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:
I have never run the element dry. There were instances where I had it in a cup of water when on and a bit of water evaporated. But element was always >90% submerged I would say.

Multimeter ohm reading on the element is 1.5 ohms.

What is the math for how you came up with what the resistance should be? I only took one circuits course in college and I remember
V = I * R
and
P = V * I = I^2 * R

But with R = 1.5, if I saw 21A from panels at one point and assume 19A was going to element, I was pushing 19^2 * 1.5 = 542 watts. Way over rating. ****, do I have too much solar for this?! Ironic...

Another element was only $15. I'll take a resistance reading on it before doing anything and see what it says. But it will be weeks before it makes it across the sea to my door.

1.5 Ohm doesn't really make since for a 300-watt element @12V.

at 12V with that resistance it will be using 8amps (which confirms your reading) but only consuming 98 watts. Remember with a resistive element you only control the voltage it will set the amperage due to its resistance. I'm starting to think you got a defective heater, or overpowering it (feeding it over 12V) has toasted it.

[ewarnerusa]

Manual has quite a bit of language about minimum and maximum diversion load elements, but I thought I was within spec. Now I'm not so sure.

https://www.morningstarcorp.com/wp-content/uploads/technical-doc-diversion-manual-en.pdf

[ewarnerusa]

I have never run the element dry. There were instances where I had it in a cup of water when on and a bit of water evaporated. But element was always >90% submerged I would say.

Multimeter ohm reading on the element is 1.5 ohms.

What is the math for how you came up with what the resistance should be? I only took one circuits course in college and I remember
V = I * R
and
P = V * I = I^2 * R

But with R = 1.5, if I saw 21A from panels at one point and assume 19A was going to element, I was pushing 19^2 * 1.5 = 542 watts. Way over rating. ****, do I have too much solar for this?! Ironic...

Another element was only $15. I'll take a resistance reading on it before doing anything and see what it says. But it will be weeks before it makes it across the sea to my door.

[StirCrazy]

ewarnerusa wrote:

StirCrazy wrote:
...so your hot rod is rated for 12V and I didn't see a +/- range on the website. how much voltage were your solar panels sending to the heater?...

I have also wondered about the voltage at the diversion load. When I connect to the SCC via laptop, the software provides me a diversion load current measurement as well as a load voltage. The current value is consistent with what I expect based on the other meters I have (one on solar array output, another bi-directional one on primary battery connection). But the load voltage confuses me as it is fluctuating at levels well below 12V and I wonder if it is not what I think it is? I have not taken a multimeter to leads, although now I have some wire exposure to take a measurement from!

During my test run, the element wiring began to feel warm as soon as diversion current began to pass to it. And it began to "cook" when there was <10A of current. This seems like well below any scenario where there could be more than 300 watts of power. The panels' open circuit voltages are 22.4 and 24.3 (2 different types of panels), so it seems to me that there isn't a V*I scenario that exceeds 300 watts.

In diversion load configuration, the wiring is for the panels to be directly connected to the batteries rather than to the SCC. The SCC leads that were formerly array input become diversion load output. A diversion load configuration is applied and DIP switches must be changed on the SCC.

you need to do an Ohm measurement on the element its self. if it is a 300watt element at 12V that means it should be a 0.48 ish Ohm element. well, if you feed 17V into a 0.48 Ohm element you get 602 watts, but that doesn't explain your current. for giggles do a Ohm reading on just the element and see what it comes back as. maybe it is a defective element or mislabeled. I am assuming you never ran it dry for even a few seconds...

[ewarnerusa]

StirCrazy wrote:
...so your hot rod is rated for 12V and I didn't see a +/- range on the website. how much voltage were your solar panels sending to the heater?...

