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Having just completed my camper van conversion I ran an interesting test today heating hot water off of battery power. Battery is a nearly new 8D East Penn Deka AGM about 230 amp hours, inverter is a Xantrex sine wave 1000, water heater is an Isotemp 4 gallon with a 750 watt element. Battery is monitored by inverter which shows voltage and current, and I have a separate voltmeter on my DC switch panel. There was no battery charger running during test. Additional non-inverter load on the battery was a couple of amps for two Maxxair fans at half speed and 8 LED lights.

I had just filled water tank and heater and I guess the water temperature was about 70 degrees F. coming from the underground pipe. Very quickly after start the battery voltage dropped from full charge 12.8 to 11.9 and the DC current jumped around 79 to 81 amps all during the test. After 30 minutes the water was very hot and usable for any hot water purpose. After 42 minutes the heater thermostat was satisfied and shut off. Water was very, very hot at least 120 degrees F.

The battery instantly recovered to 12.3 and within 10 minutes recovered to 12.5. this seems about right as some 50 plus amp hours had been taken from the battery.
 

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Having just completed my camper van conversion I ran an interesting test today heating hot water off of battery power. Battery is a nearly new 8D East Penn Deka AGM about 230 amp hours, inverter is a Xantrex sine wave 1000, water heater is an Isotemp 4 gallon with a 750 watt element. Battery is monitored by inverter which shows voltage and current, and I have a separate voltmeter on my DC switch panel. There was no battery charger running during test. Additional non-inverter load on the battery was a couple of amps for two Maxxair fans at half speed and 8 LED lights.

I had just filled water tank and heater and I guess the water temperature was about 70 degrees F. coming from the underground pipe. Very quickly after start the battery voltage dropped from full charge 12.8 to 11.9 and the DC current jumped around 79 to 81 amps all during the test. After 30 minutes the water was very hot and usable for any hot water purpose. After 42 minutes the heater thermostat was satisfied and shut off. Water was very, very hot at least 120 degrees F.

The battery instantly recovered to 12.3 and within 10 minutes recovered to 12.5. this seems about right as some 50 plus amp hours had been taken from the battery.
Nice. Have we seen any of your build? I would love to.
 

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The battery instantly recovered to 12.3 and within 10 minutes recovered to 12.5. this seems about right as some 50 plus amp hours had been taken from the battery.
Can anyone explain why the battery recovers like this? I've noticed mine doing the same.
 

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The battery really hasn't recovered anything. What has happened is the transfer of ions in the electrolyte stops and the electrical potential of the battery rises as the electrolyte stabilizes. The battery has the exact same level of depletion and will need charging. Tis ability of batteries to "return" a voltage under no load is why battery testers put a load on the battery to sense its real voltage and state of charge. Even a nearly dead battery will read near 12 volts but put a small load on it and it may return 2 or 3 volts.
 

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Thanks. I've got a volt meter installed to track my house battery voltage, but it sounds like from what you're saying that that might not be the best way to measure battery state. What would be better?

Always learning something here, thanks all.
 

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Another thing to consider is that most batteries are not able to sustain such a heavy load without greatly reducing the life of the battery. Some of the newer Lithium based batteries can, but you'll likely destroy an AGM battery of that size by repeatedly placing such a heavy and sustained load on it.
 

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Discussion Starter · #8 ·
My inverter making hot water test was just that as I would not do it that way on a regular basis. Repeated use that way will shorten the battery life. When a battery has rested with no load for several hours a voltmeter reading can give an approximate state of charge although the capacity of the battery may be very reduced. A load test is best. SoC is just that, not a measure of capacity.

When shore power is available I heat hot water that way. Isotemp tanks are very well insulated and water stays warm for at least 12 hours.

My next test might be to try heating water via the inverter off the house battery when driving and the BlueSea voltage controlled relay has combined the batteries. This would put the ProMaster alternator pumping out big time amps so the batteries are not providing all the power. HAS ANYONE TRIED THIS? My connection from the ProMaster system to my combining relay is fused at only 80 amps and wire size is about 8 feet of number 6 so I am a bit fearful of doing such a test. Would the van provide all the 80 amps? I doubt it but it might provide say 50 amps so the draw from the house battery is only 30 amps. My number 6 wire is that size so that its resistance will take the edge off of the initial surge when the relay closes. Heavy current for more than a few seconds would cause heating.
 

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My inverter making hot water test was just that as I would not do it that way on a regular basis. Repeated use that way will shorten the battery life. When a battery has rested with no load for several hours a voltmeter reading can give an approximate state of charge although the capacity of the battery may be very reduced. A load test is best. SoC is just that, not a measure of capacity.

When shore power is available I heat hot water that way. Isotemp tanks are very well insulated and water stays warm for at least 12 hours.

My next test might be to try heating water via the inverter off the house battery when driving and the BlueSea voltage controlled relay has combined the batteries. This would put the ProMaster alternator pumping out big time amps so the batteries are not providing all the power. HAS ANYONE TRIED THIS? My connection from the ProMaster system to my combining relay is fused at only 80 amps and wire size is about 8 feet of number 6 so I am a bit fearful of doing such a test. Would the van provide all the 80 amps? I doubt it but it might provide say 50 amps so the draw from the house battery is only 30 amps. My number 6 wire is that size so that its resistance will take the edge off of the initial surge when the relay closes. Heavy current for more than a few seconds would cause heating.

