Efficiency much lower when drawing from both battery and solar compared to solar only?

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randallemarshall asked Jul 4, '22

Efficiency much lower when drawing from both battery and solar compared to solar only?

I have the Multiplus 2 48v with 120v output (Model pmp482305100) that is 95% efficient. My charge controller is a 150/35 that is 98% efficient. My battery and nearby equipment is at about 90 degrees Fahrenheit or 32 degrees Celsius. I've attached 2 pictures showing common situations where the load is run off solar only and where it's combined solar and battery. I have no DC loads other than the Victron equipment itself. The battery is a 5.12kWh SOK 48v battery.

When powering a load with both AC and solar I'm at 76.5% efficiency. 577W/(255W+499W) When powering a load off solar only I'm at 92% efficiency. (155W+274W)/467W

Any ideas what could cause such a drastic difference?

Multiplus-II

solarandbattery.jpg (79.0 KiB)

solaronly.jpg (37.7 KiB)

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Guy Stewart (Victron Community Manager) answered · Jul 04, 2022 at 04:19 AM

Hi @randallemarshall@gmail.com,

A few notes;

In one case the system is charging the battery, in the other the system is discharging the battery.

Charging a DC battery from a DC MPPT is extremely efficient.

Running an AC load (especially a small load on a big inverter) from a discharging battery is much less efficient relatively.

Combining these two quite different things (running an AC load, and DC-DC charging a battery) with a single Watt rating as if they were the the same thing isn't going to give very reliable results.

Even if there was a like for like comparison, there is still likely to be sample rate variation between the components. These are indicative values only to help you understand what is happening with your system power flows, not precise measurements to be used for calibration or accounting purposes.

For a more accurate idea of your system efficiency, allow it to run for a long time (the longer the more accurate) and then use the aggregate sum totals.

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Answer 1 · 2022-07-04T02:55:40Z

kevgermany answered · Jul 04, 2022 at 02:55 AM

Do you have a shunt or are the batteries supplying the battery information?

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randallemarshall commented · Jul 04, 2022 at 03:41 PM

I am using the Victron Smartshunt. I do not currently have the battery communicating with the Multiplus.

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kevgermany ♦♦ randallemarshall commented · Jul 04, 2022 at 05:16 PM

@Guy Stewart (Victron Community Manager)

mentioned the lower accuracy of some measurements, I was wondering if this was affecting things. Make sure in Victron connect/config that the smart shunt is the master where you have the option.

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Answer 2 · 2022-07-04T10:02:26Z

seb71 answered · Jul 04, 2022 at 10:02 AM

Also, the comparisomn should be made for same (or close) AC loads.

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