How much power does my 3000 watt MSW inverter use when off but connected to the battery?

I notice, it sparks when connecting to the battery, so it must draw some small power all the time as it charges capacitors on the 12vdc side. And they dont stay charged as if I disconnect inverter from battery, they discharge by themselves after a few minutes.  Reconnect and it sparks again. 
So far no one seems to know about power used when off but battery connected.

What will happen if the 3000 watt msw is left connected to a battery for many months and battery is new and not being charged.

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  • BB.BB. Super Moderators, Administrators Posts: 32,601 admin
    Welcome to the forum sdowney717,

    You are correct, the filter capacitors on AC Inverter DC inputs can draw a pretty big spark when initially connected.

    Many inverters document their power draw when On (no load), Search Mode (power up once a second to look for AC loads), and Off... Some are "OFF" with a switch or relay, others are "electronically" off and may still draw some current:

    Just a random example:

    https://www.solar-electric.com/lib/wind-sun/SureSine.pdf
    Self Consumption
    • Inverter On (no load) 450mA
    • Inverter Off 25mA
    • Stand-by 55mA
    And you are correct to worry about "vampire" loads (loads that daw 24x7 but you may not know they are there).

    If you cannot find a specification for your inverter, you can use a DC Current Clamp DMM (digital multimeter) to see how much leakage current there may be:

    https://www.amazon.com/gp/product/B00O1Q2HOQ (inexpensive, good enough for most uses)
    https://www.amazon.com/gp/product/B019CY4FB4 (mid priced meter)

    Note there are AC Current Clamp DMMs and AC+DC current clamp DMMs... The AC versions are very good units but don't work with DC current (may be confusing descriptions as they do measure AC+DC voltages, just not DC current).

    Lets say you have a 200 AH @ 12 volt battery bank, and it sits for 6 months during the "off season". And you will accept a 10% discharge over that time:
    • 200 AH * 0.10 discharge = 20 AH
    • 20 AH * 1/24 hours per day * 1/180 days = 0.0046 amps = ~5 milliamp draw "acceptable to you" vampire load
    • 0.0046 amps * 12 volts = 0.055 Watts
    Very little current over 6 months will be an issue... And frankly, it is doubtful that a typical DC current clamp meter will even measure 5 mAmps or less accurately (the Klein can only display 60.00 amp range or 10 mA minimum).

    The typical "answer" is to use a circuit breaker (or pull a fuse, when the inverter is "off") to ensure no unplanned discharging of your batter bank (inverter, other DC loads such as a radio, lights in a closet, etc. are forgotten/left on).

    Other things to think about... Different batteries have self discharge... Flooded Cell Lead Acid--Need to be charged at least once a month. AGM once every 6 months. Lithium Ion--years.

    Also hot batteries self discharge faster--Batteries setting at 95F in a hot RV--They will discharge 2x faster than those stored at room temperature...

    If you have solar panels--They can keep up with self discharge (in general)... But some folks get bit by parking in shade/under roofs, or drifting snow in winter (solar charge controllers also have minimum current draw--And a few folks have had their cabin solar panels drifted over by snow, and the charge controller killed the bank).

    -Bill

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • sdowney717sdowney717 Registered Users Posts: 3
    Thanks for the info. Inverter is in my 37 foot boat, a 1970  Eggharbor sedan.
    Since the inverter gets little use, as does the boat, I do worry about the power cutting off for whatever reason at the marine, and the battery then getting drained by vampire electronics, like inverter, radio, small LED volt-amp meters, etc...

  • sdowney717sdowney717 Registered Users Posts: 3
    I think maybe the radio always on memory wire could be left disconnected?
    I could install a battery switch on the inverter DC wire
    I have two small LED panel meters that reads volts amps  off the battery, I wonder what each of these items draw in milli-amps.
    Other thing, I could when leaving the boat, rotate the 2 large battery selector switches to off, that leaves only the radio and bilge pumps with power available. Bilge pumps use float switches. 
    I also wonder if my VHF marine radio uses some small amount of power. It is a newer one with the MMSI broadcast security button.
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