Hi,
Quite a nice water heater -- looks like it can also be heated with engine coolant?

I've measured as high as 59 amps from the alternator to the house battery on startup in the morning when the house battery was down to about half charge. My batteries are 220 ah flooded lead acid golf cart (not AGM), so I'd think you would get more than this on your AGM with the lower internal resistance? Mine has a 50 amp breaker, which did not trip because the 59 amps settles down to about 40 amps within a minute.

You may have already looked these up, but just in case:
At 80 amps, your 8+8 ft of #6 woould have a voltage drop of 0.52 volts.
Handy calculator: http://www.bulkwire.com/wireresistance.asp So, as you say, this will lower the charge rate some.

#6 wire with 90C insulation has an ampacity of 75 amps -- pretty close to your 80 amp fuse and likely OK for your test, but I'd keep an eye on it.
http://www.cerrowire.com/ampacity-charts


If you are heating (say) 4 gallons water from 60F to 120F, it takes
(4 gal)(8.33lb/gal)(120F - 60F)(1 BTU/lb-F) = 2,000 BTU

2000 BTU is 0.6 KWH or about 48 amp-hr.
Maybe 10% more than this is if you allow for inverter inefficiency?
So, seems like on occasion using the battery to heat should be OK?
Maybe also keep an eye out for the battery heating up and for gassing even thou its an AGM.

Would like to hear how your test comes out.

Gary
 

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Discussion Starter · #10 ·
Thanks GaryBIS
I agree with everything you say. I have used Isotemp water heaters on boats for years and yes engine coolant can do the work. Because coolant can heat the water to almost 200 degrees F the heaters come standard with an adjustable mixing valve. I do not use the engine coolant heat on my set-up.

Yes, I use the number 6 wire to temper the initial surge so it is easier on the contacts of the combining relay. I have used the BlueSea voltage controlled relay in several applications and find them totally reliable. Your calculations about agree with mine but I will not have a chance to use the engine running heat via the inverter test for several days
 

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I have used Isotemp water heaters on boats for years and yes engine coolant can do the work. Because coolant can heat the water to almost 200 degrees F the heaters come standard with an adjustable mixing valve. I do not use the engine coolant heat on my set up.
I used Isotemp water heater on a previous diesel camper and 2 sailboats and I'm installing one in my gas Promaster. Have you figured out where to connect the Isotemp to the Promaster cooling system yet?
 

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I have no experience with such heaters but if there is an option to use the coolant to heat the water that is the Cats Meow!
Any chance a 12 volt heat coil is available at roughly half the wattage so the water gets hot but the amperage is reasonable say an 800 or 1000 watt one so the current would be 70 to 80 amps at 12 volts? Much easier on the battery and no inverter losses?
 

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Any chance a 12 volt heat coil is available at roughly half the wattage so the water gets hot but the amperage is reasonable say an 800 or 1000 watt one so the current would be 70 to 80 watts at 12 volts? Much easier on the battery and no inverter losses?

the 110v units are 750 watts.
 

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the 110v units are 750 watts.
All the better 60+ amps but probably 500 watts wood do it. What does that Isotherm cost? A diesel hydronic heater is probably $1000. Much better on the batteries.
 

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Discussion Starter · #15 · (Edited)
I do not plan on using engine coolant in the Isotemp as I do not want the complication or the risk of voiding the warranty if I ever have engine trouble.

Pricing of Isotemp heaters is available on the Internet.

I believe that essentially all small electric water heaters today are made in China. Getting replacement elements or changing elements to a different wattage may not be easy. Sizes may not be standard. It's not like the old days when you could go to the local store for a home water heater element and replace it yourself.

Virtually all water heaters available today, even small ones, have 1250 or 1500 watt elements. That is why I like Isotemp, the only small one I could find with a 750 watt element. What's not to like? Heats 4 gallons in around 45 minutes.
 

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Merely warm water is a health hazard - need to boost & hold 60ºC/140ºF to snub Legionaries etc., and when traveling getting water from varied sources the tank can build up it's own personality pretty quick. There are many heaters w/ many different elements - Isotemp does not offer one in 12 or 24 volts but the threaded lug for the stock element may be a standardized size to accept a 12V resistive element - google 12v water heater element, 100, 200, 300w and up units are available. Also - it'd best be treated like a 2nd or 3rd battery - have a charge minder allow current to it once the engine battery is satisfied.
 

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Discussion Starter · #17 ·
Water below 140 degrees a health hazard? In my part of the country, and I think in many parts, home water heaters come standard pre-set at 120 degrees to save energy. Building codes in some areas require that to prevent scalding. I agree that water from tanks can be bad. Municipal water usually contains some chlorine and I have a habit of adding a few drops from time to time. For drinking, cooking, making coffee I use bottled water. Tank made hot water is for washing dishes and self.
 

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Zoomyn, what about RV's in a climate where the water in the tanks are clearly in the Legionellae growth range temperature? You may heat up the hot water tank to kill what's in there but you are mixing with the "cold" supply to get your ideal shower temperature.
 
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