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Old 27-06-2014, 07:24   #16
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Re: A quick question re: fast battery discharge rate with solar

RV's usually have a dual power water heater, main is of course propane, but some have a couple of hundred watt element too. Takes a long time to heat water of course, but the total current draw is the same, just less wattage over a longer time, since I can't run my water heater and both AC's, I've been wondering if lower wattage elements existed for Marine water heaters of if there was a common thread size?
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Old 27-06-2014, 08:22   #17
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Re: A quick question re: fast battery discharge rate with solar

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I've been wondering if lower wattage elements existed for Marine water heaters of if there was a common thread size?
Thread size seems to be common around here.

Since you are on 110V, maybe you can simply swap a 230V heater element in? A 2000W element designed for 230V will be ~500W when powered from 110V.
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Old 27-06-2014, 08:27   #18
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Mrm. Don't you mean a 1000w element at 220 would be 500 at 110
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Old 27-06-2014, 09:53   #19
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Re: A quick question re: fast battery discharge rate with solar

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Mrm. Don't you mean a 1000w element at 220 would be 500 at 110
Nope,
Ohm's Law:

I = V/R


Power:

P = V * I

P = V*V / R


Resistance of the heater element will be approximately the same (ignoring differences due to temperature coefficient) so, for a given resistor (for a fixed resistive load), power is proportional to voltage squared.
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Old 27-06-2014, 10:54   #20
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K thanks for doing the math
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Old 27-06-2014, 11:03   #21
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Re: A quick question re: fast battery discharge rate with solar

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Hi
A quick question.
We have 1120W solar with max output of 85A through a victron MPPT charge controller.
I would like to plug my hot water boiler (1200W) in to the socket which is powered through the 1600W inverter
My concern is discharging the batteries too fast at around 100A
We have 4 / 140Ah flooded batteries.
It takes around 25 mins to heat the water.
So my question is, if the solar is putting in 50A, and I am taking out 100A, is that just a 50- 70A draw as far as the batteries are concerned and thus not a problem?


Thanks
Monte

Enough of all the theory answers. Did you try it yet and if so how did it work?
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Old 27-06-2014, 11:06   #22
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Re: A quick question re: fast battery discharge rate with solar

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Thread size seems to be common around here.

Since you are on 110V, maybe you can simply swap a 230V heater element in? A 2000W element designed for 230V will be ~500W when powered from 110V.
Good point, thanks
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Old 27-06-2014, 11:23   #23
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Re: A quick question re: fast battery discharge rate with solar

You are free to use the power you generate any way you like. Run some test and measure. You'll soon realise whether your system will cope.

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Old 27-06-2014, 15:40   #24
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Re: A quick question re: fast battery discharge rate with solar

Thanks and Yes I tried it. It works as expected but it is interesting to note the answers here. Interesting idea with changing the element. That would halve the amps, double the time to heat but effectively the solar would do all the work, leaving the batteries at 100%. Great idea!
résistance 500w-220v chauffe-eaux quick nautic boiler - accessoires de nautique - bueni.fr
There are also duel elements available, one from the 220V, one from 12V which would be useful as well.
The battery showed 93% charge after about 30 mins to heat the hot water. I had forgotten that the last 15% of charging can take a while with lead acid batteries so thanks for the reminder.
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Old 27-06-2014, 17:27   #25
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Re: A quick question re: fast battery discharge rate with solar.

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How many panels is 1120 W?
It might be 4 x 280w panels.
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Old 27-06-2014, 17:32   #26
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Re: A quick question re: fast battery discharge rate with solar

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Originally Posted by monte View Post
Thanks and Yes I tried it. It works as expected but it is interesting to note the answers here. Interesting idea with changing the element. That would halve the amps, double the time to heat but effectively the solar would do all the work, leaving the batteries at 100%. Great idea!
résistance 500w-220v chauffe-eaux quick nautic boiler - accessoires de nautique - bueni.fr
There are also duel elements available, one from the 220V, one from 12V which would be useful as well.
The battery showed 93% charge after about 30 mins to heat the hot water. I had forgotten that the last 15% of charging can take a while with lead acid batteries so thanks for the reminder.
I don't own a boat, but I've been thinking about getting a catamaran. I've been toying with the idea of using a large, dark water bladder on the salon roof as a large solar heated water source while at anchor. If it works, free hot water, not to mention an extra 50 - 60 gal of water on board.

This is in addition to a fairly large array of solar panels.
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Old 27-06-2014, 17:42   #27
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The problem is it will grow algae in it. that's why most people use 6 gallon sunshowers they're also clear and in cool weather being clear on one side makes a huge difference. I usually replace mine once a year in the fall because of algae growth in it, and the plastic tends to break down. I put an inline strainer in that needs to be cleaned about once a month
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Old 27-06-2014, 17:56   #28
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Re: A Quick Question Re: Fast Battery Discharge Rate With Solar.

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The problem is it will grow algae in it. that's why most people use 6 gallon sunshowers they're also clear and in cool weather being clear on one side makes a huge difference. I usually replace mine once a year in the fall because of algae growth in it, and the plastic tends to break down. I put an inline strainer in that needs to be cleaned about once a month
Ah, that's a good point! Hadn't thought of that!
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Old 27-06-2014, 19:15   #29
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Re: A quick question re: fast battery discharge rate with solar.

Direct solar heating is close to 100% efficient. Going the electric route is maybe 10% efficient, by the time all of the conversion loss is taken into effect? or less?

Our last house had both solar panels, and solar hot water. The hot water system was a much better use of roof space and $$$ then the electric panels.

Chris

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No offense meant, but that is just so wrong.
Converting solar energy to 12V DC to convert to 120V AC to heat water is a pretty darn inefficient way of doing things
Using solar to heat water may be a better way to accomplish what your wanting to do, it's not really a boiler is it?
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Old 28-06-2014, 09:08   #30
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OP its just looking for a way to put excess electricity to use. I personally like to cook with it because I can use exactly what I have in excess. And my solar shower works great. And I'm not Just saving the cost of propane but the hassel of having my tank filled
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