question

bigbadbob76 avatar image

Some thoughts on prioritising Solar.

I'm still trying to get my system to prioritise DC coupled solar when it is available to keep my batteries at 100%.

My system comprises: Multiplus 12/3000, Rpi running venus v2.66, BMV600s, DVCC on, 1400W of solar (overspec'd for winter), mppt100/50, 1480Ah UPS battery. (hence the desire to keep it charged). AC load of 1.2Kwh/day.

This is primarily a UPS system to cover power cuts and it works great for that. but I have DC solar so would like to prioritise it.

The way I understand it, the Mppt controller is a battery charger so it needs to see a load on the battery which drops it's voltage enough for the Mppt to pass current.

In order to load the battery, the multiplus ac input needs to be off (multi in inverter-only mode).

I can't feed back to the grid so can't use ESS.

I can manually switch the multi to inverter-only on sunny days via remote console and this works great but is a PITA and should ideally be automated.

Options I've tried:

lowering multi float and abs voltages by 0.1V below MPPT voltages. this works to an extent but only when the battery is under load or is partially depleted. (which makes sense)

lowering multi charge current, this still prioritises ac input for load use but does prioritise solar for charging a partially depleted battery. (which makes sense)

I've also fiddled around with venus coding to switch to inverter only when PV OC volts are high enough but I'm struggling to get it to work. I don't fully understand the venus coding.

Ideally I would do this in venus but-

Alternatively I could schedule when the multi switches between "inverter only" and "on" based on time of day or possibly using an irradiance sensor.

As a very basic way to get it working I'm going to try a time switch on the ac input to schedule the switching but this seems like a poor way to to do it when venus could schedule it.

I can't use the generator relay to do the scheduling as I use it for a generator.

I can't use the virtual switch as I use assistants for input current limit control.

I could use ignore ac input assitant based on SOC of battery but with such a big battery compared to the load it takes a while for it's SOC to drop low enough for the AC input to come on.

Has anyone else played around with this? would be good to bounce ideas off someone else.

solar
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7 Answers
JohnC avatar image
JohnC answered ·

Hi @Bigbadbob76

You seem to know your way around, so I won't bother too much with deep stuff.

To give the mppt priority it needs to see a higher V target than the Multi does. You've tried that (& presumably DVCC and temp-comp settings make it for real), but in a UPS situation 0.1V will just allow the mppt to rest the Multi charger while the sun's up. But the charge states can't be synchronized, and the Multi entering Absorb while the mppt is in Float will reverse the priority. You could set the Multi to a lower flat Storage V and let the mppt give the batts a daily stir-up.

But if you want more from the pv then you'll need to leave some space in the batts. You've explored this too, but the easiest way is just to Ignore ACIN. Think about doing this using V, not SOC (could be fun). So it will neither charge at all nor passthrough to loads. You mightn't like the batt discharge, but they'll likely enjoy the exercise. :)

Questions.. Re the 'generator relay', what's that for, auto-start? No second relay in your model Multi?

And the 'input current limit control', for the genset? Really necessary with such small loads relevant to batt capacity?

Enough for now.

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bigbadbob76 avatar image
bigbadbob76 answered ·

Thanks for your suggestions JohnC, I understand what you're saying about using battery voltage to trigger ignore AC input. that should work. :-) and about lowering the Multi charge voltage to a flat storage voltage, both these should give some room for the MPPT charger to have something to do.

Haha, I'll take my batteries for a run round the block to give them exercise, understood, they'll like to be used just not too deeply. Once we get some summer sun I'll probably be able to leave the multi in inverter only mode most of the time. it's in winter that I can't do that but still want to use my solar as much as possible on the few sunny days we do get.

Solar in Scotland is a bit hit and miss compared to countries with predictable sunshine.

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Kevin Windrem avatar image
Kevin Windrem answered ·

I've seen this request over and over again. There are some things you can do to get partially there like ignore AC input but it's not a clean solution.

The functionality within ESS does what you want but ESS can't always be used. I suggested before that ESS functionality be broken out so that pieces of it can be used (like solar energy priority in this case) without all the other baggage ESS brings to a system.

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I totally agree Kevin. ESS has some good features but comes with baggage.
bbmz avatar image
bbmz answered ·

What‘s your reason for not using the ESS functionality? ESS will offer the functions, your are describing

a) giving grid-failure protection by keeping the battery charged

b) (if available) using DC-PV via MPPT for powering devices on AC-out on the multiplus

c) no feed-in (!) There is an option to disable export to the grid.


From my point of view you should have a look into the ESS guide:

https://www.victronenergy.com/media/pg/Energy_Storage_System/en/ess-introduction---features.html

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Victron makes a firm statement that ESS can not be used in certain situations.

Maybe Victron needs to make a special version of ESS that can not feed in to the grid. Then more people could use the ESS functionality.

bbmz avatar image bbmz Kevin Windrem ·
Have you looked into the victron design guide for ESS? There is clearly described that ESS can be used with disabled grid-export. Can you maybe put a link to the statement you are referring to?

