Inverter low voltage protection shutdown

I recently bought a Cobra 2500 continuous watt inverter (yeah I know, it's cheap....both from a quality and $$$$ sense. Yes you get what you pay for.) The low voltage protection feature shuts it down after running for only a very short time...a few hours. I am only running a small 60 watt dormitory AC refrigerator off of the inverter. The inverter is hooked up to three deep cycle (109 amp hr) batteries installed in a conversion van. It runs and works fine when the vehicles engine is running and the batteries are being charged by the vehicles alternator. I believe what happens is that when the batteries start to discharge over time (vehicle turned off), there is not enough peak power to start the refrigerator. Refrigerator motors turn off and on depending on the thermostat setting. It needs an extra boost of power to start the motor. After the batteries have discharged below 12 volts (in the 11.5 to 11. 7 range, the inverter low voltage protection kicks in when trying to pull power to start the refrigerator. This would probably not happen for lighting, tv or other low draw appliances on start up. Just a thought. Would be curious if others have experienced or tested this.

Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Inverter low voltage protection shutdown

    Welcome to the forum.

    If the inverter is shutting down from low Voltage you need to check a few things, starting with what that Voltage is. This needs to be measured at the inverter and at the battery. If there is a major discrepancy (and I suspect there is because those inverters usually turn off at 10.5 Volts) then the wiring between the two is not large enough to handle the current demand without significant Voltage sag.

    Another thing to check is the way the batteries are wired together. You have three batteries in parallel, and unless they are connected properly you won't get the benefit of all three. Quite often they are 'laddered'; connections to inverter on one, subsequent batteries connected the same way so that the third is having to go through two sets of parallel wires to reach the inverter. This is particularly problematic on 12 Volt systems where slight resistance increases have a big effect at high current levels.

    See the Smart Gauge wiring diagrams here: http://www.smartgauge.co.uk/batt_con.html Method #1 is 'laddered' and no good. You really want to use method #3 for three parallel connections.

    BTW those small refrigerators aren't very efficient. We have quite an extensive thread on testing them here: http://forum.solar-electric.com/showthread.php?12272-Just-how-bad-a-small-frige-is
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Inverter low voltage protection shutdown

    3*109 AH * 12 volt batteries * 0.50 maximum discharge * 1/60 watt load = 32.7 Hours of operation

    Most refrigerators run around a 50% duty cycle--So, a closed fridge (and not adding room temperature food) should run around 60 hours on that battery bank (all things being equal, which they rarely are).

    So--It sounds like your batteries are not getting fully charged--Which is very possible when using the van alternator for charging. Most vehicle alternators charge around 13.8 to 14.2 volts--Which is pretty low for a deep cycle flooded cell battery. Most will be charged around 14.5 to 14.8 volts or so--And at 50% discharge, that would be (guessing, minimum of 10% charging current) ~33 amps for 3-5 hours with another 2-6 hours of declining charge (absorb stage) for a 50% discharged battery bank.

    So, unless you are running the van 5-8 hours or so every other day with a dedicated marine alternator (or other DC to DC charging system), it is unlikely you are getting those batteries anywhere near fully charged.

    If these are flooded cell batteries--You can get a hydrometer to measure the temperature corrected specific gravity to see what the state of charge really is.

    If the van is parked at home during the day/camping in an RV park, you should add a dedicated deep cycle battery charger to get those batteries fully charged (and floated when the van is parked).

    If you are curious to see how exactly much current the batteries are getting--You should add a current meter or battery monitor. Or you can get a DC Current Clamp DMM (like this one from Sears) for $60 or so and see what is really happening when the batteries are being charged.

    Lastly, proper wiring is important. The refrigerator compressor can easily take >300-600+ watts when starting:

    300 watts * 1/0.85 inverter eff * 1/10.5 volts cut off = 34 amps

    So, the inverter can be pulling 34-68+ amps easily just to start the refrigerator--Pretty much similar to the current needed to start your van motor (i.e., short/heavy copper cabling needed from battery bank to AC inverter). And then you need to worry about fusing/breakers to prevent a dead short/failed inverter from overheating the wiring and starting a fire.

    Doing this stuff "right" takes some research/understanding what you are doing. It is not something you want to take lightly.

    Vehicle fires can get deadly, quickly.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • jcheil
    jcheil Solar Expert Posts: 722 ✭✭✭
    Re: Inverter low voltage protection shutdown

    Not to mention some of those cheap inverters that claim to be 2500 are many time highly over-rated, especially when it comes to starting surge. My neighbors old cheap (test) 2500/5000 inverter couldn't start his fridge.

