Solar dump load

[GUMPY]

The SSR will switch AC using DC to control it.

 

The other item I would avoid like the plague.

It may well not play nicely with the inverter supplying it depending on how it chops the waveform.

 

[blackrose]

25 minutes ago, GUMPY said:

The SSR will switch AC using DC to control it.

 

The other item I would avoid like the plague.

It may well not play nicely with the inverter supplying it depending on how it chops the waveform.

 

 

Thanks. Is there a better power controller? In the original instructions I posted they seem to be using this sort of thing.

 

https://amzn.eu/d/060yuqdm

 

I thought the one I found looked better quality but perhaps not.

[GUMPY]

44 minutes ago, blackrose said:

 

Thanks. Is there a better power controller? In the original instructions I posted they seem to be using this sort of thing.

 

https://amzn.eu/d/060yuqdm

 

I thought the one I found looked better quality but perhaps not.

All depends on how they do it, the inverter may or may not play nicely.

It's down to suck it and see, 

[blackrose]

Chat GPT is also telling me not to use cheap light dimmer type power controllers and is suggesting a burst-fire/zero-cross controller.

 

I found these on eBay:

 

https://ebay.us/m/SU3MPT

 

https://ebay.us/m/CF5voz

 

 

 

Edited April 24 by blackrose

[blackrose]

I've been told that the SSR won't work from the MPPT relay output because it's just a dry contact relay , a gate which doesn't actually put out a voltage. If that's true I'm not sure what the point of that relay is exactly or how I could use it to make the SSR work.

[IanD]

21 minutes ago, blackrose said:

I've been told that the SSR won't work from the MPPT relay output because it's just a dry contact relay , a gate which doesn't actually put out a voltage. If that's true I'm not sure what the point of that relay is exactly or how I could use it to make the SSR work.

You need to connect 12V to the common terminal of the MPPT relay.

[Alan de Enfield]

57 minutes ago, IanD said:

You need to connect 12V to the common terminal of the MPPT relay.

 

I used the output on the controller (right hand side 2 terminals) for the supply for the VHF radio - radio was not installed in this picture.

 

[GUMPY]

43 minutes ago, Alan de Enfield said:

 

I used the output on the controller (right hand side 2 terminals) for the supply for the VHF radio - radio was not installed in this picture.

 

That looks like a load output, different to the relay contact that Mike has on his MPPT controller.

 

[Tony Brooks]

Just now, GUMPY said:

That looks like a load output, different to the relay contact that Mike has on his MPPT controller.

 

 

and I don't think it is an MPPT, it looks like a PWM controller to me.

[GUMPY]

Just now, Tony Brooks said:

 

and I don't think it is an MPPT, it looks like a PWM controller to me.

I wasn't going down that rabbit hole 🫣

But you are correct

 

https://www.selectsolar.co.uk/prod/569/steca-regulator-10a

 

[blackrose]

4 hours ago, IanD said:

You need to connect 12V to the common terminal of the MPPT relay.

 

Ah ok thanks, and then the relay terminals become capable of outputing 12v  current and can switch the SSR?

 

Any idea what sort of current it would be drawing and what size fuse to fit to that cable?

Edited April 25 by blackrose

[WulfNut]

I've never quite grasped the logic when these days most inverters are quite capable of dumping 1kw into a immersion heater.  Why not just flick a switch and set a timer on your phone 

[IanD]

On 25/04/2026 at 18:20, blackrose said:

 

Ah ok thanks, and then the relay terminals become capable of outputing 12v  current and can switch the SSR?

 

Any idea what sort of current it would be drawing and what size fuse to fit to that cable?

Yes.

 

Depends on the relay but current will be too small to need to worry about, certainly much less than 1A.

 

[Jen-in-Wellies]

11 hours ago, IanD said:

Yes.

 

Depends on the relay but current will be too small to need to worry about, certainly much less than 1A.

 

Using a very small current to switch a big current is what a relay does! Among other things.

[MtB]

17 hours ago, WulfNut said:

I've never quite grasped the logic when these days most inverters are quite capable of dumping 1kw into a immersion heater.  Why not just flick a switch and set a timer on your phone 

 

I think the idea is that under marginal solar generation conditions, charging the domestic batteries needs to happen first then any surplus solar energy available gets used for heating a store of water (rather than letting it go to waste), whilst at the same time leaving the batts fully charged for later.

 

A timer and a switch discharges the batts if the sun has gone in and there happens to be no solar excess.

