Missing power when charging ("losses"?)

gwatuk

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Oxford UK
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MG4 SE SR
I'm charging my MG 4 SE SR with a granny charger.
I have a Shelly EM power meter monitoring the cable that feeds the 13A socket that the charger is plugged in to.
( https://www.shelly.com/en-gb/products/product-overview/shelly-em-120a/shelly-em-50a )
(The cable is 4mm²)

I've been running a spreadsheet recording the charging data. Here's a typical entry:
Charging from 23% to 100% - this indicates that the battery's charge has increased by approximately 39.1kWh.
But the Shelly says that 44.9kWh has been used.
(A similar difference is repeated for all my charging sessions.)

Also, when charging, the car says that it is drawing about 1.7 to1.8 kW.
The Shelly says it's drawing 2.16kW.
(I've measured this with another power meter which says 2.15kW).

So where is all the extra power going? Is the car using it to heat the battery?
(The intelligent heating setting is off.)
 
It's standard losses. All EVs do this when DC, AC or even granny charging. The same goes for smartphones and power banks.

Ps. I don't think the car reports much it is drawing, it reports how much it is receiving.
 
Last edited by a moderator:
There are other posts on the forum which explain it better but in general 10% losses for a granny charger and 5% for 7 / 11 kW chargers are around normal. Expect a little less on hot summer days and more in winter.
 
But maybe the car doesn't do decimal places?
That's entirely possible.

It also may be less than expectedly accurate. I find about a 7% difference between my MG and my Nissan Leaf from the same EVSE. The EVSE is an ex-commercial one (a ChargePoint), with fancy hardware for measuring voltage, current, power, energy, etc. Unfortunately, I don't know how to scale the numbers I get, so I don't know which one is closer to the requested current. And I'm too lazy to separate out one AC wire to hook a clamp meter over it.
 
At what point of the charging cycle are you at? If the battery and electronics are a bit chilly a larger proportion of the charge will be lost as heat in warming them early on.
 
My car says it's charging at 1kW but the mains says it's delivering 1.7kW. That's a 70% difference.
But maybe the car doesn't do decimal places?
Obviously there is an initial offset too. 1kW is a very low power delivery, so a 700W overhead for a vehicle made for 11kW (AC) or 144 kW (DC) is negligible
 
I also measure charge variance between what the charger says has been delivered and change in % of battery charge.

I have put the differences down to balance charging (where you are not adding to the overall state of battery charge) and normal loss inefficiencies in the cables and connections as well as software calculation anomalies.

My biggest differential was 13kW but it is usually under 1kW
 
At what point of the charging cycle are you at? If the battery and electronics are a bit chilly a larger proportion of the charge will be lost as heat in warming them early on.
It's consistent through the whole charge.

Obviously there is an initial offset too. 1kW is a very low power delivery, so a 700W overhead for a vehicle made for 11kW (AC) or 144 kW (DC) is negligible
Mine is the Excite 51, it's 6.6 and 88, it takes about 24hrs to go from 10% to 100% with the supplied wall charger.

I'll have to do a time versus charge calculation to see what's really going on.
 
My car says it's charging at 1kW but the mains says it's delivering 1.7kW. That's a 70% difference.
But maybe the car doesn't do decimal places?
The car definitely does decimal places.
But (as usual) the app and the vehicle figures are slightly inconsistent - when the car says 1.7 kW the app says 1.8 kW.

The car saying that it is drawing about 1.8 kW when in fact what's coming out of the house is 2.16kW is IMO a really big difference.
And it means that if you do the calculations to see the mileage you're getting for how much charge you put into the car based on what the car tells you, then you're being misled. And your electricity bill is going to be significantly higher than you think it's going to be.

It would be really good if someone else could use some kind of power meter on the electricity supply to look at how their car actually charges.
(It's possible that there's a fault with my granny charger or car.)
 
This topic comes up almost once per week :)

When AC charging there are fixed losses and variable losses ... variable losses are proportional to the charge rate whereas fixed losses are, well, fixed. (Power loss through the cable, through the AC/DC converter, battery heating etc.) On a very low power charge point the fixed losses are a much higher proportion of the overall losses and so the efficiency tends to be much lower.