I have also wondered about the voltage at the diversion load. When I connect to the SCC via laptop, the software provides me a diversion load current measurement as well as a load voltage. The current value is consistent with what I expect based on the other meters I have (one on solar array output, another bi-directional one on primary battery connection). But the load voltage confuses me as it is fluctuating at levels well below 12V and I wonder if it is not what I think it is? I have not taken a multimeter to leads, although now I have some wire exposure to take a measurement from!

During my test run, the element wiring began to feel warm as soon as diversion current began to pass to it. And it began to "cook" when there was <10A of current. This seems like well below any scenario where there could be more than 300 watts of power. The panels' open circuit voltages are 22.4 and 24.3 (2 different types of panels), so it seems to me that there isn't a V*I scenario that exceeds 300 watts.

In diversion load configuration, the wiring is for the panels to be directly connected to the batteries rather than to the SCC. The SCC leads that were formerly array input become diversion load output. A diversion load configuration is applied and DIP switches must be changed on the SCC.

[pianotuna]

The power increases by the voltage squared. For example:


10 x 10 = 100

14 x 14 = 196

That's why, in the dead of winter I use my autoformer--I want all the heat I can get out of my electric heaters.

[StirCrazy]

ewarnerusa wrote:
Yes, the element says "12V 300W 22-12"
The insulation on the unit I have was white. I'll repost a pic from a page or 2 before this one. To show the writing and a better look at what the insulation looked like before it was cooked.

I have a theory, I may be wrong but its something to consider. I use 240 volt heating elements in my "water distiller" and the power is regulated and always at 240V so no issues. If I put a 120V element in there it makes so much more heat than it is rated for it destroys its self. (Had a 120V element that wasn't marked in my shelf)

so your hot rod is rated for 12V and I didn't see a +/- range on the website. how much voltage were your solar panels sending to the heater?

for resistive elements if you double the power you increase the wattage by 4x so if you have 12V rod that has a 300watt rating that will be 25amps flowing through it at max power, now let's say you feed 14.6V to that same element, which will take it to 30 amps and 456 watts, enough to cook your wires and wreck the heater eventually.
I am not sure what the output is on the diversion terminals if its panel voltage, it could be higher yet.

You could get a 24V heater but you will lose power putting it under 24V to it but you will not overheat it.

[ewarnerusa]

Yes, the element says "12V 300W 22-12"
The insulation on the unit I have was white. I'll repost a pic from a page or 2 before this one. To show the writing and a better look at what the insulation looked like before it was cooked.






The elements from the same supplier now look like this

[StirCrazy]

ewarnerusa wrote:
Success and failure today in a test run in the driveway. Nice clear sunny spring day.

I had spring fever so I dewinterized last night. Rinsed and filled fresh water system. It got down to 45F overnight and I'm assuming water started at that ambient temp.

Batteries were in 14.5V absorption by mid morning. All seemed well, but the wires got quite hot where they terminate at the element. This was with <10A current from the panels, well below the 300 watt rating. I confirmed that the element was not drawing from the battery.
I noticed the warm wires during bench testing, too. Wires feel progressively cooler the further from the element. The 8 gauge wiring I put in for supplying the current did not get warm. The heat shrinking around where the wiring terminates at the element started giving off smoke. I kept an eye on it and after 15 min or so it stopped smoking. What was once rubbery is now brittle.


I saw a max of 21A from the panels. I didn't have the laptop running at this point to see what the SCC said the diversion current was. I'm kicking myself for not putting my clamp meter on the wire at the element, but my hunch is around 19A. By early evening the water was hot! I measured 117F out of the kitchen tap.

But unfortunately the wiring looks cooked and I don't really trust it now. It needs some kind of high temperature heat shrink reapplied first. I just broke away the brittle stuff that was formerly the heat shrink and this is what is beneath it.

Did you pull the element out and see what the coating looks like? if could be they just used the wrong wire insulation on a cheap build but I am almost wondering if they sent you the wrong element. is there a voltage/power marking stamped on it anywhere?

