Paladen56Paladen56 Registered Users Posts: 1
I am trying to figure out how to use my extra voltage during the day my batteries charge up and I have a couple of hours that I use that power on. Of course I could by more batteries but I have enough and they are so expensive. So I thought if I had a small hot water tank and changed out the elements to DC I could at least put that voltage to work. I have am mppt charger but I don't really know how to make a dump load . The power needs to divert from the batteries to the small water tank. Thanks for any help!


  • Marc KurthMarc Kurth Solar Expert Posts: 835 ✭✭✭✭
    edited October 2019 #2
    How are you determining that your battery bank is reaching 100%?  My first reaction is let your batteries have some extra float time.
    I always have more questions than answers. That's the nature of life.
  • BB.BB. Super Moderators, Administrators Posts: 31,182 admin
    For standard stuff... AC or DC, heaters and Calrods (the heating elements typically found inside a water heater) don't really care if AC or DC.

    You did not state the voltage or wattage you were looking for... Here is a 12/24 volt 2x elements (you can wire one, or two in series or two in parallel depending on voltage and wattage):

    You can also use a standard element if you can find one that meets your electrical needs:

    Here is a 2,000 Watt @ 120 Watts... At 24 volts (for example):
    • P=V^2 / R
    • R = V^2 / P = (120 Volts)^2 / 2,000 Watts = 7.2 Ohms
    • P = V^2 / R = (24 Volts)^2 / 7.2 Ohms = 80 Watts
    Some other random comments:

    As stated above, you need to know your battery bank is charged first, before diverting loads. It can be done, but you don't want to "murder" your battery bank to electrically heat your water.

    Another issue is normal electrical contacts for water heaters are designed for AC loads. AC current is much easier to "break' vs DC current. If you use an AC rated switch/breaker/relay on a DC circuit (not DC rated switch), it is very likely that your switch will arc a lot and fail fairly soon.

    You can use DC silicon relays as switches, or use relays that are DC rated for your needed voltage.

    Other options include using your 120 VAC inverter output to heat the water (~85-90% efficiency) and the standard water heater thermostats and a simple AC relay.

    You can also just get a few extra (now relatively cheap) solar panels and heat the water that way (no battery issues).

    Make sure your setup is safe. Don't want to overheat the water tank, need TP (temperature/pressure) safety valve, and usually a second over temperature cutoff switch.

    If you run a "very hot" water tank (>>120F), you probably want a mixing valve to prevent scalding.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • PhotowhitPhotowhit Solar Expert Posts: 5,426 ✭✭✭✭✭
    We don't know much about your system. The simplest way to start a load  if you maintain some minimal loads on AC all the time, is to use a timer. On known sunny days just take a good guess as to when your batteries will reach float and start a load no more than 60% of your array's rated output. 

    Using a bread machine is fun and you can schedule fresh bread for about the time you arrive home!

    As Bill stated, Heating elements have a 'rating' but really don't care what you are driving them with. I've been using a 3600 watt 240 volt element on 120 volt AC for 6 years now. it draws about 900 watts.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do
  • NANOcontrolNANOcontrol Registered Users Posts: 194 ✭✭✭
    If you are thinking just some on/off heating, you need batteries because the current draw may be more than the panels can supply directly. One simple way to know if your batteries are "charged" is to monitor the panel voltage.  Any time the panel voltage goes above the power point voltage excess energy is there to use. One simple system would be a voltage monitor relay looking at the panels.  That would turn on a AC SSR from an inverter. It is important to use a wattage under 1KW (as low as 400W) for a longer sustained heating more in line with excess. As batteries demand a charge, the water heater will turn off. Lead batterie charge stages are especially wasteful of energy.  That can be as short as a couple of seconds.  I use a proportional system that sends enough power to maintain PPV be it 5W or 500W going to the heater. Unfortunately that requires building something not off the shelf. Typical PV solar systems are great energy wasters.
  • fratermusfratermus Registered Users Posts: 44 ✭✭
    I am dorky so I ran a separate circuit that is only energized when there is "excess power" available.  Some controllers can signal when they have dropped to float but mine doesn't. 

    There's a little voltage-triggered switch/timer doodad that powers a solenoid to activate the circuit.  The switch/timer kicks in at something like 13.4v (just below my Vfloat) and waits 30mins or so before engaging the solenoid. I don't remember the exact settings since It Just Works for the last year I've had it in.  The voltage is set high enough to signal we are in later bulk (or in float), and in my case a half hour is long enough to get far enough into Absorption where charging demands are dropping. When we drop below float the circuit turns off again.  My system is heavier on panel (2.5w:1Ah);  longer delay might be necessary for folks with more balanced systems.

    It charges laptops, feeds a little power donation station just inside the sliding door, even runs a small litecoin mining rig.  :-)  I am absent minded and it keeps me from leaving "niceties" on at night, stealing power from necessities.
    2017 Promaster 159" DIY camper
    570W mono / 220AH GC
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