Voltage drop...inverter trips

Mike
Mike Registered Users Posts: 2
in Off Grid Solar & Battery Systems #1
In my VERY far off grid cabin, I have a bare bones system. It is not solar..yet. Currently I use the system when I'm at the cabin then I bring the batteries home and charge them. My loads are minimal with no intention or desire to increase them.
I have ten 7.5w 120vac LED lights. Max usage is probably 5 at a time for 5 hours.
I have one 12vdc water pump, 5.0 amps max. Max usage is probably 2 hours.
And the problem child is my small chest freezer rewired to work as a fridge...120vac. It's locked rotor current is 6.2amps. From testing at home, it only runs 2 hours per day... So lets say 3 hours/day in summer. It's operating current is 1amp.
I have an 1100w inverter and four 12v 35a hr batteries. Wire between the two is 4awg THHN and is about 5 ft long

Now the problem: Whenever the fridge tries to start the battery voltage falls and the inverter trips off. Every time without fail. I know it's because of the high starting current of the motor pushing the compressor. But I don't know what to change to fix it. So what's my best bet to fix this?
Thanks!
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Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Voltage drop...inverter trips

    Welcome to the forum.

    What you have there is a classic case of Voltage drop under load.
    When the 'frige demands start-up it is probably asking for 1100+ Watts from the inverter momentarily. On a 12 Volt system this is over 100 Amps of current. This presents two problems.

    The first is the V-drop across the 4 gauge inverter wires. That size wire isn't able to handle that much current. You would be better off with at least 0 AWG. That may sound extreme as the current isn't continuous, but it is the difference between wire that can take 60 Amps and wire that can take 150 Amps. Less Voltage drop under the current demand.

    The second is the batteries' ability to supply that current. Four 35 Amp 12 Volt batteries in parallel will have problems keeping the current even between the four batteries; inevitably some will be doing more work than others. Then there's the total Amp hours issue: you've got only 140 Amp hours there. Asking that to supply 100+ Amps even for a moment is asking for more than 50% of the total capacity. This is what is known as "flat-lining the batteries"; their Voltage, even without wiring difficulties, will drop drastically under the strain of trying to supply that power.

    When fully charged it may work, but the more power used the lower the batteries SOC and the worst the situation gets. The two things to do would be increase the battery wire size and change the batteries for larger capacity but fewer number. You might try picking up a couple of 'warehouse' golf cart batteries; at 220 Amp hours & 6 Volts it will increase the capacity and eliminate the current sharing problems.
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  • Mike
    Mike Registered Users Posts: 2
    #3
    Re: Voltage drop...inverter trips

    Thanks for the quick reply. Unless I'm mistaken the inverter should handle the load because 6.2a * 120v = 744W, which is why I opted for the 1100w inverter. I'll try both of your other suggestions. Thanks!
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
    Re: Voltage drop...inverter trips

    The inverter probably has surge capacity as well, but what you're dealing with is a "double Voltage drop" at the inverter in put due to the combination of small battery capacity and small wires.

    BTW, 6.2 Amps @ 120 Volts is 744 Watts, but the inverter has a conversion efficiency which will effectively increase that Wattage and the inverter itself needs to be powered so the Watt demand from the batteries is more like 900. When this demand (75 Amps) is placed on the batteries their Voltage falls causing the current demand to go up even further making the effect of the 'choking' of the small wires even worse.

    Sort of a self-compounding problem.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #5
    Re: Voltage drop...inverter trips

    One more thing: if this is an MSW type inverter the refrigerator will demand more current and the efficiency will be even worse.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #6
    Re: Voltage drop...inverter trips

    that freezer conversion doesn't sound like it is working out for you as new full sized refrigs do better than that. in any case you are putting to much burden onto a small capacity battery bank with wires that are too small to handle large current draws which all leads to voltage sag. even if the voltage did not sag, those batteries would not last long as they aren't properly sized to carry your loads. batteries that have an ah capacity rating does not mean it will indefinitely carry that much current. usually it is referenced to a 20hr rate and for each of your 35ah batteries this means it would be designed for a draw of 35ah/20hr=1.75a. higher than this 1.75a and the capacity will start to drop and you won't have 35ah to work with.

    the way to fix this is to add up all of the load current over a 24hr period to get this into ah. for instance the 10 7.5w leds will be a 75w ac draw. the inverter will add about 20% (varies) to that for a total of about 90w load presented to the battery. the battery is 12v so this is 90w/12v=7.5adc. over 5hrs this is 37.5ah. each load needs calculated out over 24hrs and added together to arrive at the total ah needed by all of the loads. once this is done you need to multiply this by at least 2 to arrive at a suitable battery ah capacity so that battery life is considered. sometimes if the loads are stretched out over more than a day without receiving a proper charge then it may need to have a larger battery capacity to accommodate the extra use before being fully charged. this bigger battery bank will need a larger charge source. we generally recommend at least a 5% rate of charge, but that does not always work out well. at the other end of things we say up to about a 13% rate of charge as some batteries may not always handle more than this well. some can handle a larger charge rate and battery manufacturers will state what they feel the best top end charge rate will be for any particular battery. for fla types it may also mean more maintenance.
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