    And as mentioned those little fridges are very inefficient. Just checking, it does have a compressor right? It is not one of those all electric cooling type? (which are even worse)
    And also as mentioned, I hope you have some large wires going from the batteries to that inverter otherwise the voltage loss/sag could be killing you also.
    Off-Grid in Central Florida since 2005, Full-Time since June 2014 | 12 X Sovello 205w panels, 9 X ToPoint 220w panels, 36x ToPoint 225w panels (12,525 watts total) | Custom built single-axis ground mounts | Complete FP2 Outback System: 3 x FM80, 2 x VFX3648, X240 Transformer, FLEXnet-DC, Mate-3, Hub-10, FW500 AC/DC | 24 x Trojan L16RE-B Batteries 1110ah @ 48v | Honda EU7000is Generator and a pile of "other" Generators | Home-Made PVC solar hot water collector | Custom data logging software http://www.somewhatcrookedcamp.com/monitormate.html
  • 1sharpguy
    1sharpguy Registered Users Posts: 2
    Re: Inverter low voltage protection shutdown

    I hope this post makes sense. I posted earlier today, but despite seeing the composition auto-saved the post ended up lost in cyber space. Am now unable to retrieve it. It was a rather lengthy response to your replies and suggestions. Perhaps it is for the best. Saved you from needing to read a lengthy tomb. Suffice it to say that much of the advice and suggestions you made I have tried or taken into consideration. The bottom line is that I own a cheap piece of crap inverter (which I am determined to make work for this application) and a refrigerator that sets off the low voltage protection shutdown on the inverter because the inverter is a cheap piece of crap. So now that we all understand what we are dealing with here (and oh btw, you can and should assume that batteries, cabling, alternator, connections, wiring scheme, etc etc are not the problem (this is what I tried to explain in my lengthy post that is now star dust) we can get to my point.

    On another forum, it was recommended that I should install a Hard Start Boost Capacitor (https://www.arnoldservice.com/air-conditioning/compressor-hard-start-boosters/) to compensate for the surges the inverter is experiencing from the refrigerator's start run stop cycle. For a variety of reasons (economics being primary), this makes the most sense to me. My question to you and this forum is "Is anyone familar with a Hard Start Boost Capacitor and if so, have you ever installed one on a refrigerator?" Based upon the manufacturer's instructions and a youTube video, it appears to be an easy installation, but not sure if it will work on a mini refrigerator.

    I apologize if this post seems confusing. My original post had a lengthy explanation about my rational, but unfortunately that is lost and I do not have the energy nor the where with all to attempt to rewrite it. Unfortunately, like a post lost in cyber space, once I do a brain dumb, it's gone. :(

    Thanks for understanding and any input you may be able to offer on the Hard Start Boost Capacitor.
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: Inverter low voltage protection shutdown

    cheap mos-sine inverters do not handle I-V imbalance well. A proper sized hard start cap, could possibly help, by bringing the I-V loading closer to 1:1, possibly in a range where the inverter can manage it.

    But if the shutdown is a Low Voltage condition, it's not likely to help. It could be that the internal inverter wiring is too small and causing the 12V to sag enough to trigger the lo battery shutdown.

    Looking at the link you sent, can you tell if your motor circuit is a PSC or CSIR style configuration ?
    https://www.arnoldservice.com/spp6e-compressor-hard-start-capacitor/

    Maybe worth spending the $ on a inverter with higher quality ?

    And you know that motors run on mod-sine, consume 20% more power and have to reject that heat somewhere ? And that 20% more power comes from your batteries ?

    And if the fridge starts ok when the batteries are full, or engine running, and fails when the batteries are "tired", it's purely a DC voltage problem, and not much is going to help .
    Powerfab top of pole PV mount | 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 ||
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  • jonr
    jonr Solar Expert Posts: 1,386 ✭✭✭✭
    Re: Inverter low voltage protection shutdown

    I measured my refrigerator yesterday (using a current clamp and the audio input in my laptop). 1.3 amps running, 14.3 amps when starting. Duration of start is around 600 msec.

    I am available for custom hardware/firmware development

  • PNjunction
    PNjunction Solar Expert Posts: 762 ✭✭✭
    Re: Inverter low voltage protection shutdown

    More details on the batteries please:

    Are they the same age? Were they all installed at the same time, or did you make additions afterwards? Was each one charged to full individually first before paralleling them all together - possible balance issues....

    And, are you physically disconnecting the inverter after use? The manual states <600ma draw when the button is off. Let's say it draws 1/2A (500ma) in the standby/off state. If you don't physically pull it from the circuit, that means in the space of 24 hours of non-use, you have drawn 12Ah just sitting there. If used only on the weekends, you can see how this can get you into trouble, especially if you aren't starting from a fully charged battery in the first place.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Inverter low voltage protection shutdown

    Even a cheap inverter will not shut down for low Voltage due to a high low if the DC side is correct. It may shut down from overload, but that is not the same thing as LVD. So which is actually what is going wrong? If you check the V at battery and inverter input at the time of (before) shutdown you can see if there really is a low Voltage problem.

