Feedback on 12V water heater element as solar diversion load

[ewarnerusa]

My solar charge controller has a diversion load feature where excess current can be diverted to a load once batteries are full. The manual discusses using a water heater element and this sounds like a clever way to do something with excess solar harvest that would otherwise be wasted. I recognize that this is not a primary method for heating the water, just a way to doing something rather than nothing with excess solar harvest. Does anyone do this and have feedback? I found an old thread posted by "msiminoff" with a cool project doing this with a custom element.
https://www.rv.net/forum/index.cfm/fuseaction/thread/tid/25824208/srt/pa/pging/1/page/1
Otherwise, there are existing 12V heating elements that can do this.
https://windandsolar.com/9-3-8-inch-dual-power-submersible-water-heating-element/

[StirCrazy]

ewarnerusa wrote:
Wow, it is working great! I'm shutting this test down as the water is getting hotter than expected and I'm not available for continuous monitoring. 66F to 130F in a couple hours. I realize this is just a tiny volume compared to 6 gallons, but this little sucker is cranking out heat just using current that the battery isn't needing for absorption charging. Panels are harvesting 15 amps, looks like between 7-10 amps are going to the diversion load element, the remainder is going to maintaining the batteries at absorption setpoint, the laptop charger to run the laptop in order to monitor stuff, and any other camper parasitic loads (not that there is much, it is sitting in storage at home).

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Laptop hooked to the SCC to monitor things. Interesting that when wired up for diversion load, the charge current shows 0. I believe this is because the panels are not actually hooked to the SCC in diversion load mode. They are hooked directly to battery and SCC monitors battery voltage, allowing anything in excess of charging setpoints to flow to the diversion load.

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Full current from panels right now is 15 amps:

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It's too bad we can't get a water temp gauge into the hot water heater. When you finally hook it all up, I would recommend putting a temperature operated shutoff in line with the heater. The last thing you want is it to get too hot and blow your safety relief valve.

[ewarnerusa]

Wow, it is working great! I'm shutting this test down as the water is getting hotter than expected and I'm not available for continuous monitoring. 66F to 130F in a couple hours. I realize this is just a tiny volume compared to 6 gallons, but this little sucker is cranking out heat just using current that the battery isn't needing for absorption charging. Panels are harvesting 15 amps, looks like between 7-10 amps are going to the diversion load element, the remainder is going to maintaining the batteries at absorption setpoint, the laptop charger to run the laptop in order to monitor stuff, and any other camper parasitic loads (not that there is much, it is sitting in storage at home).

Click For Full-Size Image.

Laptop hooked to the SCC to monitor things. Interesting that when wired up for diversion load, the charge current shows 0. I believe this is because the panels are not actually hooked to the SCC in diversion load mode. They are hooked directly to battery and SCC monitors battery voltage, allowing anything in excess of charging setpoints to flow to the diversion load.

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Full current from panels right now is 15 amps:

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[ewarnerusa]

pianotuna wrote:
ewarnerusa,

I for one, will be waiting with baited breath on the results!

I've got a test scenario all rigged up right now! Should be a nice sunny morning but clouds and wind forecast for later. Sun has not yet crept over the trees and hit my panels yet, so nothing to report.


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When I first got to wiring up the diversion load circuit to the SCC, it wasn't what I expected since I had not read the manual for that part carefully yet. I know, I know, RTFM... But I got it figured out now. It requires connecting the panels directly to the batteries and the former panel input to the SCC is now the diversion load output. Thankfully I had purchased a pack of several Anderson style connectors and could make several convenient disconnects for swapping back and forth between normal setup and the new diversion load setup.

[pianotuna]

ewarnerusa,

I for one, will be waiting with baited breath on the results!

[ewarnerusa]

Spring projects weekend. I got 3 projects done: installed auto transfer switch for the inverter, replaced main 12V feed to control center with 4 gauge wire, and got the diversion load water heater element wired in. No testing yet, 2 full days of crawling around on the floor and ground and I am done working for the weekend! I'll do some testing some evening this week or next weekend.

[ewarnerusa]

I did some "bench" testing on the heating element today. It pulled 24 amps and got nice and hot. I had the element in a mug of water and it heated it up quickly.

