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12v / 240v Relay to use solar to heat water


jono2.0

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Hello,  I have 750w of solar panels going into a Victron system with 4x 110Ah batteries.  I'm finding my batteries are fully charged by about 8.30 in the morning on sunny days.  I'd like to use some of the remaining day's solar power to heat the water in my tank. That way when moored up for a few days out in the wilderness everything remains charged and water hot.

The immersion heater in the tank is 1Kw and is 240v.  I have a Victron MultiPlus that can more than cope with that.  I'd like to use the switching facility in the Victron controller.  When the batteries are full and happy I'd like the controller to switch a 12v relay which in turn would switch the 240v on to the immersion heater.  If the batteries go below 85% the immersion relay turns off and the solar goes back to charging the batteries.  Basically using the lost solar power to heat my water.

My question is, can I use the 100A type 12v relays used in automotive industry? Like the one in the picture, or does anyone know of a DIN rail mounted version as we are talking of 240v and I would like it all to remain safe!

Thanks Jono

 

relay.jpg

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You could swap for a 12v immersion heater. Wide range of power and thread options so you'll need to search for the right one. They're used for dump load applications from wind turbines but also for solar dump loads, exactly as you're suggesting. If you use shoreline mains, the immersion will be fed via the batteries whilst the charger feeds them.

Edited by stegra
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11 hours ago, stegra said:

You could swap for a 12v immersion heater. Wide range of power and thread options so you'll need to search for the right one. They're used for dump load applications from wind turbines but also for solar dump loads, exactly as you're suggesting. If you use shoreline mains, the immersion will be fed via the batteries whilst the charger feeds them.

Also a couple of options from Bimble:

http://www.bimblesolar.com/index.php?route=product/search&search=Heater

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Are you sure your batteries are fully charged by 8:30 on a sunny day? How are you measuring the state of charge? I have a Victron battery monitor and one of its settings is a threshold voltage above which itassumes the batteries are fully charged and jumps the display to 100%. My mppt was slightly exceeding this voltage on sunny mornings and misleading me on the state of the batteries. Get up at 6, batteries at 80%. Come 8:30 the display says 100%, but nowhere near the number of amp hours have gone in for that to be real. If yours is all Victron you might be experiencing something similar. What sort of current are your panels producing early in the morning? Is it realistic to fuly charge your batteries from their overnight use? I increased the fully charged threshold on the  bmv501 by 0.2V to solve this. I have a lot less solar capacity than your boat though.

Jenny

Edited by Jen-in-Wellies
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21 hours ago, jono2.0 said:

Hello,  I have 750w of solar panels going into a Victron system with 4x 110Ah batteries.  I'm finding my batteries are fully charged by about 8.30 in the morning on sunny days.  I'd like to use some of the remaining day's solar power to heat the water in my tank. That way when moored up for a few days out in the wilderness everything remains charged and water hot.

The immersion heater in the tank is 1Kw and is 240v.  I have a Victron MultiPlus that can more than cope with that.  I'd like to use the switching facility in the Victron controller.  When the batteries are full and happy I'd like the controller to switch a 12v relay which in turn would switch the 240v on to the immersion heater.  If the batteries go below 85% the immersion relay turns off and the solar goes back to charging the batteries.  Basically using the lost solar power to heat my water.

My question is, can I use the 100A type 12v relays used in automotive industry? Like the one in the picture, or does anyone know of a DIN rail mounted version as we are talking of 240v and I would like it all to remain safe!

Thanks Jono

 

 

Definitely  no, don't even think about. As NMEA has said the insulation properties are not suitable for 240 volts. I've used one of these on a similar application on a 1Kw immersion for several years without any no problems.

http://www.ebay.co.uk/itm/30-Amp-1-Pole-Normally-Open-Surface-Mount-Power-Relay-12VDC-BTA5-1XS-SC1-12VDC-/171174035931

Edited by Flyboy
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I have found that you will not be getting 100% out of your solar panels even under the 'best conditions', passing clouds, angle of the sun, angle of the panels etc all make them less than 100% effective.

Personally (others experience may vary) I 'work on' 50% (maybe 75% on a perfect Summers day)

 

With my 'method' your panels will effectively be about 400w.

Your 1kw immersion heater will draw approximately 100 amps, your solar will be replacing approximately 32 amps given you a discharge of 68 amps whilst the immersion heater is operating.

The length of operation of the immersion heater will depend on water temperature (start & finish) and volume of water to be heated.

Starting from 'cold' our 1Kw heater takes almost 3 hours to heat to 85 degrees.

 

If yours is similar, then you will need to replace around 200Ah with your solar.

Ie - you need to ensure that the solar is producing for approximately 3x the duration of the immersion heater being powered.

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1 hour ago, Jambo said:

I'd suggest buying a 500W 'summer' immersion heater so that you don't hit your batteries so hard. They're not particularly cheap but probably more cost-effective than shortening your battery life. 

You can get a device that halfs the wattage, however I'm not sure if it's inverter friendly so it's much better to install an additional boss and heater at the top of the calorfier of theirs room.

Link to example boss; http://www.ebay.co.uk/itm/Essex-2-25-E2-SX-Immersion-Heater-Boss-Flange-CURVED-surface-cylinder-tank-e2-r-/291209819359

Edited by Robbo
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There are plenty of Din rail mounted contactors available with 12 volt DC operating coils.

For an immersion heater you need a contactor rated for at least 20 amp AC load . RS list Schneider Electric types LC1D098JD or LP4K1201JW3 . The latter has a lower inrush  current when used on DC.

Depending on the switching device in your Victron, I would fit a low power automotive DC relay on its output and then use this to switch the larger coil on the contactor. Also fit a back EMF diode  ( IN 5000 series possibly ) across the contactor coil to kill the spark you get on the controlling relay when it switches off - the controlling relay will last longer. Fit the correct low power dc fuses in the wiring of the 12 volt circuits.

I suggest that you mount this contactor in a small metal earthed enclosure with a 20 amp 2 pole RCCD on the incoming supply, and switch both the live and neutral using only 2 poles of the contactor ( note the 4th pole on most contactors are only rated at 6 amp so no good for a high load).

Edited by adrianh
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