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My Victron inverter/charger has the most efficient delivery at lower power settings. I believe it is based around switching and toroidal transformers but excuse my ignorance here. So although an 8kW unit it is probably most efficient around 1kW. This graph is for a similar unit. Unfortunately they don't publish them for all units.

I have read that car chargers are different and are most efficient at full power. However this wasn't backed up by any evidence and I suspect it might have something to be the unit being 3-phase and being less efficient when at lower powers on single phase. Does anyone know if I am better off charging my single phase ac car at 1.3kW or 7kW?

I cannot comment on efficiency.

However, most of us are more concerned with cost than efficiency and may have TOU tarrifs, such a s Octopus Go or Intelligent Octopus. In my case Octopus Go gives me 4 hours charging per night at a cheap rate. Charging at 1.3kW would just be to slow unless I am hardly using the car.

John_S wrote: ↑Sat Mar 04, 2023 9:41 am I cannot comment on efficiency.

However, most of us are more concerned with cost than efficiency and may have TOU tarrifs, such a s Octopus Go or Intelligent Octopus. In my case Octopus Go gives me 4 hours charging per night at a cheap rate. Charging at 1.3kW would just be to slow unless I am hardly using the car.

I am using a combination of both and am soon going to be off grid. Then efficiency is king. If the two inverters are opposed then that isn't ideal. By opposed I mean their most efficient power points. The aim is to use 15-20kWh from the batteries to charge the car during the night. Blasting out 8kW from the inverter instantly loses about 1kW and then more again in the EV. However if both are happy at the low end I can just run it gently over the whole night.

Andy wrote: ↑Sat Mar 04, 2023 10:33 am The aim is to use 15-20kWh from the batteries to charge the car during the night.

That will be very inefficient.

Tinbum wrote: ↑Sat Mar 04, 2023 12:57 pm

Andy wrote: ↑Sat Mar 04, 2023 10:33 am The aim is to use 15-20kWh from the batteries to charge the car during the night.

That will be very inefficient.

Yep. Which is why I want to try and optimise it a bit. Unfortunately we often aren’t around in the day to charge when the sun is out which is why I have put in 18kW of panels

Andy wrote: ↑Sat Mar 04, 2023 1:25 pm

Tinbum wrote: ↑Sat Mar 04, 2023 12:57 pm

Andy wrote: ↑Sat Mar 04, 2023 10:33 am The aim is to use 15-20kWh from the batteries to charge the car during the night.

That will be very inefficient.

Yep. Which is why I want to try and optimise it a bit. Unfortunately we often aren’t around in the day to charge when the sun is out which is why I have put in 18kW of panels

Andy, what's your total PV now, I've still got that down as shown on your sig, PM me if you want.

EDIT - Scrub that, you already PM'd me and I did not notice the increased PV.

I can't comment on ev chargers specifically, but as a general rule of thumb, most power converters run less efficiently at max power due to resistive losses and cooling fans, and generally improve down to the 10-20% of rating ball-park, below which the fixed losses start to dominate and the efficiency curve drops away again.

Even if the manufacturers don't sypply an efficiency curve, they ought to supply a max efficiency and might specify the power output that that corresponds to, which would give you some idea.

Marcus wrote: ↑Sat Mar 04, 2023 2:18 pm I can't comment on ev chargers specifically, but as a general rule of thumb, most power converters run less efficiently at max power due to resistive losses and cooling fans, and generally improve down to the 10-20% of rating ball-park, below which the fixed losses start to dominate and the efficiency curve drops away again.

Thanks, I just took the time to change the max charge rate on my Zappi and then see what was going into the battery from the cars dash. Not terribly accurate as the kW I was using was good to 1 decimal place but its a start. I ranged from 1.3kW to 7kW.

It was as you describe with a sweet spot of about 3.5kW.

3.5kW was 94.5%, 6.9kW - about 89%. 1.3 and 1.8 where odd readings but really quite low, approx 80%. Aiming for the sweet spot when discharging the battery should save 1 kW for each drive into work.

I have a 22kW Zappi (3-phase) charger for my Tesla MX and was considering dropping my charging from the car's max of 17kW to 10kW. This is more and experiment to see if I can get IO to more accurately charge to full SOC. It is particularly annoying to wake up for a long journey to 80% odd instead of 100!

The charger profile charges at 17kW for 30 minutes then ramps down to 10kW so I actually don't loose anything time-wise.

Interesting thread here: https://teslamotorsclub.com/tmc/threads ... st-1773479

..upshot of which is that (at least in 2016) you could expect around 90% efficiency at anything above a very low current charge up to full power. There are also comments about temperature - keep the car above 10C and you are not wasting power warming up the batteries.

It now depends on the inverter/charger of the home battery system. I know my Sofar ME3000SP get quote hot at full power and so I suspect efficiency drops.

My approximate findings agree that car charging at the lowest rates is inefficient, but quite quickly improve. I get about 70% @1.7kW, & 80% @2.1kW with our variable rate granny charger, but if I want to use mostly surplus & not drain the house battery too much I sometimes have to put up with this.

On the subject of Victron inverter ratings, car charging has been quite a test of continuous load capabilities. We have a Multiplus II 3000, rated 2.4kW continuous @25*C however if run @2.4kW for a couple of hours it will overheat and cut out. My workaround & reason I went for the Multiplus II is using our original Phoenix 1600 as a slave, turned on on ac in when load>2kW, & limited to 5.3A, it’s continuous rating. It seems better at not overheating than the Multiplus, & when it does the multiplus is capable of taking up the slack while it cools down!

Ph.