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valipopescu avatar image
valipopescu asked

DC excess solar power diverting

Hello everybody,

I have a system composed of

  • Multiplus 48v 5000;
  • Victron solar charger 250/100;
  • Color control GX;
  • 6 pylontech batteries;

and I want to divert the excess solar power from the 48V DC bar into the 48V hot water boiler.

The diverting system worked very well when I had lead batteries and the diversion start and stop conditions were dependent on the battery voltage - the charger controlled the charging of the batteries.

After I changed the lead batteries to Pylontech batteries, the BMS controls the chargers. The problem occurs when the SOC is 100% and the BMS sets the CCL (charge current limit) to 0. At this moment, the charger no longer deliver all the available solar power.

Does anyone know a solution to divert excess solar power from the 48V bar if the charger is controlled by the battery BMS?

Thank you!

dc systemHot Water Diversion
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6 Answers
matt1309 avatar image
matt1309 answered ·

Hi @vali.popescu


I would use node red to detect if SoC is 100% (and if CCL is activate meaning the solar is being limited). Then turn on the 48v load via node red (either via a 48v smart switch or a 48v relay) and then add a wait into the node red checks to allow the MPPT to ramp up to check if there's enough excess power to cover the load.

If not and the battery is being used to power the load then node red turns the load (relay off) and waits a fixed length of time before doing those checks again (to prevent the heater turning on and off constantly if there isn't enough excess power) .


I'm guessing you're not grid tied/exporting as this is easy in that scenario as you can check if amount being exported is greater than the amount the water heater uses, so you can just check that rather than turning on the load to see if there's enough excess power.


One thing to note when you make these checks in node red is to add buffers so the load isnt turning on and off repeatedly.

ie turn on -> wait for solar to ramp up -> then check if there's enough solar.

Rather than turn on -> check if there's enough solar. As this will likely instantly turn off.

You can add in these buffers either in the form of time, or in the form of watts, ie if load is taking less than x watts from the battery. Or a combination of both.


EDIT: I have a setup you describe but I do it with AC immersion heater. And rather than a relay i use a shelly smart switch which is turned on via node red when i have excess power.

I do have grid exporting though so my checks are slightly simpler.

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Fideri avatar image Fideri commented ·
@matt1309

I'm planning to do exactly what you did in Node Red. My project is not mission critical. Rather than reinventing the proverbial wheel, are you fine with sharing your solution as a json file here?

Fideri


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valipopescu avatar image
valipopescu answered ·

Thank you for the suggestions @matt1309 ,

I am connected to the grid but I do not have export authorization.

As long as the BMS is connected to the CCGX, the CCL is automatically activated and controls/limits the charger.

If I turn on the 48V load when CCL=0, the 48V load will consume 100% from the battery, even there is solar power available. If I continue to discharge the batteries and the SOC drops to 99%, the BMS resets the CCL value to 10, 20 or 30 A and the charger is reactivated.

I was thinking that a solution would be to start the power deviation when SOC is 98% (for example) when the CCL value is 30/40A and to stop when SOC = 95%. In this case, I have to pay attention to the balancing of the batteries.

Another solution is to request export authorization and control the power available through export, as you suggested. I think this is the best solution.

I prefer to insist to finding a diversion solution on DC than on AC.

Thank you again

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matt1309 avatar image matt1309 commented ·

Hi @vali.popescu


I might be wrong here but CCL just limits the power going into the battery doesn't it, not the output of the MPPT? (if you have no loads on system it would be the same thing)

For example if you had no loads on the system and CCL was set to 0 then the MPPT would ramp down to 0 output but if you have some loads the MPPT shouldn't ramp down all the way, it should ramp down so that battery has 0 going into it but the loads are covered by the solar.


Which is what we want to happen here, the battery will have CCL of zero but MPPT should ramp back up to cover the load when it's turned on. The only additional check you'd need to do in node red is to see if the MPPT when fully ramped back up has enough power to cover the load.

You could do this by just waiting a few seconds after load turns on to see if the MPPT has enough solar by checking if there's any current leaving the battery after you've given it a few seconds to ramp back up (maybe add a buffer of a few watts just incase the system isn't balancing it perfectly).




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valipopescu avatar image
valipopescu answered ·

HI @matt1309 ,

I did some tests under the following conditions:
- around 4000 W available solar power (sunny day);
- SOC=100%, CCL=0;
- AC loads around 500W;
- DC load 1000W;

First test - I set the inverter "Off Grid"

Remarks:
- the DC load was supplied from the battery;
- the AC load was supplied from the inverter, using solar energy from the charger;
- I increased the AC loads to 1000W and all the power was supplied from the inverter, using solar power from the charger;

Second test - I set the inverter "On Grid"

Remarks:
- DC load was supplied from charger - solar power;
- The AC load was supplied from the inverter, using solar energy from the charger;
- the inverter exports the rest of the available solar power, around 2200 W = (4000 W solar production - 1000W DC load - 500W AC loads - internal consumption)

My conclusions:
- in "off grid" configuration, DC diversion is not feasible. The BMS have partial control over the charger;
- in "on grid" configuration, DC diversion is a good option. The BMS has les control to the charger.

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matt1309 avatar image
matt1309 answered ·

Hi @vali.popescu


That's a shame i was convinced that would work.

One more question/thing to try. Is the DC load being monitored?

Your conclusions make sense (and i dont know enough about the calculations running in background to give you concrete answer) however could it also be the case that in on grid mode the MPPT just isn't reducing output at all. And the excess is just flowing through the inverter ie no system calculations

So it's not that the system is actively calculating the excess, it's just not limiting the MPPT.


The reason I mention this is if the DC load is metered (via a smart shunt) I thought the GX device would instruct MPPT to cover it, the same way AC load is.

Also worth checking do you have "Has DC system" enabled in gx device (In System setup). Be interesting to see if that cause any changes also? (try this one first as smart shunts aren't cheap).



EDIT:

Just checked my smart meter, you can set smart shunt to monitor DC loads. But hopefully just enabling "has DC system" adjusts the calculations in the backend and behaves as you require.

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valipopescu avatar image
valipopescu answered ·

@matt1309 All tests were performed with "Has DC system" function enabled.

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matt1309 avatar image matt1309 commented ·
Damn,


Hopefully someone with knowledge of the backend calculations will comment to confirm if smart shut as dc load would work. As its a lot of money if it doesn't work.


I was almost certain that feature would exist. Seems such a common thing folk would want

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taylortops avatar image
taylortops answered ·

I wonder if my little Raspberry Pi project might align to your needs?
https://community.victronenergy.com/idea/160811/hot-water-heater-opportunity-load.html

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