[ewarnerusa]

pianotuna wrote:
Hi temp:

https://www.amazon.ca/010832-Black-High-Temperature-Shrink-Tube/dp/B001HYQQ9W?th=1

Thick wall construction and remains flexible

Operating temperature range: -67°F to 275°F; apply at 120 to 180°F using heat gun

Thanks, I'll try that stuff out. The heat shrink costs about the same as another element, and the pics of the elements suggest a different type of heat shrink/insulation at where the wire terminates at the element. Maybe I got old-tech before and now they're using new tech? :h Only $15 and a month waiting for shipment to find out.

Anyone experienced with heating element wiring? Does that wire connection look normal and is it normal for the wiring to get so warm as well?

[pianotuna]

Hi temp:

https://www.amazon.ca/010832-Black-High-Temperature-Shrink-Tube/dp/B001HYQQ9W?th=1

Thick wall construction and remains flexible

Operating temperature range: -67°F to 275°F; apply at 120 to 180°F using heat gun

[ewarnerusa]

Success and failure today in a test run in the driveway. Nice clear sunny spring day.

I had spring fever so I dewinterized last night. Rinsed and filled fresh water system. It got down to 45F overnight and I'm assuming water started at that ambient temp.

Batteries were in 14.5V absorption by mid morning. All seemed well, but the wires got quite hot where they terminate at the element. This was with <10A current from the panels, well below the 300 watt rating. I confirmed that the element was not drawing from the battery.
I noticed the warm wires during bench testing, too. Wires feel progressively cooler the further from the element. The 8 gauge wiring I put in for supplying the current did not get warm. The heat shrinking around where the wiring terminates at the element started giving off smoke. I kept an eye on it and after 15 min or so it stopped smoking. What was once rubbery is now brittle.


I saw a max of 21A from the panels. I didn't have the laptop running at this point to see what the SCC said the diversion current was. I'm kicking myself for not putting my clamp meter on the wire at the element, but my hunch is around 19A. By early evening the water was hot! I measured 117F out of the kitchen tap.

But unfortunately the wiring looks cooked and I don't really trust it now. It needs some kind of high temperature heat shrink reapplied first. I just broke away the brittle stuff that was formerly the heat shrink and this is what is beneath it.

[StirCrazy]

ewarnerusa wrote:
StirCrazy,
It has definitely crossed my mind from the start of this little project. I admit that I have been hoping that the 6 gallons of water as a heat sink will never be able to reach the blow off valve setpoint with this diversion load. Assuming I am leaving the propane water heater burner off. But if the diversion load gets disconnected, then I have all my panels directly wired to the batteries with no control. Which scenario is worse?

I also don't know why my sig won't show up? The box is checked.

Ya if Pianotuna's figures are accurate, I would add a temperature controlled switch to it as what I think you could do is warm up your water initially with propane or even just the solar if you're not using the water and it would come up to full temp (might take a bit longer to get to temp)

but if he is calculating 350 watts loss over 4 hours then you have enough power, potentially to increase the temperature significantly.

I'll do some snooping around tonight and see if I can find something cheap that would work. might be as simple as a 12V switch that has a temp pad you put on the bare tank itself.

[pianotuna]

ewarnerusa,

These figures might help a bit.

The ten gallon unit in my rv takes 90 minutes before it cycles the first time.

There after, if no water is drawn, it will cycle on for 15 minutes every 4 hours.

It has a 1400 watt electric heater built in.

From that I can infer that heat loss is about 350 watts over the 4 hour time frame.

[ewarnerusa]

StirCrazy,
It has definitely crossed my mind from the start of this little project. I admit that I have been hoping that the 6 gallons of water as a heat sink will never be able to reach the blow off valve setpoint with this diversion load. Assuming I am leaving the propane water heater burner off. But if the diversion load gets disconnected, then I have all my panels directly wired to the batteries with no control. Which scenario is worse?

I also don't know why my sig won't show up? The box is checked.