[ewarnerusa]

Check this thing out. 300 watt 12V element that is swappable with the hot water tank drain plug. It dry fit well in my tank. Hopefully with thread tape it seals up perfect. The white plug is one of my spares.

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[pianotuna]

I too just used the inverter, but powered it from twin #8 wire from the chassis battery and solar.

I'd disconnect the charging path when the starting battery reached 12.3 volts (which took about 20 minutes), and then reconnect 40 minutes later, when the chassis battery had recovered.

[Rice]

ewarnerusa wrote:
To me, the cool part about the diversion load idea is that 100% of solar harvest capacity is used throughout the entire solar day.

Our setup is significantly different from yours, but you might be interested in what a fellow traveler did to use excess solar to make hot water. We have 1,050 watts of panels and six 6-volt golf cart batteries. Not huge by today's standards, but it was considered a lot of solar when we installed it in 2005. And since we still have a Norcold and not a residential refrigerator, it's still enough.

Our motorhome came with a gas/electric water heater that was installed on a non-inverter circuit. We moved it to the inverter circuit, so it would work on the batteries.

(And actually, we tested everything before moving the water heater breaker by necking down the shore power cord to a 15-amp plug, and plugging it into one of the 15-amp plugs on the coach. That meant the water heater "saw" shore power and would work, but the shore power was actually coming from the batteries. It looked very strange, having the shore power cord go to an outlet on the side of the coach (I can only imagine what our neighbors thought), but was easier than moving the breaker only to find out the whole plan was bogus.)

The basic idea was to rewire the AC power to the water heater through a solid state relay that comes ON when the battery voltage is at float level, and turns OFF when it drops below that, which happens almost instantaneously once the water heater begins to draw current. So the water heater is on for just a fraction of a second, and the battery voltage recovers just as quickly, and then it does the ON/OFF thing again.

This happens dozens, even hundreds, of times a second. The fraction of time ON versus OFF tracks the amount of incoming solar power available constantly, via a process called pulse width modulation ("PWM"). And since the water heater is a simple resistive load, it doesn't mind being rapidly pulsed like that.

Best of all, we put in a little LED light that flashes when the water heater is going on and off, and you'll probably appreciate how much fun it is to watch that light flash like mad. And it really does heat the water, to the point that we've boondocked for weeks at a time not running the generator and not using any propane for the water heater, taking daily showers.

We currently have a Midnite Classic controller, which has an AUX output and a high-speed FLOAT-tracking PWM mode that works great for this purpose, by connecting the controller's AUX output to the high-current solid state relay that switches the water heater's AC on or off.

We originally had an Outback MX-60 controller. We did get this scheme to work with that controller, but it was kind of tricky and took some monitoring that's not needed with the Midnight, which is basically "set and forget." However, with the Midnight's "set and forget," it does it only when the batteries are in float mode, and not when they're in absorb, when there's also excess solar that could be used.

So with the current controller, we're not using 100% of the possible solar, because it doesn't get diverted while the batteries are in absorb stage. Wringing out every electron with the old controller had its charms, but really, this is all just icing on the cake, so I've decided I'm okay with sacrificing some output.

[ewarnerusa]

Cptnvideo wrote:
"IMO, in order to actually use excess solar capacity, add battery storage capacity so the excess power can be used when needed."

Exactly. That is the correct thing to do.

I completely agree that the idea is to save the energy for later rather than "waste" it. Unfortunately I have no more room on the trailer tongue for more batteries. I don't want to relocate batteries inside as that would require moving to a different battery technology than my flooded lead acid GC batteries. A full battery technology swap is a very expensive alternative. Adding more batteries also requires that I have more solar than needed for my current battery capacity and that I could properly charge more batteries if I had them. Or that I need more of both batteries and solar for my needs. Neither of those situations are true, although admittedly I am thinking of a bit more solar as I explore remote work possibilities.

A heating element adds heat energy to water that I want heated, so it is a way of saving the energy for later as well. All of the daily energy not used for charging and other loads is still being saved in a heat reservoir vs a battery chemistry reservoir. I am disregarding the insulation of the hot water tank and other variables that allow for that heat energy to escape.

