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Old 04-10-2015, 07:50 PM   #1
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12v charge level dependant switch for 120v outlet.

I would like to be able to run my 120v water heater off of my solar panels with a switch that can detect the charge level of the batteries and turn on when the charge level of the batteries is up to a programable or pre-set point and off when at a certain point. Does such a thing exist? If not why not? Seems like a good idea. That way i could heat the water when off on my bike without stressing the battery. It would turn on when there was power from the sun and off when there wasn't. It could either turn on the inverter or just be plugged into the 120 outlet.
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Old 04-10-2015, 10:25 PM   #2
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Re: 12v charge level dependant switch for 120v outlet.

Short answer is your solar panels won't put enough power to run an electric water heater.

Here's a typical RV water heater. It uses 1440 watts. Just figure out how many solar panels that would take. And don't forget that your inverter is probably only 80% efficient.
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Old 04-11-2015, 06:16 AM   #3
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Re: 12v charge level dependant switch for 120v outlet.

Yep, kind of like the idea of "why can't you pump up water to the top of a hill they let it flow to turn a generator so it could be pumped back up and use the spare electricity to run other stuff". The pumps draws more that what the generator can put out. Not that power companies don't do that but it's only to produce peak power.
However with a insulated hot water tank, solar could be used to offset the hot water cooling. I just think it's a waste of space for what you would get out of it. A solar water heater might be a better choice.
There is a reason why there are so many propane water heaters in the RV world.
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Old 04-11-2015, 07:10 AM   #4
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Re: 12v charge level dependant switch for 120v outlet.

Not so fast guys, I do not have time to post the detailed response but this is the very approach boywonder is looking at. Some mods like a 12v heating element and tank size need to be considered. The idea is to "dump" the excess solar not needed for the batteries, once they are charged, into the heating element for water. At least the solar PV is being used to bring the water temp above ambient to some degree.

Heating the water directly by the sun energy striking a black tank is more efficient than converting to electric, but it can still be done to some degree of not "wasting" sun energy.

Pretty sure others will jump in on this one.
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Old 04-11-2015, 12:13 PM   #5
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Re: 12v charge level dependant switch for 120v outlet.

After a day of riding my bike I use one of these:

Coleman 5-Gallon Solar Shower

Works fairly well, as long as I remember to lay it out before starting.
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Old 04-11-2015, 12:47 PM   #6
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Re: 12v charge level dependant switch for 120v outlet.

Some solar controllers have a connection that will output the excess power not needed to charge the batteries. The trouble is the output is 12 volts. This output is looking for something useful to do.
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Old 04-11-2015, 10:48 PM   #7
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Re: 12v charge level dependant switch for 120v outlet.

The answer may be in the perfect switch. They will make you a switch that can be pre-programmed to turn on and off at certain power thresholds. In other words it would be wired between the battery and the inverter. When the battery is up to say 13.8 v in my case it would allow power to the inverter which would then run the heater. When the battery went down to say 12.2v from the draw then it would shut off the switch and the panels would charge the batteries again until it hit 13.8. I suppose one would wire up some kind of manual switch bypass as well. Am I missing something? It doesn't seem that tricky.
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Old 04-11-2015, 11:06 PM   #8
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Re: 12v charge level dependant switch for 120v outlet.

By the way I have two 150w panels on the roof and two house batteries with the vehicle battery on a separator. With decent sun I can easilily heat my 700w 4 gal water heater. It's an Isotemp. I just have to manually monitor it and turn it off and on to ensure the barriers stay in the optimal charge zone. The switch should do it for me.
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Old 04-12-2015, 05:08 AM   #9
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Re: 12v charge level dependant switch for 120v outlet.

Quote:
Originally Posted by Madsadsalad
By the way I have two 150w panels on the roof and two house batteries with the vehicle battery on a separator. With decent sun I can easilily heat my 700w 4 gal water heater. It's an Isotemp. I just have to manually monitor it and turn it off and on to ensure the barriers stay in the optimal charge zone. The switch should do it for me.
I agree you can do that. It will depend on the tank and/or battery power plus charging to offset any losses. I mean I can run my microwave periodically throughout the day and still have enough battery reserves at night provided I don't go past a certain threshold. Myself I find I want hot water at times where my battery power is low and/or there isn't much sun producing solar energy. The reason I have two 135w solar panels instead of one isn't to run the microwave (it can't), it's to help supply battery charging in lower light conditions. Seems like an adjustable AC timer switch could work for you. But you should take into account that constant higher amp discharges and recharging cycles the batteries and shortens their lives. Just to have on demand hot water that you use now and then? I'd rather use a low amp approach to maintain heated water in a well insulated tank using excess solar to power keep up the temps while maintaining a plus 12.8 voltage on the battery bank. (I can't wait until lithium batteries come down in price)
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Old 04-12-2015, 08:00 AM   #10
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Re: 12v charge level dependant switch for 120v outlet.

At the bottom of this page are some voltage controlled switches. These should enable you do what you want. If you had a Battery Monitor that had relay outputs you could program that also. This would be something like the Xantrex LinkPRO Battery Monitor or VICTRON ENERGY BMV 70X PRECISION BATTERY MONITORs. Both of those have relays that you can set to voltage levels or even SOC. They would be more money than the voltage controlled switch.
http://www.solarseller.com/dc_photos...h_dc_relay.htm

Controlling the inverter, would depend on your inverter itself.

hope this helps.

greg
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