Using excess power from the PV array to power a 12v water heater once the batteries are full.

[steve w]

I have done a bit of a search and a lot of what I read is way over my head so here's my situation. I am in the process of adding solar to my sailboat. I currently have 2x 6v golf cart batteries in series to give me 225 ah at 12v as well as a 12v lawn tractor battery as a starting battery for the small outboard auxiliary, charged by shore power when at the dock and the outboards small alternator while motoring and nothing while sailing. I am going to install 2 x 400watt LG neon panels and while on the surface it may seem like way more solar than I need for the battery bank I have they do serve another purpose being as a hard top to provide protection from sun and rain as the panels are quite large. I have purchased 2 x Epever Triron 4210N mppt charge controllers (100v/40a). So, it seems to me that with 800 watts of solar and only a fairly small ( but adequate ) battery bank i'm thinking they will be charged fully well before noon if the sun is out and then I will have a lot of energy going to waste which is why I would like to use it to heat a 6 gallon marine/rv water heater. These heaters come with a 110v element but there is a work around to also install a 12v element so I can use either. So this is what i'm working with, none of the solar components are installed yet. So, my question to you folks is this, what do you think would be the best way to utilize the excess power from the panels to achieve this? my initial thinking has been to power the element from the load terminals on the charge controllers but I can not find anything in the user manual that tells me that I can do this or not. Anyone done something similar? I could possibly install an inverter and just power it that way with the 110v element and not bother with installing the 12v one but it seems that it would be cleaner to not go through the batteries at all and go direct from the panels , but I am somewhat open to suggestions as I am not at all well versed in electricity. The other part of this puzzle for me is that I am not sure of what wattage 12 v element I would be needing, I would think I would want one that was a little over the 800 watts of the two panels combined but i'm not sure.

Steve.

 

[svetz]

I've been investigating the same thing, although my purpose is turn the hot water tank/eCar on/off when the grid is out depending on if there's excess solar power or not.

In your case deciding to turn them on/off might be pretty easy as you could use the battery's state of charge. Going DC makes sense for the big amp draws on a boat, but I'm not sure if you could can get inexpensive commercial devices to control them like you can from the Smart Home market. You can definitely go DIY. For example you could use an RF controlled DC contactor to turn the hot water heater on/off (similar to this). Going AC means you can use the devices/techniques in the links below.

In a nutshell, it takes a brain (e.g., SmartThings/hubitat or MCU de jour) to read a sensor (e.g., battery monitor) and then the brain transmits to a remote switch to turn the water heater on or off. Although, in a boat the proximity is pretty close so you might be able to skip RF and just run wires to trip relays. The RF thread is probably the better of the two threads if you're thinking about DIY.

Smart Solar/Battery
RF and the Smart Solar Home

Update: If you can have or can find a high-voltage cut-off switch (flips a relay when the battery is full), you wouldn't need the brain, just wire the relay to control the water heater. This way the hot water tank would only heat when the battery was full.

 

[svetz]

This is the converse of the idea in the update above...
Not sure if they make them, but a low-voltage cut-off switch that you could program the "low-voltage" to be, say, 95% of your battery bank could be wired to the hot water tank. So, as long as the battery was over 95% if would power the tank. But below that the hot water tank would cut off.

 

[svetz]

...The other part of this puzzle for me is that I am not sure of what wattage 12 v element I would be needing, I would think I would want one that was a little over the 800 watts of the two panels combined but i'm not sure....

Watts = amps x volts. So, 800 watts = ? amps x 12V, solve for amps = 800 / 12 = ~67 amps

 

[steve w]

Thanks, I guess I was hoping that a charge controller would know when the batteries are full and automatically switch over to the load circuit and send power to the water heater. I would have thought that the thermostat on the water heater would switch it on and off based on temperature but maybe that's too simple . I had assumed that the purpose of the Load terminals on the charge controller was for something like this but maybe not, they always show a symbol of a light bulb but I really don't see another purpose for it.

Steve.

 

[Knighthawk]

You could use the load setting to control a contact/relay on the pv side. Would need to be a good quality relay. Those high dc volts from the panel could weld the contacts closed. Someone here I hope knows of a good high voltage DC relay?

 

[svetz]

...was hoping that a charge controller would know when the batteries are full and [have a relay]...

I'm not a hardware guy, but such a thing might exist and that would work.

...I would have thought that the thermostat on the water heater would switch it on and off based on temperature...

That's the way it works... what I was talking about was turning on/off the power before that based on if excess power was available. It should still be temperature controlled so as to not overheat the water.

 

[steve w]

Thanks for pointing me in the right direction, I have checked with Epever and the charge controller does not function as I had hoped but I think I will be able to achieve what I want the way you guys have suggested. It looks like I will be able to get the Dc water heater element from Missouri wind and Solar with a high enough wattage but only at 48 volts but my panels have a VOC of 49.3 so that should work. They also have a mechanical relay that will work, looks like an automotive starter solenoid. I will continue looking for a solid state relay for 48v dc but so far no luck but the mechanical one will get the job done.

 

[svetz]

Please post pictures when you get it! Since I'm still in analysis mode for my issues I'll be curious as to what you do.

 

[dashdrum]

The wind power industry has controllers and other devices that divert excess power to dump loads. Would these work in this scenario?

 

[MisterSandals]

The wind power industry has controllers and other devices that divert excess power to dump loads. Would these work in this scenario?

Check out the Morningstar Tristar MPPT charge controllers. They are awesome SCC’s and also have a dump load function.

 

[steve w]

The wind power industry has controllers and other devices that divert excess power to dump loads. Would these work in this scenario?

I think so, ( see my last post) One problem I see with the mechanical contactor I mentioned for my use is that I think it may be a bit noisy as its switching which is why I am searching for a solid state version.