The ESS manual

https://www.victronenergy.com/media/pg/Energy_Storage_System/en/ess-introduction---features.html

states:

When not to use ESS

  • Off-grid systems - either with or without generator.

  • Marine systems.

  • Automotive systems.

  • Inverter priority, also known as 'Intentional islanding“ or 'Ignore AC' input systems.

Also this document

https://www.victronenergy.com/live/ve.bus:grid-codes-and-loss-of-mains-detection

states:

Q2: Can I use ESS in a vehicle or a boat?

A: No you cannot. After unplugging the shore cord it can take up to a few seconds for the system to detect the loss of mains and open the back-feed relay. During those seconds, the shore power plug will be `live`: there is 230 Volts AC on the terminals. The exact number of seconds differs per country and depends on the local requirements.

The same applies to other solution where the wiring is not protected against accidental removing, for example a simple cable with end-user removable plug, such as a portable power supply.

Q4: I use ESS, but have disabled feed-back, is a grid-code still required?

A: Yes, also in this case LOM is required to make for a safe and also local regulations compliant system. Running ESS with disabled-feed-back is still not the same as using a Multi in its default configuration without ESS.


I'd like to see where Victron says it IS ok to run ESS with feed-in disabled and how to do that. I'd like to take advantage of functionality only offered through ESS.


bbmz avatar image bbmz Kevin Windrem ·

Ok - that‘s the same document I was referring to.

Sorry I overread that you are talkin about a mobile setup (car / boat)...


You can maybe think about using external control for the setpoinf of the multiplus via Modbus or NodeRed but honestly I don‘t even know if these options are available without using an ESS.

I'm not sure the original poster is a mobile system.
Assistants, controlling AC In work as well.
ESS always stays connected to the grid if grid is available, I don't want to feed anything back to the grid so I would need to set a minimum power draw from the grid in ESS to prevent this. that defeats the object of using solar if available.

to put it another way- ESS always either draws from grid or feeds back to grid. albeit in small amounts.

bigbadbob76 avatar image
bigbadbob76 answered ·

Having failed to get Venus to switch the multiplus mode depending on pv output, I'm now looking at this from a different angle. I understand hardware better than software so...

I have a 12v 1.5W pv panel that is supposed to keep a car battery topped up when not in use for extended periods, frankly it's useless for that as it only supplies 100mA max.

I'm thinking I could use it as an irradiance sensor to switch a relay which I would connect to the aux or temp inputs of the multiplus and set up an assistant to ignore ac input if the relay is closed for a few minutes, and not ignore ac input if relay is open for a few minutes.

I'll have to play with the load resistance across the panel to get it switching at an irradiance level that will supply sufficient power from my main panels to keep up with demand from the loads on the inverter.

The relay coil might be enough load on the panel to do this but Ideally it should be adjustable.

I could also have an over-ride switch in series with the relay for when I'm running the generator.

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Why not use an assistant to control the Ignore AC IN based on SOC or voltage. Also some of the MPPTs have a relay, which you could use to control the Aux input on the MultiPlus as well.


You may find the question here interesting

"Ignore ac input" based on SOC or voltage cuts the ac input at night after a full charge during the day, then allows AC input the next morning after the slight overnight discharge.


I just want it to cut the ac input when solar is available and let it charge from grid at night.

Interesting idea on using an mppt driven relay, thanks, that bears closer study. :-)

my mppt 100/50 doesn't have a relay but maybe one could be added. possibly driven by one of the LEDs.

Unfortunately my 12v 1.5W panel is not man enough to switch a relay, the voltage gets dragged down to a few volts as soon as I try to draw any current. no wonder it never worked as a battery charger. hahaha. it might work better in summer but I'll not hold my breath.


bigbadbob76 avatar image
bigbadbob76 answered ·

Another thought on prioritising solar. Thanks for the hint bathnm. :-)

My MPPT 100/50 has the "streetlight" function which is designed to Automatically switch streetlights off during the day. It learns when sunrise and sunset are and corrects for changing seasons.

The panel voltages used to detect sunrise/sunset are configurable. :-)

This seems like it could be very usable for prioritising solar use and switching "ignore ac input" assistant. unfortunately it uses the same i/o port as the ve-direct port so I would lose the data I'm currently sending to my Venus device.

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bigbadbob76 avatar image
bigbadbob76 answered ·

I now have my system prioritising solar. :-)

But it's not very elegant. I use a separate time switch on the ac input which at the moment switches the grid supply off at midnight and back on at 4:30 pm. with current solar availability my batteries are discharged to 97-98% SOC by the time the sun comes up and then PV charges the battery and keeps up with load demand during the day while i'm at work but not in the evening, so I allow grid use in the evening. This will improve as we come into summer and I can extend the grid off time.

This keeps my UPS batteries well charged to prolong their life and provide plenty of capacity in the event of grid failure while maximising solar use.

Having the ability to schedule "ignore ac input" via venus would be nice, then I could adjust the times remotely without having to re-program my time switch manually. I might have a go at programming that in venus but if anyone else fancies the challenge I'd be very interested.

Scheduled charging is an option in ESS but It would be nice to have some of the options from ESS available separately as Kevin says.

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