To me, the cool part about the diversion load idea is that 100% of solar harvest capacity is used throughout the entire solar day. If a solar charging system is designed to be able to fully charge your batteries, then for a good chunk of time your charge controller is going to be tapering the amps to the batteries. Whether you're using 280 watts of solar into a 200 Ah battery bank or 1600 watts of solar into a 1000 Ah battery bank, at some point the CC begins to taper amps. A heating element diversion load changes this tapering from on/off (use/waste) of the solar input to splitting it between charging & loads and water heater. 100% use, zero waste.

[StirCrazy]

Cptnvideo wrote:
"IMO, in order to actually use excess solar capacity, add battery storage capacity so the excess power can be used when needed."

Exactly. That is the correct thing to do.

Not realy though, that only works if you also increase your daily usage.. for instance if I say doubled my 280AH on my camper for 560AH all it would do is let me go 18 days with out the solar producing at all instead of 9 days, the chances of that happening are close enough to 0 that it would just be a waist of money and equipemt. extra weight and so on. you have to decide how much reserve you need and if you don't have enough this would be a good idea but once it is topped up your in the same boat, so unless you plan to use that exra capacity its not realy that good of an idea.

[StirCrazy]

Grit dog wrote:
^You’re still barking up the wrong tree. No, 1 280W panel won’t power a space heater either.
At best 100% solar power being absorbed, it will kick out 8-9amps at 12v +-
Good luck. I know you so want to “make this work” but the practicality of it is of seriously diminishing returns.

I have a 325 watt panel (24V split cell) and I get 22amps out of it for at least 4 hours and it climbs and drops before and after. the thing about electric resistant elements any power put into them will have an effect, sure it might only be warm early but if you size it right, in my case a 300 watt 12V heater it will fully power it. in his case I would do a 200 watt element weather it is for the water or the space heater.

Steve

[StirCrazy]

ewarnerusa wrote:

StirCrazy wrote:

ewarnerusa wrote:

Chum lee wrote:
As previously hinted at, if you have a 2/3 way absorption type fridge, why not divert the excess power generated to that and save some propane? Clearly, if you have a 2 way fridge, you'll need an efficient inverter capable of generating at least +-500 watts on a duty cycle of at least 50%. Just keeping your 1500 watt PSW inverter online full time may be enough to waste enough excess power to solve your issue doing nothing else. IMO, 280 watts of solar wont be enough to run the fridge full time, but hey, it's a start. You'll be spending a lot of time doing load management.

Chum lee

A different take on this idea^^ I wonder if instead of a 12V water heater element I could explore a refrigerator 12V element? Is there harm in adding variable/inconsistent heat (less than the OEM 300 watt AC power element) to the ammonia in an absorption fridge? This is just a brainstorm, I definitely don't want to swap out the AC side of the fridge.

EDIT: I clearly don't know exactly how the ammonia is heated in an absorption fridge, just that this is part of the process. But first google search showed a promising success for a similar idea. This is a Dometic fridge, mine is Norcold. https://www.escapeforum.org/forums/f8/adding-12v-heating-element-to-rm2510-fridge-16224.html

my 3 way on 12V is 12 amps. thats a lot of power to be chucking out for the return on propane savings.. I can go probably between 1 and 2 months on just my fridge with a 20lb bottle, but as soon as your solar stops producing over the amprage for the 12v heater then you have to turn your fridge back to propane or you start eating battery capacity. Idealy you want somthing that you don't have to interact with in my opinion. the hot water element would be great, in my case the space heater would be good but in the summer you would have to be able to turn it off and you would still be not using all your potential when its hot... maybe a combanation of the water heater and space heater so you can use which ever one you want.

Steve

Thank you for this. Do you think both a water heater element and a space heater could be paralleled on the diversion load circuit each with an on/off switch?

run one output to a bus bar and then run each one with a switch off the bus bar or ya you can do it with out the bus bars by just splicing the wires to two switches but the bus bars will make a cleaner install and there cheep.

[StirCrazy]

ewarnerusa wrote:
Then we have the kind of online gambling shopping when trying something from Ali Express. The 1/2" NPT size means it could swap with my 1/2" NPT thread drain plug, right? This would be ideal, maintaining my OEM 120V AC 1400 watt heating capacity.

Dernord Immersion Cartridge Heater Hot Rod Heating Element Replacement with 1/2"NPT Thread 12V 24V 1...

that should work if thats the size of your plug.

Steve

[pianotuna]

Unfortunately batteries are quite expensive. I started out with 875 amp-hours.