1.8 / 2.16 = ~83%
 
Plus, the MG4 shows the charge going into the battery. That means that on top of the AC/Granny losses, the other loads (lights, BMS, etc) are also subtracted and the number you see is what goes into the battery.
Some other vehicles show the power after conversion from AC to DC, but do not show the whole story.
A bit like the speedometer: on the MG4 it’s spot on with GPS true speed, whereas on other cars it’s somewhat higher (eg 70 displayed, 67 real)
If you charge while sitting in the driver seat at night, with HVAC and lights on, you may well see 0.5 to 3kW less.
 
It's easy to see where losses can soon add up...take the 5m cable from charger to car. Assuming it a fairly standard 6mm² conductor then at 32A the power lost in the cable will be about 30W.
 
It would be really good if someone else could use some kind of power meter on the electricity supply to look at how their car actually charges.
I have dedicated power/energy monitoring on my EVSE circuit and also the SAIC integration with Home Assistant which tracks the DC kWh into the car's battery but as yet have not bothered with doing the efficiency calculations.

I'll look into it when suitably motivated to do so.

This topic comes up almost once per week :)

When AC charging there are fixed losses and variable losses ... variable losses are proportional to the charge rate whereas fixed losses are, well, fixed. (Power loss through the cable, through the AC/DC converter, battery heating etc.) On a very low power charge point the fixed losses are a much higher proportion of the overall losses and so the efficiency tends to be much lower.
Aside from AC to DC losses, the car is also consuming power during a charging session.

Systems are monitoring things, plus the 12 V auxiliary battery is typically being charged as well and that can consume 100-200 W, tailing off its charge rate as it approaches full charge.

As a result, the DC charge rate into the drive battery can increase during a charge session once the auxiliary battery has completed charging.

Here's a snapshot chart from yesterday. We had an outing in the morning which involved a few stops, then back home and middle of the day the car was charging. Charging power supplied by my charging circuit shown in orange, the auxiliary battery voltage samples are the white dots.

Screen Shot 2024-06-17 at 8.06.09 am.png

When driving the car charges the auxiliary battery, when parked/off, the auxiliary battery is left to rest.

Can see on the left the voltage jumps up and down, this was during the outing with a few stops.

Then the charging session started at noon (free energy for 2 hours, then reverts to solar only charging) and the auxiliary battery voltage jumps right up during that time. Once charging stops is reverts back down to resting voltage (which frankly is a bit lower than it should be, I need to keep an eye on that).

Anyway, all that just to show that while charging the car, you are also charging the auxiliary battery, so it accounts for some of the power demand.
 
I'll look into it when suitably motivated to do so.
Efficiency is something needing a more careful analysis method and longer period but for example here are some numbers picked from some days over the last few weeks.

DateDC Energy kWhAC Energy kWhEfficiency
19-May
14.5015.30
94.8%
25-May
14.1014.40
97.9%
26-May
13.9016.00
86.9%
1-Jun
9.4010.10
93.1%
2-Jun
6.20 7.28
85.2%
9-Jun
14.0015.20
92.1%
11-Jun
7.70 8.46
91.0%
15-Jun
7.90 9.13
86.5%
16-Jun
15.8015.34
103.0%
17-Jun
5.12 5.64
90.8%

All over the shop.

Since the > 100% efficiency as suggested on 16 Jun is physically impossible, this tells me the measurements are not entirely reliable. I am more inclined to believe my AC measurement than the DC values from the car's API as the API's data sampling is infrequent.
 
I've finally got the app working. Makes a difference with glasses on! I could see the tiny little check box at the bottom to say you agree with their terms. Once I hit that it registered quickly.

So now I know that the car display doesn't show decimals. Because it's still showing 1kW but the app says 1.4kW, so 1.4 to 1.7, is a much better ratio than 1.0 to 1.7
 
Thanks everyone for your comments.
.... On a very low power charge point the fixed losses are a much higher proportion of the overall losses and so the efficiency tends to be much lower...

So I've been wondering if I should install a proper EVSE instead of using the granny charger so as to get a better charging efficiency.
But given that we only use the car for low mileage I reckon we'd only break even with the cost of installing one after about 10 years :ROFLMAO:
 
It'll be more convenient and quicker, more efficient and as its now part of your property should increase its value and saleability should that need arise. Amazon have many on offer so you don't really need an all singing dancing one, but you will need a trusted accredited sparky to install it.
 
Plus if you're considering a Time of Day tariff (e.g. Octopus Go / Intelligent Go) then having a 7kW EVSE will make that more practical.
 
the government ending the grant on installing EVSEs was a very bad decision.
For the miles I do when at home, I might as well just charge at work and save myself the trouble.
 

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