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  • acceptable voltage sag during use?

    hello,
    thanks to everyone who contributed here. i learned a lot and was able to jump in based off all the info shared

    a few months ago i moved to a small urban farm part time, and put together the following solar rig:

    2 x 260 watt REC panels
    rich solar 40 watt mppt
    4 x interstate batteries in series (6v, 210ah)
    wzreb 2kw 24v puresine inverter.

    the set up seems to work well for charging devices (phone, tools) and runs an apartment-sized fridge without issue. my issue is when i use the induction cooktop (1200 watts i believe), at level 3 or 4 out of the 10 settings, my battery voltage swings wildly, going from 26v+ down to 20v. if i disconnect the fridge i think it doesn’t go as low, perhaps 21/21v. as soon as i turn it off, the voltage picks back up to 26v+ and shows 100% charge. i know fully discharging lead batteries is bad, am i a bad man and doing that? or is that swing somehow acceptable?

    thanks!

  • #2
    Hello against88 and welcome to Solar Panel Talk

    Based on a lot of data, big loads like a cooktop will easily drain a set of batteries. Most people that live off grid will use another power source to run their cooking or cooling equipment.

    You may want to look into some other power source to cook with or you will kill your batteries pretty quick.

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    • #3
      I guess you are discovering the limitations of a very small solar array. Double your batteries and quadruple the solar panels then you might get away with cooking on that induction thingy. Just make sure it is used during very bright sunny days.

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      • #4
        What size wire is going between the batteries and the inverter? Undersized cables or long cable runs can cause voltage drops.

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        • #5
          perhaps i should’ve mentioned that i’m using the induction cooktop for a few minutes at a time (this scenario was to boil a single cup of water)? i’m also in southern california and typically use this only in bright sun. i guess i just assumed a 5kw battery bank could power a 1200 watt device via a 2k watt inverter for short periods. and the batteries being full immediately after i stop using the device seems to indicate that i haven’t fully drained the batteries, otherwise wouldn’t it take the 520 watts of PV ~10 hours to fully recharge a flat battery?

          the battery bank to inverter cables are under 3 feet and 2 gauge. they don’t get warm to the touch.

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          • #6
            Originally posted by against88 View Post
            perhaps i should’ve mentioned that i’m using the induction cooktop for a few minutes at a time (this scenario was to boil a single cup of water)? i’m also in southern california and typically use this only in bright sun. i guess i just assumed a 5kw battery bank could power a 1200 watt device via a 2k watt inverter for short periods. and the batteries being full immediately after i stop using the device seems to indicate that i haven’t fully drained the batteries, otherwise wouldn’t it take the 520 watts of PV ~10 hours to fully recharge a flat battery?

            the battery bank to inverter cables are under 3 feet and 2 gauge. they don’t get warm to the touch.
            1200 watts divided by a 24 battery = 50 amps. That's well within specs for a 3 foot long 2ga wire. Did you check your connections? Anything getting warm where the bolts and screws tighten the fittings down?

            Are your batteries new?

            What you're describing doesn't make sense. If you're battery wires were 15 feet long and you were running an 1800 watt load on a 2k (continuous) inverter, then it would. But 1200 watts drawing a 50 amp load shouldn't do that. I know Chinese inverters are mostly junk, but that still doesn't explain it.

            How and where are you checking the voltage? Use a multimeter and check the voltage at the battery terminals, then the inverter input terminals. It is possible that a built-in digital meter on the inverter might be off as well.
            Last edited by Salts; 07-04-2020, 03:28 PM.

            Comment


            • #7
              Being new at this, I’d consider checking each battery cell individually with a hydrometer.

              The rest of this is just me trying to figure out how I would start to size a system for a burner.

              For using an electric burner or electric kettle, it’s not supposed to draw extra energy above what its rated like an air conditioner since there’s no motor to spool up, so you don’t lose watts mysteriously to large peak watts above what the device is rated. I found this that I’m going to look at and help me plan what I would need to boil water: https://www.plotwatt.com/best-way-boil-water/

              When I started my system, I was hoping to get enough energy to power something like a electric burner for several hours a day, and I saw what it would take for this and I’m far underpowered, so I adjusted a little, but am interested to see what it would take to run a 1500 Watt electric kettle, safely about 16 times a day, so I could boil 2 gallons of water total. The electric kettle is supposed to be more efficient at boiling water than an electric burner. An electric kettle takes 25 watts a minute to boil a cup of water, and there’s 4 cups in a quart. THe burner is supposed to take about 50 watts and two minutes for the first cup, and then 25 watts and an extra minute for each additional cup.

              Also note that that 25 watts a minute comes from dividing 1500 watts by 50 minutes. I have always wondered if you measured the wattage draw, how close a device is to its rated consumption. Maybe I have the purchase of a kilowatt meter in my future.

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              • #8
                An electric kettle - submerged element, with an insulated pot, is the most efficient.
                Then the induction hot plate.
                then microwave
                then glow coil heater element.

                The voltage sag is acceptable, as long as it does not trip the inverter off line. Sag is also power loss, either internal resistance in the battery or in the wiring. Reduce the sag = reduced losses
                Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
                || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
                || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

                solar: http://tinyurl.com/LMR-Solar
                gen: http://tinyurl.com/LMR-Lister

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                • #9
                  When you fire up that induction stove is just like cranking a car. If you take a reading while cranking may see a drop down to 8-10 volts. It is almost a dead short. The battery comes right back up. Just a hard load. Any load will cause a voltage drop. A hard load will yield a very large voltage drop. Just because your battery voltage returns doesnt mean they are still fully charged.

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