Determining the potential power of the MPPT charger

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rwarps asked Feb 11, '21

Determining the potential power of the MPPT charger

Hello,

I was surprised to not find this question, feel free to link if you happen to have seen this being asked before.

So I have a multiplus 2 inverter, venus GX, and a Bluesolar charger. They are hooked up to a battery bank and charge it during the day discharge during the night and switch to mains if the battery voltage is to low.
Pretty vanilla stuff so far.

Now I can determine from the VenusGX data what the state of the system is. And what I see is that during a sunny day I the charger will charge the battery's and then only provide the power the inverter needs at that particular moment.
Makes sense as it has no place to put the power.

So the panels can supply more power than is being used at that particular moment. I would like to determine how much more potential power there is in order to decide whether to switch on additional loads.
For instance one could switch on a additional water heater. But you would not want that to start discharging the battery.

Now I am thinking if you could very gradually increase the load on the inverter, you can determine the "sweet spot" at that time. But that really limits the type of load you could add as it would need to be a rather variable (PWM?) load.

Feeding back to the grid would also be an option but with the battery bank I don't quite see how the Mlutiplus 2 would be able to know how much to back-feed as to not discharge the battery.

Any other suggestions for maximizing the use of your panels?

MPPT Controllers

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Answer 1 · 2021-02-11T16:36:47Z

eerskie answered · Feb 11, 2021 at 04:36 PM

I am actually considering this exact question: my thought was to use the bmv relay to trigger a circuit with higher-usage loads once the battery is full. I was thinking of adding an a/c unit or a supplemental hot water heater - these would switch on only when the battery is full, which by definition will only be when the sun is out. Once the sun disappears or loses power the batteries would start to drain and the bmv relay would switch the higher-usage loads off, leaving only the essential circuits on.

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Answer 2 · 2021-02-11T18:58:46Z

rwarps answered · Feb 11, 2021 at 06:58 PM

As I also use OpenHab I have come up with an idea to make the load variable.
I already have OpenHab monitoring the system. It can therefore take battery state into account. It can then turn on a variable load and increase the load as long as the charge-controller output also keeps increasing (meaning there is a power surplus). And of course decrease the load as power is being drawn from the batteries.

As for making the load quite so variable in real time? I figure I can use a dimmer usually used for entertainment lighting.
PWM controllers are usually not this high power and would also require some way of interfacing with OpenHab. There is quite a bit involved in regulating large electrical loads and these single channel commercial dmx dimmers are fairly affordable. They can manage a 2300Watt load and cost about 75 euros.
So I add an dmx bridge to control a dimmer that will then control the power going to the water heater as calculated real time by OpenHab. OpenHab is at that point aware of how much surplus power there is and can determine what other static loads could be added. Surplus of 1000 watt send turn on the 800 watt AC and set the water-heater to 200 watts.

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