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Thread: Low Voltage Disconnect

  1. #1

    Default Low Voltage Disconnect

    Hi,

    So I have a Xantrex C12 controller, a BP350U solar panel, a 12V deka marine battery model 8G24M, a 12V 150W pure sine-wave inverter and a hot water kettle load. The system is originally supposed to support a computer but since the kettle was smaller and easier to move around during testing, it is being used as a substitute.

    The problem that I am encountering is when I connect the kettle load to the inverter the voltage across the battery decreases really quickly up to about 11.5V then the inverter trips. The battery before I connect the load is at a solid 12.4V or so. I also looked up the spec sheet of the inverter and there is a low voltage input shutdown at 10.5V so I don't know why it's disconnecting at 11.5.

    Could the wire size that I'm using be the problem? The battery is connected to the controller with 16awg and the inverter is connected directly to the battery with 10awg which came with the inverter. Or is the kettle load too big? Because apparently it draws 1000W. Although this is a high number would my setup be able to power a desktop computer which should draw about 300W? Or is it just that the battery I'm using isn't providing enough power?

    I have links below for the spec sheets of the components I'm using. Any help would be appreciated. Thanks!

    Solar panel: http://www.solarelectricsupply.com/S...r/BP_350U.html

    Inverter: http://www.samlexamerica.com/custome...ifications.pdf

    Controller: http://shop.solardirect.com/pdf/sola...ies-manual.pdf

    Battery: http://www.dekabatteries.com/assets/base/epm0909.pdf

  2. #2
    Join Date
    Sep 2006
    Location
    Willits, CA
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    5,750

    Default Re: Low Voltage Disconnect

    A hot water kettle is a LOT more than a 150W load, usually 700W min, and 1200W normally. Your inverter is overheating and shutting off.

    And it's likely, the battery cannot sustain that much power for very long either.
    http://tinyurl.com/LMR-Solar
    http://tinyurl.com/LMR-BigLug
    http://tinyurl.com/LMR-NiFe

    Grundfos 10 SO5-9 with 3 wire Franklin motor (1/2hp 240V 1ph # 214505 ) on a timer for 3 hr noontime run - Runs off PV
    Powerfab poletop PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe battery | 15, Evergreen 205w "12V" PV | Midnight ePanel || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT

  3. #3

    Default Re: Low Voltage Disconnect

    So that means the inverter won't be able to support a desktop computer either? So the only way to fix this is to get a new inverter with a higher wattage rating?

  4. #4

    Default Re: Low Voltage Disconnect

    Wattage rating on an inverter is its maximum output (excluding momentary surge rating). You can't get 1000 Watts out of a 150 Watt inverter; it will shut down.

    Likewise it will not supply 300 Watts for your computer, although you may find that it will run it: the Wattage rating of a power supply is again its peak output, not its average consumption. I have measured desktop computer consumption and found it tends to be under 200 Watts. It varies too; might be 90 or 190 depending on what the computer is doing.

    If your going to try and figure out what will run on any given inverter, invest $30 in a Kill-A-Watt meter and measure the device's actual use. Then you'll have a much better idea how much power you need to run it with.

    Also, be advised there are two different types of inverter: the "pure" sine wave and the "modified" sine wave (both technically misnomers). Something that draws 'X' Watts on a pure sine wave inverter may draw more power on a modified sine wave. This is the Power Factor issue, and the K-A-W will give you a PF number as well. Some things, like induction motors, do not run on MSW inverters at all. There again, it depends on the particular motor and the particular inverter.
    1220 Watts of PV, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

    Ohm's Law: Amps = Volts / Ohms
    Power Formula: Watts = Volts * Amps

  5. #5

    Default Re: Low Voltage Disconnect

    hyp009,

    I have a small setup on my travel trailer, a small 20 watt panel connecter in parallel with a 80 watt panel. I also have a small Samlex 150 watt pure sine wave inverter which is not hard wired but plugs into a DC socket. It is the PST-15S-12A. The DC socket is wired to the trailer converter panel with 12 gauge wire. I have no problems running a laptop with it. During the daytime with sun on the panels I see no voltage drop with the laptop on. The only thing that I have noticed is when the laptop has been on for a few hours the DC plug on the inverter to the DC socket is a little warm, not hot.

