I did see some of the videoes where you go trough Fronius and Victron where you discuss AC and DC coupled systems. In the video you say the Fronius is very efficient at doing PV DC to AC whereas Multi is not or that's how i understand it, why is that ?
/Peter
Hi @peternielsen,
In the Fronius to Load transform,
150-500V DC Solar -> AC PV Inverter -> 230V AC load
For the MultiPlus;
120V DC Solar -> Victron MPPT -> 48V DC battery -> MultiPlus -> 230 V AC load
Those extra steps of voltage transformation make the complete process from panel to load less efficient.
The Multi itself is quite efficient, but as a round trip from the supply to the load, it is inherently going to lose a bit more on the way.
Now if the intention is to charge a battery, the situation is reversed.
In my system I see MP-II's inverting efficiency around 86%. 1087/1258=0.8641
It is not close to what the specifications says. (95%)
Hi @MarekP,
The word maximum is important there. There are other factors too, such as the type of load (reactive power factor), battery voltage, DC ripple effect, etc. Also the accuracy of the test equipment/built in monitoring.
Here are some of the lab test results and further details about efficiency and temperature derating if you are interested.
@Guy Stewart (Victron Community Manager)
Hi,
Battery (48V) was fully charged and staying at 52.19V under this load.(DC23.90A)
Load was pure resistor (electric heater) (PF=1).
Ambient temp was around 8C.
MP-II 48/3000 could not get very hot after 3 minutes of this 1kW load. (fan did not start).
MP-II was the test equipment. :)
Best Regards,
Marek
@Guy Stewart (Victron Community Manager)
Any explanation for this low efficiency?
To follow a similar test protocol as would be used for the spec document, use a calibrated clamp meter on the AC output, and DC shunt (like a BMV) on the DC input side. Also AC and DC cables should be very large, and load and battery supply very close (to take out voltage drop loses), and accounting for fusing, switches and circuit breakers.
Then record a range of measurements at different loads to find the sweet spot.
The built in readings are indicative, and perfect for day to day use, but not intended as test or calibration equipment.
@Guy Stewart (Victron Community Manager)
Can you provide me the data on the accuracy of the measuring equipment installed inside MP-II?
The VRM graphs are based on the data from MP-II.
The DC or AC cable sizes have no bearing on the measurements done by MP-II. Measuring components of MP-II see only voltage and current on DC and AC terminals of MP-II.
@Guy Stewart (Victron Community Manager) thank you and yes the intentions is ALSO to charge battery therefore i will go for a DC AND AC coupled installation as you have advised in some of the webinars i have seen which seems like a good solution.
In the case of the MPPT does it mean it will wear out the battery in time when using PV power directly because it has to go thru the battery ?
Great, happy to hear that you are getting some value from the online training resources, have you found the Victron online training program itself yet?
Short answer
During the day the power will flow across the DC bus, so there is no additional cycling wear on the battery. But that additional step of transformation I mentioned will still happen.....
Long Answer
For technical completeness, but note, you really should not worry about this.
There is an additional effect of a battery inverter on a battery called DC ripple. There is more information about what ripple is in the Wiring Unlimited book, but basically there is a gap between the AC sine waves peaks that can appear as a current draw fluctuation or 'ripple' on the DC bus and batteries. It is best to keep DC ripple as low as possible with large batteries, good DC wiring, and to some extent AC coupled PV will help to reduce this effect too.
@Guy Stewart (Victron Community Manager) no i didn't know about the online training, it looks really interesting. I would have a look at it and maybe i am able to pass an exam now :P
Thanks for the detailed explanation so i understand now that this extra DC (120V) -> DC (48V) step-down generates a loss when again once has to use the 48VDC to convert to 230VAC.
