 35 KW per year? Isn't that equivalent to about 100 miles of driving? That is a bit of a phantom. (My meter bounces around between 2 and 6 watts per hour while the wall charger is not charging).

Sorry, being sloppy with my units.

My readings show the HPWC (idle, not plugged into car) consumes 0.04kWh of energy each day.
Converting energy to average hourly power
0.04kWh / 24 hour = 0.00167 kW or 1.6 watts (average each hour of the day) or 40 watts total each day
Total annual energy consumption is 0.04kWh (per day) * 365 (days) = 14.6kWh

Your reading of 2 - 6 watts per hour is higher than what I'm seeing - don't know why.

Hope I haven't confused things more

 

[ZeroDarkSilver]

I keep pretty detailed logs in order to keep my charging provider honest.

I charge from a 208V 3-phase at 30A, which yields a nominal 6.2kW. But I measure only 5.3kW going into my battery. So on average, the car is charging at 85% efficiency.

 

[AWDtsla]

Should be well over 90%, not including vampire loss. I've measured dozens of times. 85% sounds like either the measurement is wrong or something else is wrong. It seems hard to believe Tesla would go backwards with the new 48/72A charger.

 

[ZeroDarkSilver]

Should be well over 90%, not including vampire loss. I've measured dozens of times. 85% sounds like either the measurement is wrong or something else is wrong. It seems hard to believe Tesla would go backwards with the new 48/72A charger.

Here's what I'm measuring during charging:

The car is reporting a Current/Pilot Current of 30A and a Voltage of 208VAC. It also reports Charger Power of 6kW (it looks like it only reports whole integers), so those numbers are all consistent (30 * 208 = 6,240).

The car is also reporting a Battery Current of 15.3A. According to the manual, the battery's voltage is 346VDC. So that yields 5.3kW (15.3 * 346 = 5,300).

5,300 / 6,240 = 85%

What am I doing wrong?

 

[AWDtsla]

Battery voltage depends on rate of charge and state of charge, so that's very imprecise.

Using the energy added reported by the car, or using rated mile math versus metered input will give you a better number.

 

[ran349]

Here's what I'm measuring during charging:

The car is reporting a Current/Pilot Current of 30A and a Voltage of 208VAC. It also reports Charger Power of 6kW (it looks like it only reports whole integers), so those numbers are all consistent (30 * 208 = 6,240).

The car is also reporting a Battery Current of 15.3A. According to the manual, the battery's voltage is 346VDC. So that yields 5.3kW (15.3 * 346 = 5,300).

5,300 / 6,240 = 85%

What am I doing wrong?

Your method is imprecise, but your number (85%) is not too far off, but probably a little bit high. As mentioned up thread, there are two main components that determine charging efficiency: 1) the battery, which is about 90% efficient and 2) the car charger itself is 90 - 95 % efficient. So that gives a combined efficiency range of between 81% - 85%. Other losses, such as line losses, etc., will make the number worse.