[blackrose]

On 26/04/2026 at 18:01, WulfNut said:

I've never quite grasped the logic when these days most inverters are quite capable of dumping 1kw into a immersion heater.  Why not just flick a switch and set a timer on your phone 

 

My inverter doesn't have that feature, but I think you're missing the point. I have a timer on my immersion heater control but the immersion heater doesn't know how well charged the batteries are or how much the sun is shining. So I wouldn't want the immersion heater switching itself on for 2 hours when sun isn't shining and the batteries aren't well charged.

9 hours ago, MtB said:

 

A timer and a switch discharges the batts if the sun has gone in and there happens to be no solar excess.

 

Yes that's it.

Edited April 27 by blackrose

[WulfNut]

I wasn't suggesting having it auto set to come on for 2 hours.  In houses that are all electric they have a "boost" which switches off after a preset time.   So you could go "by heck the sun has got his hat on, and me batteries are full to the brim, I think i will hit the old boost button and change some of that stored energy into hottish water"  that sort of thing.  Because it is simple and cheap all be it not smart. 

Edited April 30 by WulfNut

[blackrose]

10 hours ago, WulfNut said:

I wasn't suggesting having it auto set to come on for 2 hours.  In houses that are all electric they have a "boost" which switches off after a preset time.   So you could go "by heck the sun has got his hat on, and me batteries are full to the brim, I think i will hit the old boost button and change some of that stored energy into hottish water"  that sort of thing.  Because it is simple and cheap all be it not smart. 

 

To "hit a boost button" one needs to be at home. I already switch my immersion heater on when the sun is out and the batteries are charged, but that relies on me being at home on the boat to manually flick that switch and more importantly switch it off again to protect the batteries.

 

Here we're talking about a solar dump load which would switch the immersion on automatically after the batteries have charged and reached a certain voltage, and then switch it off when that voltage drops. The idea is to have a free tank of hot water from solar when you get home from work, etc, when weather conditions permit.

_______________________

 

I'm just trying to map out the system using the MPPT relay, a DC to AC SSR and a variable AC regulator. Am I thinking along the right lines here? 

 

I don't really understand why the regulator only has one cable gland? Is this used for cable entry & exit? Seems odd. 

 

Also it only has one E terminal so do the input and load earth cables both go to the same terminal?

 

I haven't included the AC  load side to the immersion in my diagram. Should I have another fused switch on the load side of the regulator?

 

Edited May 1 by blackrose

[wakey_wake]

3 hours ago, blackrose said:

Am I thinking along the right lines here? 

If we accept that there is no 'right' way to do it, then yes. 😁

IMO. I'm not a 'qualified' electrician and I tend to overthink stuff.

 

2 hours ago, blackrose said:

I don't really understand why the regulator only has one cable gland? Is this used for cable entry & exit? Seems odd. 

5-core mains cable does exist, but your outgoing load is going to have unusual colours (probably grey & black).

Here's one but twice the mm² you need: https://www.essentialsupplies.co.uk/2-5mm-ho7-rubber-5-core-16a-3-phase-rubber-cable-5g2-5.html

Otherwise I guess you're trying to stuff two flat-ish cables into a round gland which seems a bit hackish.

 

Assuming you want to continue with design by committee 😁 I would suggest re-drawing the diagram with 5-core in mind, and a junction box or terminal block inside the existing switch so it can meet the 3-cores in and out.

 

3 hours ago, blackrose said:

Also it only has one E terminal so do the input and load earth cables both go to the same terminal?

Earth is common all over.

 

3 hours ago, blackrose said:

I haven't included the AC  load side to the immersion in my diagram. Should I have another fused switch on the load side of the regulator?

I believe you wanted a manual override? That's going to need switching.

Also is there a twiddly on the outside of the regulator, or would you want it bypassed?

I see no reason for another fuse. Fused for the thinnest cable is fused.

 

I am curious about that ?gold plated 30A fuse. It has a diode symbol on it... 🤯

[GUMPY]

What is that thing you are connecting the SSR to?

If the MPPT has relay terminals that can be controlled by the SOC then it's not needed.

 

[MtB]

3 hours ago, blackrose said:

Here we're talking about a solar dump load which would switch the immersion on automatically after the batteries have charged and reached a certain voltage, and then switch it off when that voltage drops.

 

There is a problem here you may have already anticipated. Turning the immersion ON will lead to battery voltage drooping a bit, to under that "certain voltage" you mention. So the immersion immediately turns OFF, and the battery terminal voltage with no load on it once again rises above the "certain voltage". You need to consider how to stop the immersion cycling on and off by this mechansism. Possibly very fast! 

 

You need to make sure the on/off 'hysteresis' is large enough. 

 

[wakey_wake]

Fair point I had not considered - control systems fighting with each other, if measures are not taken to avoid that.

 

19 minutes ago, MtB said:

You need to make sure the on/off 'hysteresis' is large enough.

