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What is normal parasitic current drain?

I have a 2004 Chevy TrailBlazer that recently refused to start for my wife. I've got it going again and with a new battery. but I checked the key-off battery current drain with an ammeter (negative lead disconnected, meter between the cable and the battery) and the drain is 92 milliamperes. That seems high to me.
<br/> Does anyone know what a "normal" key-off electrical current from the battery should be? I've pulled every fuse, every circuit breaker, and have disconnected the alternator, and no matter what I do, the meter still shows right around 92 milliamps.
<br/> The only reference I've found to tell me what's "normal" is the document at this site: { -- which says on the second page that "A full option GM vehicle can experience a parasitic load of 25-35 milliamps." But does anyone know it that is correct and if 92 is too much?
<br/> Secondly, supposing ~30 milliamperes is about right, does anyone have an idea where the extra 60 milliamperes is going? Since it is not going through the fuses or the circuit breakers or the alternator, What's left??
<br/> Thanks.


  • edited October 2008
    This is Dave again. If that link mentioned above does not work, try going to Google and typing in: "Parasitic Current Drain". That should bring it up.
  • edited October 2008
    To me, 92 doesn't sound that high---that's less than 1/10 of an amp, or about 1.1 Watts of power. If the car has keyless entry, a stereo, Onstar, an alarm, in addition to the vehicle's own computers, it's possible that just maintaining the memory of these devices is using this much power. Of course I don't know what the draw should be, so I can't answer your original question. What kind of meter did you use to measure the current draw? The cheapie analog meters (the kind with the needle that swings) have pretty lousy accuracy at measuring low voltages and currents.
  • edited October 2008
    92 milliamps sounds high. I think it should be around 50 milliamps.

    Did you wait about 30 minutes before checking the amp draw? When the battery is disconnected then reconnected it takes about 30 minutes for everything to power up then shut down. So if you didn't wait long enough you could be getting a false reading.
  • edited October 2008
    Just opening the door can wake up some microprocessors in some vehicles. That puts you in the awkward position of having to crawl in the window to pull fuses after you wait for everything to go asleep. 90-100 mA seems a bit high, but look at it this way. I presume that your TB has a pretty big battery. The reserve capacity might be 100 Ah. If you do the calculation it would take 500 hours to discharge the battery 50%. That is less than a month so it seems kind of high.

    Are you sure that your ammeter is correct?
  • edited October 2008
    Thanks for the reply. The meter I used was a pretty nice one -- a digital multimeter with three current ranges available. I have a DC Clamp ammeter ordered so I can check things with the cables connected normally. That will be about 5 days from now, though.
  • edited October 2008
    Dear Willey, Yes, I checked the current over a period of several hours and then again after sitting all night. Thanks.
  • edited October 2008
    Dear Beadsandbeads - Your reserve calulation is interesting. I expect the battery gradually lost ground after many weeks because my wife usually makes short trips with the car. On her last trip, as I mentioned, it just couldn't crank the engine again. That fits with your reserve calculation at least in a general way.
    Ammeter Calibration -- Good idea -- I'll go to Radio Shack, buy a resistor or two, and double check the calibration of the meter at near this current level and on the same scale.
    By the way, I did a side-by-side test with my two cars the other night. First I topped off both batteries with a trickle charger until there was almost zero charging current draw for both batteries. Then I let both cars sit overnight (about 10 hours) with the key off and no charger. Then I put the trickle charger on each car, one at a time. The Buick needed only a brief surge (1/4 scale) of current in about TEN SECONDS the charger's ammeter was back to virtually zero . Then I did the same with the TrailBlazer. The TrailBlazer pulled the charger's ammeter fully to 1/2 scale and it took over a half hour to get it gradually back close to the "zero" level that it had showed when topped off the night before. This is an indirect indication, but pretty strong evidence that there's a whole lot more parasitic draw on the TB than the Buick.

    Thanks, Dave
  • edited October 2008
    Regarding accuracy of ammeter -- I just checked it with known resistive load and it checks out just about perfectly. Three 470 ohm resistors in parallel make 156.7 ohms. Battery voltage of 13.55 should give 86.5 milliamps. And that's what it reads. - Dave
  • edited October 2008
    Here is a TSB that might apply:

    Bulletin No.: 02-01-39-007B

    Date: November 05, 2007


    Automatic Dual Zone HVAC Battery Draw

    2004-2007 Buick Rainier
    2003-2008 Cadillac Escalade Models
    2002-2008 Chevrolet TrailBlazer
    2003-2008 Chevrolet Avalanche, Silverado, Suburban, Tahoe
    2002-2008 GMC Envoy Models
    2003-2008 GMC Sierra, Yukon Models
    2002-2004 Oldsmobile Bravada
    2003-2008 HUMMER H2, H3
    2005-2008 Saab 9-7X


    This bulletin is being revised to add models and model years. Please discard Corporate Bulletin Number 02-01-39-007A (Section 01 - HVAC).

    When diagnosing battery draws on trucks equipped with the automatic dual zone HVAC controls (RPO CJ2), technicians should keep in mind that the control head does not completely "go to sleep" until after 150-250 minutes, or up to 4-1/4 hours, from when the ignition key is turned OFF. This is a normal condition. In these cases, DO NOT replace the control head.

  • edited October 2008
    Before anyone can tell you if 92ma is normal, you need to provide more information about your particular Trailblazer. Is it 4x4? Is it equipped with OnStar? Do you have any aftermarket accessories?

    I own an '04 LS 4x4. I can check my normal draw as a comparison but I want to know if we'll be comparing apples-apples beforehand.

    Your TB is equipped with a power saving feature that is supposed to prevent the battery from being discharged enough that it cannot be started. However, this system only works if the battery is in good shape and the charging system is working and allowed time to replenish the lost charge from starting. Infrequent, short trips are killers for batteries and as it sulfates, it will spiral downward rapidly. 4-5 years is not outside the unexpected time for a battery replacement in those situations.
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