Charging losses

[Julian Edwards]

I recently upgraded my MG5 from a 22 plate to a 24 plate FL. Now I have the app I have noticed that when charging with a 10 amp that should be pulling 2.3 kW at least, the app tells me it is charging at 1.6 kW. Surely there is not a 25% efficiency loss, or is there?

 

[Coulomb]

Surely there is not a 25% efficiency loss, or is there?

The 1.6 kW is the net power into the battery, after AC→DC conversion losses, but also discounting all the 12 V loads (computers, lights, pumps, fans), the DC→DC converter losses, and any HVAC high voltage power use.

The overheads and losses are striking at granny "charger" levels. When rapid charging or even when faster AC charging, a higher percentage of input power makes it into the battery. In addition, the AC→DC conversion and most of the cable losses are the charger company's problem, and are not usually visible.

 

[Jaumg]

Just taking a snapshot from my full charge yesterday, my FL was turning 2.3kW (9.7A at 237V) into 1.95kW to the battery (at 401V/97% SoC), so a 15% loss.

Do you know what power your charger was actually drawing from the grid? Is it possible that it has a switch between 8A and 10A? Alternatively, is it a very long cable? You could be looking at a voltage drop along it.

 

[Julian Edwards]

Thanks, now I will dig deeper, but accept that there will be losses.

 

[MickeySw]

Thanks, now I will dig deeper, but accept that there will be losses.

Don’t forget the BMS will activate the battery heater during AC charging sessions when temperatures drop low enough.

 

[Jaumg]

Don’t forget the BMS will activate the battery heater during AC charging sessions when temperatures drop low enough.

Has that been seen when charging at 10A or less? I've seen that with my 32A charger certainly, but I've not noticed it when charging slowly, and I wonder if it even makes sense to use all the available power to heat the battery pack

 

[MickeySw]

Has that been seen when charging at 10A or less? I've seen that with my 32A charger certainly, but I've not noticed it when charging slowly, and I wonder if it even makes sense to use all the available power to heat the battery pack

Charging NMC batteries below freezing can damage the cells permanently.

I would be surprised if MG doesn’t try to prevent this from happening, possibly by use all available power initially to heat the battery first and then taper it down as the cells warm up during the charging process..

 

[Green Salop Man]

Charging NMC batteries below freezing can damage the cells permanently.

I would be surprised if MG doesn’t try to prevent this from happening, possibly by use all available power initially to heat the battery first and then taper it down as the cells warm up during the charging process..

So are you saying people shouldn’t charge their cars when it’s below freezing? How does that work in Scandinavia?

 

[MickeySw]

So are you saying people shouldn’t charge their cars when it’s below freezing? How does that work in Scandinavia?

No, EVs with NMC cells generally have battery heaters that are automatically activated by the BMS. To prevent damage, the cells are warmed to about 5°C before charging commences.

 

[Alibro]

No, EVs with NMC cells generally have battery heaters that are automatically activated by the BMS. To prevent damage, the cells are warmed to about 5C before charging commences.

Not all EV's have temperature control. I took apart an eGolf battery pack and it had no heaters and no cooling plates. I'm not sure what happens to them in cold countries but I'm pretty sure they were sold all over Europe.

 

[MickeySw]

Not all EV's have temperature control. I took apart an eGolf battery pack and it had no heaters and no cooling plates. I'm not sure what happens to them in cold countries but I'm pretty sure they were sold all over Europe.

I know a guy who got a VW ID4 without a battery heater (it was an option??). It AC charges (if at all) at minimum rates during freezing conditions..

Just to be clear, there is no problem driving below zero (as long as regen is turned off)…

 

[gwatuk]

@Julian Edwards this is what my Home Assistant system reports for the car's power. This is also with a granny charger.
(This may not be exactly what the power is going into the battery but it's probably fairly close.)
It seems to charge at a higher rate at the start of the charging period and reduce over time:

There's more information available from Home Assistant so I've been doing some maths in order to work out the charging efficiency.
Over this 11 hour charging period:
My Shelly relay (which switches the mains supply to the granny charger) reports energy figures starting at 1417.86kWh and ending at 1442.44kWh so that's 24.58kWh from the mains.
The car's SOC starts at 33.4% and ends at 74.4% so that's 20.83kWh (50.8 x 0.41).

Based on these figures the charging efficiency is 84.74%

 

[MickeySw]

The cars SOC starts at 33.4% and ends at 74.4% so that's 20.83kWh (50.8 x 0.41).

Based on these figures the charging efficiency is 84.74%

The usable capacity is around 49kWh for a PFL NMC pack, but that sounds about right for a granny.

Efficiency is better using a 7kW EVSE and generally improves in warmer climate.