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Problem with load from inverter.

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  • Problem with load from inverter.

    Hey guys, to start I'd like to say thank you, I've got a lot of great information from this site over the years. I haven't really been able to find a reason for my most recent problem, though, so I thought I would ask.

    Pretty simple setup:
    '05 Cobalt with an aftermarket head unit (7" screen, AUX input, etc.)
    1000w (1500w peak) rated inverter.

    So, I connected the positive wire from my inverter to the battery and grounded to the frame (clean connection). I plugged the PS3 into that, and the PS3 into the screen and turned it all on. Success! It worked great. I turned it all off, and started my car. Turned the inverter and head unit on, turned on the PS3 and my inverter kicked out and reset itself. So I tried it again, same thing. So I turned off my car, turned on the PS3 and it fired right up.
    I'm trying to make sense of this. The output of the battery while the car is off is maybe slightly over 12v. When I measured it while the car was on, it was getting the ~13.5v from the alternator, which was well within the inverters rating of up to 15v input.

  • #2
    So more information:

    I tried another similar inverter today, same results. I also looked up the PS3's peak watts used is approximately 380. The inverter I'm using states that it is rated for 1000w constant, 2500 peak. Any ideas yet?


    • #3
      Do you have a mix of grounds - ie, inverter powered AC loads to DC powered HU etc?
      If so, noise & spikes could be triggering the inverter protection.

      I'd also suggest the inverter GND be taken from the battery- rather than the chassis (better noise suppression).
      And maybe too the other grounds from the same (ie, battery-).

      You are right about 13.5V being withing normal inverter range...

      However your 13.5V alternator output is low (unless idling) - it should be at least 13.8 if not over 14.0V to 14.4V (max long-term). 13.5V is not sufficient to properly recharge a battery (subjective comment) and even 13.8V is not considered sufficient for good battery life. (If your alt is only 13.5V, put a diode in its Sense line to boost output to ~14.2V unless there is a valid reason for its low output.)

      And "slightly over 12v" is indicative of an undercharged battery. It should be 12.6-12.7V fully charged and rested (overnight etc) with no load.
      However, with a ~30A load (ie, 300W inverter output), a fully charged 12.7V battery could reasonably be expected to read ~12.2V (ie, battery internal resistance ~0.15 Ohm; 30A x 0.15R = 0.45V; 12.65V-0.45V = 12.2V)

      The combination of all the above can do strange things. VIZ - without upgrading the Big-3 and using a chassis ground with an extra 30A-100A load means a big battery to chassis voltage drop.
      An undercharging alternator resulting in an undercharged battery can mean higher than usual alternator currents (going flat out to recharge the battery). Hence its mere 13.5V output...
      Actual alternator outputs depend on type. The single-wire D+ types won't exceed their setting (typ 14.2V) hence less than that at the battery. Other types will try to regulate the battery to typically 14.2V which can mean the alternator puts out over 16V is there is a combined voltage drop of 2V between the alt & battery (ie, +12V & GND path).

      Anyhow, hence why I suggest battery-grounding the inverter.
      After that - even if it fixes the problem - you can check for other issues - like undercharging or stuffed battery etc.

      And IMO the big-3 are always worthwhile. At least running extra engine to chassis/body grounds for redundancy in case one fails. (I suspect engine to chassis ground failures are the biggest cause of equipment and wiring destruction, and maybe several fire else smoke sessions... )

      PS - I was going to say it could just be a random occurrence (poo happens). But if you describe typical behavior... see above.
      Last edited by OldSpark; 10-27-2011, 08:20 PM. Reason: ps...


      • #4
        So after chasing around the ground issue with no luck I decided to try something else; I put a diode in between my battery and the inverter and tada! it worked. Do you think that I might have a bad diode in my alternator, and how would I know?