    The first thing I see is you have 16 gauge wire from the controller to the battery. But this should not be your problem if you are taking the power from the battery with a 10 gauge wire.

    I wired my controller to the battery with 6 gauge wire which was what the manufacturer recommended. I have 10 gauge wires from the panels to a terminal block, an Edison PB1042 where a 6 gauge wire is then connected to the controller. I have fused the controller about a foot from the battery with a Cooper Bussmann HEB-BB in line fuse holder with a 30 amp AGU fuse. This also was recommended by the manufacture also.

    A 150 watt inverter should have no trouble running a laptop.

    Keith
    Last edited by Crotalus; June 2nd, 2011 at 9:34 PDT. Reason: bad spelling

  6. #6
    Join Date
    Sep 2006
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    Default Re: Low Voltage Disconnect

    Quote Originally Posted by hyp009 View Post
    So that means the inverter won't be able to support a desktop computer either? So the only way to fix this is to get a new inverter with a higher wattage rating?
    I belive so. 150W will run a laptop, but not a desktop, with a seperate monitor. Desktop CPU's average about 75W, with their big heatsinks, and they you have the motherboard, drives and such, and you are at at least 200W before you turn on the monitor.

    Morningstar has a nice 300W inverter, Suresine, wihch is pure sine, and rated for 600W for 10 minutes. A nice inverter, and would be able to supply most desktop and monitor setups, and could peak power for 5 min or so, when burning a CD.

    Some other good quality sine inverters are here : http://www.solar-electric.com/exsiwain.html

  7. #7
    Join Date
    Sep 2006
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    Posts
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    Default Re: Low Voltage Disconnect

    regarding your chargeing source, only 100W of panel. That means for every 4 hours of sun, you can only run the computer for about 1 hour. you will need to increase your PV array, or charge from an alternate source, because you need to recharge the battery to above 80% charge daily. Leaveing a partial charge will ruin the battery in a short time.

    Read up some about batteries and charging here:
    http://www.windsun.com/Batteries/Battery_FAQ.htm
    http://tinyurl.com/LMR-Solar
    http://tinyurl.com/LMR-BigLug
    http://tinyurl.com/LMR-NiFe

    Grundfos 10 SO5-9 with 3 wire Franklin motor (1/2hp 240V 1ph # 214505 ) on a timer for 3 hr noontime run - Runs off PV
    Powerfab poletop PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe battery | 15, Evergreen 205w "12V" PV | Midnight ePanel || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT

  8. #8
    Join Date
    Mar 2006
    Location
    SF Bay Area (California)
    Posts
    21,546

    Default Re: Low Voltage Disconnect

    It sounds like you have several issues...

    First, 12.4 volts resting for a room temp battery is around 75% state of charge--So, your battery should be recharged fairly soon to reduce the chances of sulfating the battery (if stored for days/weeks/months at this low of state of charge).

    Second, you have a single 50 Watt solar panel, but how long the is cable--16 awg is probably too small of wire for your 12 volt battery charger. Small gauge wire and long lengths add up to lots of voltage drop--which reduces the charging current and takes much longer to recharge a battery bank. The charge controller "sees" the higher voltage from the wiring voltage drop and things the battery is more fully charged than it really is.

