Battery low voltage disconnect

solarfatcatsolarfatcat Registered Users Posts: 12
Hello. I was recently looking at my inverters specs. I noticed that my inverter will shut down if the voltage drops below 9.5V. It says this will protect the battery from being over-discharged. That seems kinda low to me. I thought anything under 11.5V ( or so ) was bad for the battery ? Why would this inverter be set up to shut down only when it seems that it's too late ? It is a Cobra CPI880. Is there a way I can change this setting ? Or, is there a product I can buy that will do this for me ? Thanks for your help.

Comments

  • waynefromnscanadawaynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Re: Battery low voltage disconnect

    Well, this is not something for which a simple answer works. Take for example a short time with a real heavy load, such as a high power microwave oven. It will drag the voltage way down while it's on, and if only on for a short time, would perhaps take only a little power from the battery. In this case the inverter could shut down while there's still plenty power left in the batteries.
    On the other hand, a small load for a long time could practically do a complete drain of the batteries before the inverter shut down. In this case, the batteries could be well into the danger zone even if the inverter were to shut down at 11.5 volts.
    Clear as mud?
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Battery low voltage disconnect

    You are absolutely correct. 9.5 Volts is in fact below the usual dead battery shut-off point of most inverters, which is 10.5. 12 Volts no load is about 50% DOD. There usually isn't any user-adjustment for this. Why they did this I have no idea. Maybe they think there's some advantage is squeezing every last drop out of the battery. Maybe they think flowing extra current is a good idea. Maybe they're just stupid. :p

    What can you do about it? Buy a different inverter. Failing that you'd need something like a Voltage controlled switch to detect a programmable minimum Voltage and turn the inverter off. That's about $130 for the VCS and some re-wiring on your part (you may have to open the inverter case and tap the on/off switch - a few inverters have remote switching ability).
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Battery low voltage disconnect

    actually, i wouldn't get another inverter, but would rather just not depend on that feature to protect your battery(s). operate things like that feature does not exist and watch your batteries that they do not overly discharge.
  • vtmapsvtmaps Solar Expert Posts: 3,739 ✭✭✭✭
    Re: Battery low voltage disconnect
    You are absolutely correct. 9.5 Volts is in fact below the usual dead battery shut-off point of most inverters
    <snip>
    Why they did this I have no idea.

    Nor do I know why in this particular case. Sometimes these low voltage disconnects are to protect the load. Some inverters may draw too much current as the voltage gets too low. The low voltage disconnect on the Danfoss refrigerator compressors (Sunfrost, Sundanzer, Steca, and others) is set at 10.5 volts to protect the compressor motor.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • BB.BB. Super Moderators, Administrators Posts: 32,027 admin
    Re: Battery low voltage disconnect

    I would suggest that low voltage disconnects are for protecting inverters (and loads from browning out). Not to protect the battery bank.

    AC Inverters are "constant power" devices... I.e., the equation is Power=Voltage*Current. If the voltage falls, the current must increase to maintain the output power levels... For example, a 1,200 watt "12 volt" inverter current requirements at different input voltages (excluding losses):
    • 1,200 watts * 1/15 volts = 80 amps
    • 1,200 watts * 1/12 volts = 100 amps
    • 1,200 watts * 1/10.5 volts = 114 amps
    • 1,200 watts * 1/9.5 volts = 126 amps

    There are also issues with wiring voltage drop... So, having an inverter that can run at a lower input voltage with a lower cutoff voltage can make wiring a little bit easier (11.5 volt battery - 1 volt drop or - 2 volt wiring drop).

    On the downside, a 2 volt drop would give you wiring losses of:
    • 126 amps * 2 volt drop = 252 watts of heat in wiring

    Not an insignificant loss of power...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Battery low voltage disconnect

    If the inverter is sized right and if the wiring is sized right and if the battery bank is sized right there is not a problem with 12 Volt systems for the right application. If you get all that right the load should not drop the apparent Voltage at the inverter to where it shuts down.

    But that's four "ifs".

    What Bill said is the key reason why large capacity (over 1kW) 12 Volt inverters are such a bad idea: the current draw enters the realm of unreal electrical engineering. :p

    Normally I'd disagree with his POV that the LVD is to protect the inverter; that's what the fuse/breaker should be there for. But in this case 9.5 Volts obviously isn't going to protect the battery since it will already be dead when it gets there.

    Many if not most 12 Volt inverters are designed for mobile applications, wherein there is typically a second battery or running alternator to provide additional power and hitting the low Voltage means things have gone seriously wrong. Of course they can go seriously wrong with two batteries available and you can't get the camper started again because you've just drained both down to dead.
  • BB.BB. Super Moderators, Administrators Posts: 32,027 admin
    Re: Battery low voltage disconnect

    "Fuses" protect wiring (and the input connections of loads)... They do not protect the loads themselves.

    Electronics are, usually, very sensitive to over current and over voltage conditions. Over voltage because the physical dimensions of transistors are so small, that they arc over with "small" over voltages. Currents, heating is I2R -- so, even a short time of doubling the current will give you 4x the local heating.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Battery low voltage disconnect

    Usually we think of this from the point of view of "overload" as that is typical with most systems. If the load is drawing too many Amps the fuse will blow to protect the wiring. With a battery system you can have the same problem created the other-way around: too low a Voltage creating too high a current even though there is nothing wrong with the load. In that case both the wiring and the battery are in danger.

    Letting the Voltage go down to less than 10 while still trying to supply maximum Watts is a recipe for disaster. If you set system Voltage as the minimum and expect the current to be in line with that (1200 Watts / 12 Volts = 100 Amps) then you can protect everything: inverter, wiring, and battery with one piece of circuit protection.

    Trying to run any component at or beyond its limit is a big mistake.
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