direct PV dump when batteries are full (Off grid)

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sean2222 asked Feb 26, '24

direct PV dump when batteries are full (Off grid)

Hi, has anyone tried one of thees? I'm looking to utilise excess solar to heat water but don't like the idea of cycling my batteries or burning up my other electronics powering a 2kw heater, I've see a bunch of diy projects etc but this seems like an off the shelf option. any input appreciated..

https://www.ebay.ie/itm/263523458861?itmmeta=01HQG1CBNGPXRAK1C1AP71ZYAS&hash=item3d5b38df2d:g:tbEAAOSw9BBkwqt2&itmprp=enc%3AAQAIAAAA4EMoKoqzxltvIdjgPmf4MgLB1dDwxyZwyd%2FuGEZY6vvzvxw237b2OR7UNTiRK3yquwCoWTkZ6AtN%2F4Vlxs7RlyoTCFDCDxci0OvpjIu30yz%2FLBsYgS6DuVYmPTvpva1Q%2BCedZ%2Bg%2FYB0B2jWeug5jbGpInei0VgRjo490AnZ59dEkiYysyq8z1DQMUxtqMx3kRQruIJ3mJI6PYoPZaaXc2i67F9zAt2%2FmMwncZhEhVL6KLVrYxDXJ3oTQCUwX9XaP9CMCcKpa%2BpljWXY0Zvqh%2BmDRXyZtfM3YX%2FXvwLPLGfU%2F%7Ctkp%3ABk9SR-i6sYG8Yw

Hot Water Diversion

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matt1309 commented · Feb 26 at 03:05 PM

Hi @Sean2222

I've not tried those before. However you'd probably have to get quite creative with the install. You'd have to automatically disconnect the victron mppt to allow the water heater mppt to work. If you kept them both connected to the PV at the same time (ie in parallel with PV panels) then the maximum power point tracking of each MPPT would likely conflict and cause all sorts of weirdness. ie victron mppt would confict with the hot water mppt.

Which if you create automated relays/safety features then i suppose is fine however you then wont be able to have loads covered by PV and have excess go to hot water. It would be one of the other. Unless you split the PV strings up.

ie if you've got 500w of loads, and 3000w of PV coming in, battery at 100% in float.

You cant power the 500w of loads with PV and have hot water on at same time as the victron MPPT would conflict with the hot water one. So you'd have to only have one conencted at once. Either the victron mppt. or the hot water one.

Personally i would just have a 48v hot water heater after the mppt on battery side.

And just programmatically schedule when it turns on so that it doesnt cycle the battery.

ie in absorption or float phase and only when > 2000w is being exported.

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sean2222 matt1309 commented · Feb 26 at 03:36 PM

How would you add (2000w being exported) as a condition for a dump load? I know this is standard for grid tie but off grid I don’t know how to do this.

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matt1309 sean2222 commented · Feb 26 at 04:14 PM

Ahh i see. that is a tough one. Tried guessing an answer below.

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Modifications Space

Node Red Space

Answer 1 · 2024-02-26T15:24:19Z

sean2222 answered · Feb 26 at 03:24 PM

For context I’m Runing a smart solar mppt and multiplus in an off grid installation,

I’m curious how this component will work with the smart solar? from what I understand I should wire the pwn controller between my panels and Smartsolar, and the output from the pwm controller should go thru a dump load relay controlled by the Smartsolar and then on to my heating element, the pwm controller will always be on but have no current flowing thru to the heater until the smart solar triggers the dump load and closes the relay, when this happens i imagine the PV voltage will start to fluctuate rapidly due the the pwm controller drawing current?, would this PWM confuse the smart solar?

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matt1309 commented · Feb 26 at 04:13 PM

If it's just PWM/static load then i think the MPPT of the victron unit wont be effected after the initial adjustment of turning hot water charge controller on. However if the hotwater heater controller is MPPT charge controller ie the PWM duty cycle varies to get maximum PV output, then i think it'll conflict with the victron one.

Ngl this is really testing my understanding of charge controllers so would prefer if someone could confirm that my logic is correct. Generally speaking however i cant imagine it's advised just incase it does impact it.

Apologies didnt realise it was offgrid. I agree it is harder as you dont know the max available solar.

I would still configure this is having some sort of smart relay rated for the size of the hot water heater that takes 48v input so after the victron mppt. To avoid the risk of conflict with other MPPT's. I've put the smart relay behind a smart shunt so you can monitor the load of the water heater inside vrm etc.

I would do the logic for turning on/off in node red. And you're right it's difficult to know what solar is available given you've not got a grid to export to. Only way i can think of to see what you max potential solar is, is to turn on hot water heater and check periodically. If you're in the situation where you've got excess PV.

Knowing if you've got excess PV is easy. The smart charger node in node red will output 1 if you've got excess PV (or if current/voltage is being limited), hard bit is knowing how much you've got spare.

So i would turn on the hot water heater. see if there's enough power. if there is then keep it on. if not turn it off and disable the check for the next x mins. So it's not constantly turning hot water heater on and off. and that x mins you'll just have to vary.

Think the only other way you'd get the data you'd need to work out if there's lots of excess pv is if you could somehow get the duty cycle data that's being used inside the MPPT, to see just how far it is from maximum power point.

Or if you implement your own small solar sensor. Some sort of solar sensor that you know the max output of that connects to node red/gx device. Some sort of device that says solar = 100%. then you'd be able to calculate and approximation for the amount of solar that's excess without having to increase load to find out.

EDIT did more reading pretty sure PWM would conflict with MPPT even though it's not MPPT it's still not static.

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Answer 2 · 2024-02-26T21:32:38Z

Michelle Konzack answered · Feb 26 at 09:32 PM

Since the power of this regulator is limited to 2000W, the best combination would be 5 Solarpanels 400Wp in series.

This give 2000Wp and you can use the SmartSolar 250/70 which give 2000W max at 27,6V. The bigger SmartSolar/BlueSolar have a relais output which can be setup to do nice things.

All what you have to do is setup the relais to use "HighVoltage" (or any suitable options from the dropdown menu, e.G. "Float") and if you have reached the high Voltage, switch on the relais und use a bigger Relais (NC/NO) to change the PLUS conductor of the Solarpanels from the SmartSolar over to the PWM/MPPT-HeaterController. also set a "minimum time" the Relais should be switched on, but do not make it shorter then 30min.

This should definitiv work, but put a High-Voltage Ceramic Capacitor over the relais contacts to avoid burning it off with up to 250V DC from the Solarpanels.

Have fun.

Hmmm, maybe I should buy such controller too and try it with my 1860Wp but then I would need a 2000W/100V Heating element with 1" BSP since my panels deliver only 96Vmpp

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