    If not then the inverter is faulting due to the load. Different problem, which requires a different solution. Frankly even the surge on a dorm 'frige should not shut down a 2.5kW inverter (unless other loads have it near peak capacity anyway).
  • jcheil
    jcheil Solar Expert Posts: 722 ✭✭✭
    Re: Inverter low voltage protection shutdown
    Even a cheap inverter will not shut down for low Voltage due to a high low if the DC side is correct. It may shut down from overload, but that is not the same thing as LVD. So which is actually what is going wrong? If you check the V at battery and inverter input at the time of (before) shutdown you can see if there really is a low Voltage problem.

    If not then the inverter is faulting due to the load. Different problem, which requires a different solution. Frankly even the surge on a dorm 'frige should not shut down a 2.5kW inverter (unless other loads have it near peak capacity anyway).

    I know that my neighbor's old CHEAP 2500w MS inverter (that he had to use in a pinch when his main one died) would not start his fridge either. There is no reason it should not be able to. It claimed 2500/5000 but nothing would get it to start. Everything was fine on the higher-quality 2500 inverter he had before (until it died). He bought another higher "quality" MSW 2500 and it starts fine. Not sure if it was a bad inverter or just an over-rated-claims inverter. I bet the OP is having the same issue. Just cause it "says" 2500 on it, doesn't mean it will actually be able to do it (on these cheap ones).
    Off-Grid in Central Florida since 2005, Full-Time since June 2014 | 12 X Sovello 205w panels, 9 X ToPoint 220w panels, 36x ToPoint 225w panels (12,525 watts total) | Custom built single-axis ground mounts | Complete FP2 Outback System: 3 x FM80, 2 x VFX3648, X240 Transformer, FLEXnet-DC, Mate-3, Hub-10, FW500 AC/DC | 24 x Trojan L16RE-B Batteries 1110ah @ 48v | Honda EU7000is Generator and a pile of "other" Generators | Home-Made PVC solar hot water collector | Custom data logging software http://www.somewhatcrookedcamp.com/monitormate.html
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Inverter low voltage protection shutdown
    jcheil wrote: »
    I know that my neighbor's old CHEAP 2500w MS inverter (that he had to use in a pinch when his main one died) would not start his fridge either. There is no reason it should not be able to. It claimed 2500/5000 but nothing would get it to start. Everything was fine on the higher-quality 2500 inverter he had before (until it died). He bought another higher "quality" MSW 2500 and it starts fine. Not sure if it was a bad inverter or just an over-rated-claims inverter. I bet the OP is having the same issue. Just cause it "says" 2500 on it, doesn't mean it will actually be able to do it (on these cheap ones).

    This is oh-so-true. I tested a bunch of them about six years ago; not one could meet the supposed output specs. And not all MSW inverters are alike: their output waveform can range from strictly square wave to something that approximates a sine wave fairly well (around 5% THD). You spends your money and rolls the dice.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Inverter low voltage protection shutdown

    MSW inverters (modified square wave) are not very "inductive load" friendly... It is possible that the inductive kickback from the refrigerator motor starting is causing the MSW inverter grief.

    However, my first guess would be to measure the DC input voltage at the inverter's DC bus connection. It sure sounds like long/small gauge wire and/or a battery bank that is not able to handle the surge current.

    If you have an electric space heater--Try putting that on the inverter and crank of the heater to full (usually around 800-1,500 watts) and see if the inverter can support that load.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: Inverter low voltage protection shutdown

    Just put a load of meat into the freezer and connected the kill-a-watt for kicks. When I turned it on, I got real lucky, the KaW caught a 600W starting surge, and I'm sure it's easily double that. running is 150W. This is on pure sine.
    Powerfab top of pole PV mount | 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 ,

  • jcheil
    jcheil Solar Expert Posts: 722 ✭✭✭
    Re: Inverter low voltage protection shutdown
    mike95490 wrote: »
    Just put a load of meat into the freezer and connected the kill-a-watt for kicks. When I turned it on, I got real lucky, the KaW caught a 600W starting surge, and I'm sure it's easily double that. running is 150W. This is on pure sine.

    I've seen that too, but it was rarely the "correct" number. It just happened to "grab" that number perhaps while it was ramping up or ramping down. The KaW only samples maybe once a second. The surge may only last for 1/2 that and it can "miss" the real number. So your true surge might (and I would expect) to be more than that. or you could have been lucky and got it at the EXACT peak of the surge - in which case I would go buy a lottery ticket :)
    Off-Grid in Central Florida since 2005, Full-Time since June 2014 | 12 X Sovello 205w panels, 9 X ToPoint 220w panels, 36x ToPoint 225w panels (12,525 watts total) | Custom built single-axis ground mounts | Complete FP2 Outback System: 3 x FM80, 2 x VFX3648, X240 Transformer, FLEXnet-DC, Mate-3, Hub-10, FW500 AC/DC | 24 x Trojan L16RE-B Batteries 1110ah @ 48v | Honda EU7000is Generator and a pile of "other" Generators | Home-Made PVC solar hot water collector | Custom data logging software http://www.somewhatcrookedcamp.com/monitormate.html