Sufficient hysteresis will do. I guess that LFP will need it wider as the charged knee is approached, but I don't have numbers.

We discussed earlier about hysteresis of CV charging voltage, and there is potential for interference here, e.g.

 

Another possibility is to size the load towards what the PV might be supplying, so you don't push the batteries into discharge... as averaged across the relevant number of cycles. The chemistry may not see the averaging that way, depending on inverter topology.

 

Inconveniently, this variation in voltage is likely going to be too fast for multimeters to see.

Perhaps perversely conveniently, the variation in voltage may be too fast for the MPPT relay output to notice.

 

If it's a mechanical relay, it might be worth listening for 'chatter'.

If it's not actually a relay but a transistor pair of push-pull FETs then one would need something like an LED + resistor off the 'NC' output, to see if it blinks lit (representing immersion being off) during use.

Edited May 1 by wakey_wake
transistor

[blackrose]

2 hours ago, wakey_wake said:

I believe you wanted a manual override? That's going to need switching.

Also is there a twiddly on the outside of the regulator, or would you want it bypassed?

I see no reason for another fuse. Fused for the thinnest cable is fused.

 

I am curious about that ?gold plated 30A fuse. It has a diode symbol on it... 🤯

 

Thanks. Yes, the regulator has an external dial to adjust the draw of the load.

 

2 hours ago, GUMPY said:

What is that thing you are connecting the SSR to?

If the MPPT has relay terminals that can be controlled by the SOC then it's not needed.

 

 

The SSR is connected to the MPPT relay terminals. Surely the MPPT relay terminals can't switch AC? Hence the DC to AC SSR.

 

2 hours ago, MtB said:

 

There is a problem here you may have already anticipated. Turning the immersion ON will lead to battery voltage drooping a bit, to under that "certain voltage" you mention. So the immersion immediately turns OFF, and the battery terminal voltage with no load on it once again rises above the "certain voltage". You need to consider how to stop the immersion cycling on and off by this mechansism. Possibly very fast! 

 

You need to make sure the on/off 'hysteresis' is large enough. 

 

 

Yes I had considered that. It's going to need some careful adjustment of the MPPT relay settings. However, if people are achieving this using cheap external voltage controlled relays from ebay as in the method I posted in the OP (presumably because they don't have a relay built into their MPPT), then surely I can achieve the same thing using a better quality Victron MPPT relay?

Edited May 1 by blackrose

[wakey_wake]

  

44 minutes ago, wakey_wake said:

I am curious about that ?gold plated 30A fuse. It has a diode symbol on it... 🤯

Possibly a 'protistor', a fuse that claims it can compete on speed with "the fastest fuse on three legs".

https://www.mouser.com/datasheet/2/1449/DS_Semiconductor_Protection_Cylindrical_Fuses_Prot-3597045.pdf

says it can begin to open in 0.1ms at 30x rated current.

 

1 hour ago, blackrose said:

The SSR is connected to the MPPT relay terminals. Surely the MPPT relay terminals can't switch AC? Hence the DC to AC SSR.

It's pedantry but electrons do play exactly by the rules... I don't think the issue is it switching AC, so much as being safe at 240V and being able to sustain 4~8A; bearing in mind that although you're reducing the average draw with the regulator, the peak draw remains.

 

1 hour ago, blackrose said:

if people are achieving this using cheap external voltage controlled relays from ebay as in the method I posted in the OP (presumably because they don't have a relay built into their MPPT), then surely I can achieve the same thing using a better quality Victron MPPT relay?

I believe it's not about the quality of the relay, but the flexibility of the control system that drives it.

I have no Victron experience yet and was planning to avoid the Bluetooth side of it.

 

Anyway, if the MPPT won't do what you need then it's not a show stopper. All it needs is a couple of op-amps and a pair of potentiometers to set the thresholds.

[blackrose]

2 hours ago, wakey_wake said:

 It's pedantry but electrons do play exactly by the rules... I don't think the issue is it switching AC, so much as being safe at 240V and being able to sustain 4~8A; bearing in mind that although you're reducing the average draw with the regulator, the peak draw remains.

 

I believe it's not about the quality of the relay, but the flexibility of the control system that drives it.

I have no Victron experience yet and was planning to avoid the Bluetooth side of it.

 

Anyway, if the MPPT won't do what you need then it's not a show stopper. All it needs is a couple of op-amps and a pair of potentiometers to set the thresholds.

 

The SSR is rated at 40A so surely it can sustain 4-8A?

 

I think the quality of any electrical components is important, but since you mention flexibility, is the relay in the method I posted in the OP (shown below) that others are using any more flexible than the Victron relay in my MPPT? 

 

 

Edited May 1 by blackrose