Decreasing amp draw of an electric element

[AZJoe]

I'm having problems trying to run a small electric watet heater with a 2000 watt pure sine wave inverter. The inverter is supplied by six 6v golf cart batteries wired in series parallel. If I switch over from shore power to run the load off of the inverter I have voltage drop and after about 45 seconds the voltage drops below 12v and the inverter powers down. The batteries are rated for a 75amp draw (individually and I'm doubting they stack). I know that I'm trying to draw too many amps in a short period of time.
The water heater is a 120vac unit with a 1440watt heating element that is proprietary and can not be switched out with a lower wattage unit. The water heater is set up on a shore/off/inverter switch to toggle between running off of the ambilical power or off of battery backup.
My original solution was to install a beefy enough dimmer switch intended for indoor grow houses and I was not thrilled by the idea of the switch overheating. After I ruled that out, I decided after talking it over with my brother with HVAC/electrician/engineering background to opt for an infinite switch like you would find on an electric stove top rated for the maximum draw of the water heater. I will make a custom enclosure and wire it between the inverter and the power source selector switch. It should limit down how many amps are drawn by the wafter heater down to about half (or the best settling that makes the batteries and inverter happy) and when I switch to a regulator plugin, I get the full 1440watts.

[PNPmacnab]

You could use a transformer. Half the voltage would be 1/4 the power. Your element is 10-11 ohms. A common 24V 250W buck transformer would reduce the voltage to 100V or 1,000W to your element which the inverter might like more.

[Sunking]

I'm having problems trying to run a small electric watet heater with a 2000 watt pure sine wave inverter. The inverter is supplied by six 6v golf cart batteries wired in series parallel. If I switch over from shore power to run the load off of the inverter I have voltage drop and after about 45 seconds the voltage drops below 12v and the inverter powers down. .

So what is the problem? That is what you asked for using 12 volt toys trying to do real work. There are no 6-volt batteries that can supply 75 amps for any meaningful amount of time without excessive voltage sag. It would have to be a 6-volt, 250 pound. 750 AH battery.

Couple that with undersized wire that is too long and poor connections and you planned for failure. 12 volt systems should be no larger than 500 watt anything, You are asking a 12 volt toy to do the work of a 48 volt system.

Here is what will work and what you should have done to start with. Buy 2 more 6-volt gold cart batteries for a total of 8 wired in series to give you 48 volts. It will require a minimum 2000 watt solar panel, 40 amp charge controller, and a 2000 watt 48 volt Inverter.

There is nothing wrong with your batteries other than the way they are being used. You are trapped inside a 12-volt toy box. Your idea of reducing voltage is NOT GOING TO WORK.

[jflorey2]

The water heater is a 120vac unit with a 1440watt heating element that is proprietary and can not be switched out with a lower wattage unit. The water heater is set up on a shore/off/inverter switch to toggle between running off of the ambilical power or off of battery backup.

You don't need AC for such an element. Consider a boost converter that will boost 12V up to (say) 60V. That would give you 1/4 the wattage (and 1/4 the draw from the battery.) You could also run it on 12V but then you'd get 14 watts, which I assume is too low for your purposes.

Arrange a relay so that when shore power is available it switches back to AC.

[AZJoe]

I don't have enough room to add two more batteries and redo my entire system (everything else works great), so I decided to order an Atwood combination propane and electric 6 gallon water heater and replace the electric element with a 1000 watt version on place of the 1500 watt. I can use the electric more during the day or when I'm plugged in and gas at night if the 1000 watt is still too much for my system.

[NukeEngineer]

Just use a standard 240Vac water heater and feed it 120v. These typically have 4500 watt elements. 4500/240 = 18.75A, 240/18.75 = 12.8 ohms per element, and they only run one at a time.

With 120v feed, you'd have 120/12.8 = 9.375 amps, times 120v = 1125 watts. Problem solved

[PNPmacnab]

OR.........
Just use a standard 2,000W 120Vac water heater and feed it from the 36V PV string. These are 7.6 ohm element. When panels are operated at power point, I get all the hot water I want from just excess PV and a 10 gal tank. Your results may vary.