    The battery is a Deka group size 24, 12 volt 73.6 Ah gel battery. Gel batteries are not great for high charging/discharging currents. A good rule of thumb is to limit maximum current draw to C/2.5 for surge current and C/8 for continuous loads (8 hour discharge rate):

    • 12 volts * 73.6 AH * 1/2.5 surge current = 353 Watts maximum surge load
    • 12 volts * 73.6 AH * 1/8 surge current = 110 Watts maximum continous load

    And for proper charging, a good starting point is 5-13% rate of charge:

    • 14.2 volts * 73.6 AH * 1/0.77 solar charging derating * 0.05 rate of charge = 68 watts minimum solar array
    • 14.2 volts * 73.6 AH * 1/0.77 solar charging derating * 0.13 rate of charge = 176 watts cost effective solar array

    Your 50 watt array is on the "light side" of rate of charge... It should work, but it will take a while to recharge you battery after use.

    For the inverter--as others have said, we really need to know the load wattage rating (a Kill-a-Watt meter is really handy for measuring 120 VAC loads). Anyway, the wiring for a 150 Watt inverter should be rated to:

    • 150 watts * 1/10.5 volts cutoff voltage * 1/0.85 inverter eff * 1.25 NEC safety factor = 21 Amps minimum wiring/fusing/breaker rating

    So, 10 AWG is "safe"--but the other issue is how long is the wiring... Again, long wire with added voltage drop will make the cutoff activate at a higher voltage... For a 1 volt drop, using a generic voltage drop calculator and ~17 amps maximum current (above equation without the 1.25 safety factor):

    • 10' of 10 AWG copper cable and 17 amps => 0.4 volt wiring drop

    If you are drawing ~300 watts of power (some inverters will supply 2x power for a few seconds to a few minutes), you would be looking at 0.8 volt drop--very close to an expected 11.5 volt cut off at the battery because the inverter is "seeing" nearly 10.5 volts because of the wiring drop.

    -Bill
    20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

  9. #9

    Default Re: Low Voltage Disconnect

    The 10 gauge wire from the inverter to battery is about 2'. The 16 gauge wire from the battery to the controller is about 6'. The wire going from the solar panel to the controller is 22 gauge and is about 3'.

    So I ran some tests with the actual computer desktop without the monitor and without the solar panel connected to the controller. I wanted to see if the inverter's low voltage shutdown of 10.5V actually works or not.

    There are a few concerns though. Referring to some of the replies, a computer desktop shouldn't work with my setup since the inverter is only supplying a continuous 150W when the power supply in the desktop ranges from 200-250W. I then added another load, a soldering iron station with is rated at 60W. So with the desktop, it should be drawing at least 260W from the inverter. How is this possible when the inverter is only 150W?
    *Edit: Okay so I reread Cariboocoot's post which answers this question. Sorry about that.

    Also, when I had the setup with the desktop and soldering station without the solar panels hooked up, I had a voltmeter connected to the battery and once it dipped below 10.5V the inverter didn't trip like it did with the kettle. Does "low DC input voltage shutdown" mean the same thing as low voltage disconnect or was I wrong in assuming they were one and the same? If not, then is there something I can add externally to make a low voltage disconnect?

    Any suggestions? I appreciate all the responses so far. Thanks!
    Last edited by hyp009; June 2nd, 2011 at 13:51 PDT.

  10. #10

    Default Re: Low Voltage Disconnect

    You "ran some tests". Did you plug the computer in to a Kill-A-Watt? If not, how do you know what it was drawing?

    The kettle, being 1000 Watts, puts a much bigger drain on the batteries than even the peak inverter capacity:

    1000 Watts @ 12 VDC = 83 Amps. Your 10 AWG wire can not carry that much Amperage, therefor the Voltage drop seen by the inverter would be much greater than merely maxing its capacity (which is surge to 200 Watts).

    200 Watts @ 10.5 VDC = 19 Amps, well within the wire's capacity and easily delivered by the battery.

    In short, plugging that electric kettle in would look like a dead short to the whole system. The low Voltage shutdown is meant to turn things off when the battery Voltage drops to a point where it is no longer feasible (nor safe) to keep the inverter running. That is different from an over-load shutdown, however the inverter "reports" it.
    1220 Watts of PV, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

    Ohm's Law: Amps = Volts / Ohms
    Power Formula: Watts = Volts * Amps

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