Forums General Discussion Hot water heating w/battery

This topic contains 3 replies, has 3 voices, and was last updated by  typhoontye 4 months, 4 weeks ago.

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  • #223621


    Hello all,

    Thought I would pass along a query I had for Nigel Calder and his response (The response is directly below; my question to him is below the response):

    You are not harming your batteries and we do the same! The discharge rate is only around 20% of the rated capacity of the batteries which will not bother them at all. The voltage drop is quite normal – any time you apply a relatively high load to a lead-acid battery it will pull the voltage down. Because you have deep cycle wet cell batteries you will have more of a voltage drop than if you were using, for example AGM batteries – it’s a function of the way the batteries are built. The key thing is to ensure the batteries do not get left in a discharged state for extended periods of time, and that they regularly get a full recharge cycle, which is obviously happening in your case. You will find they will use some water and so will need topping up monthly. You should use only distilled water.

    My wife and I often anchor our 42 ft sailboat for long periods while living aboard and rely almost exclusively on solar and wind to provide for most of our electrical needs. Our house battery bank consists of four golf cart equivalents (six volt, from West Marine) rated at 460 amp hours in total. We are really happy with our system and rarely need to augment our ‘renewables’ by running the engine driven alternator. We often make enough energy to have an excess, and have taken to making hot water through our a powerful house inverter that easily handles the heating element in our hot water heater. My question is, are we harming our batteries with the high amperage required to run the inverter and power the water heating element for 30-45 minutes at a time? We never discharge the battery beyond 50%, and usually 35-55% is more the norm. But the rapid discharge rate of around 80 amps concerns me, as does watching the battery bank voltage drop sometimes to 11.9 or 11.8 volts. The voltage comes back up to normal levels after the hot water heating element is turned off and we begin to see power going back into the batteries from our renewable charging sources. Logic tells me that we aren’t abusing the batteries with this practice but I’m not sure. Can you advise? Thank you!

    David Tye
    S/V Blue Moon
    Pearson 424

  • #223674


    This is something I’ve wanted to do.

    What hot water heater are you using?

    Pearson 424 cutter - "Effie"

    • #223675


      If you have a 6-gal water heater and want to raise the water temp from 60degF to 105degF, you’ll consume about 480WH. At 12.0V, that’s 40AH. Assuming a 1500W heating element, an inverter capable of producing 1500W, and a 90% efficiency, it would take about 20 minutes. Your batteries should be able to handle the load if you don’t otherwise have them heavily loaded. I’d be careful of overloading your inverter. Read your manual.

      I have a 15gal water heater. I’d need 100AH, which is a lot of power. My battery bank is 440AH. By practice, I don’t discharge my batteries below 50%. This would consume half my daily budget. I put a Honda inverter-generator onboard. I use it to make hot-water when needed. I start it up and leave the boat for an hour.


  • #223740


    Sorry Effie, just saw your query from Oct. To answer your question, ours is an Isotemp Basic, 6 gallon. I can generally get cool water plenty warm for two showers in about 40 minutes, burning about 70 to 80 amps, or around 45 amp hours. If the wind is up and the batteries are topped its a good way to use excess available power. If the renewables are just keeping up with normal demand, I can run the engine for a bit, making hot water with both the inverter and the heat exchanger function from engine coolant. Works pretty well, though I will often run the engine with a bit of load on it, in reverse, to bring the heat up.


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