[bcroe]

OR.........
Just use a standard 2,000W 120Vac water heater and feed it from the 36V PV string. These are 7.6 ohm element. When panels are operated at power point, I get all the hot water I want from just excess PV and a 10 gal tank. Your results may vary.

Keep in mind, the thermostat switches in AC water heaters are not intended for breaking
DC current. Generally their voltage rating would need to be drastically reduced. Bruce Roe

[Sunking]

I have never heard such poor advice anywhere.

[sensij]

I have never heard such poor advice anywhere.

If I try to turn this comment into a productive contribution, it is probably to point out that while reducing the voltage can address the *power* problem, water heating also has an *energy* problem to solve too. Reducing the current draw by reducing the supply voltage will reduce the voltage sag and peukert loss, but has the OP attempted to calculate the how much energy is needed to heat the required volume of water, or quantify the heat loss of the system? A system that is so mis-matched in power requirements is probably going to be mis-matched in energy requirements, too.

[Sunking]

Senji the issue is there is no fix for the problem. You cannot fix an undersized system. If Sag and Peukert is causing problems, that is a design issue and the only way to fix it is start over. Sure you could use a transformer to cut the power by 75%. All that means is it takes 4 times longer to heat the water. In the end you use just as much energy and when done either way the battery is drained. Slowing down the process is most likely going to make it inefficient meaning more energy required and bigger problems. .

I am not even going to touch the compatibility issues with lower voltage and electrical control circuits/switches. If you are so concerned tell the OP the TRUTH for a change.

[jflorey2]

Senji the issue is there is no fix for the problem. You cannot fix an undersized system.

And you don't know the system is undersized. He may have a very limited need for hot water when operating away from shore power, in which case energy requirements go way down. And if he can solve the power problem, he's solved his problem.

If you are so concerned tell the OP the TRUTH for a change.

I have a feeling the OP is just going to do it and get something that works for him no matter what sort of names you call him. Which is a good outcome.

[AzRoute66]

I have a good feel about it. The OP knew what the problem was, overdrawing the battery bank, and decided to solve that by buying a propane/electric combo water heater, and to dump his excess daytime solar watts into a heating element half the size of his previous troublesome one. What's so terrible? The worst that can happen is he still trips out with a 1000W element and decides to put his spare daytime watts into something different while taking a nice proprane enabled hot shower.

[AZJoe]

I have a good feel about it. The OP knew what the problem was, overdrawing the battery bank, and decided to solve that by buying a propane/electric combo water heater, and to dump his excess daytime solar watts into a heating element half the size of his previous troublesome one. What's so terrible? The worst that can happen is he still trips out with a 1000W element and decides to put his spare daytime watts into something different while taking a nice proprane enabled hot shower.

Thank you for actually reading my post. I'm happy with my solar project even if it'still not perfect, it works for the space constraints I had and will do just about everything I need. The new 6 gallon water heater replaces my Zodi setup and sloves my probless. Since you can use both gas and electric together, it should take stress off of an electric only system.

[Sunking]

And you don't know the system is undersized.

I am absolutely certain his battery is undersized and his panel wattage. No question about it. If you had read what he says you would know it to. Either that or you do not know what you are doing. What is so hard to understand?

12-volt 2000 watt rectifier requires a minimum 12 volt @ 1600 AH, 1100 pound battery with short 4/0 Copper cable between the battery and Inverter. Otherwise when he tries to draw 150 to 200 amps of current the voltage is going to sag and cause the inverter to trip off-line. That is if his undersized wire does not burn up first. That simple. To keep that battery charged up will require 1500 watts or more in panel wattage, and two very expensive 60-amp charge controllers. He is no where close to that.

That is why I asked what he is complaining about. It is doing exactly what he planned for it to do. Fail. FWIW a 12 volt 2000 watt Inverter is a ticking time bomb and you know it. I am a PE and like a doctor if you are going to smoke, all I can say is quit or you will die,. Get out of my office, I cannot help you unless you quit what you are doing.