Parasitic draw & various other draws on 12v system...

Discussion in 'Prime Technical Discussion' started by Insighter, Aug 2, 2018.

  1. Insighter

    Insighter Active Member

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    Location:
    SoCal
    Vehicle:
    2017 Prius Prime
    Model:
    Plug-in Advanced
    I recently had trouble with my 12v battery on my 2017 Prime Advanced discharging. It turned out it was a heavy parasitic draw (which is the draw on a car's battery when the car is off). I described that in a separate recent post which many of you were good enough to help me with.

    This dead 12v battery issue I had has prompted me to fully test my parasitic draw, and what certain items add to that draw if connected when the car is off. I should note that I can test the draw of various items when the car is off because I had my stereo/alarm installer install a 12v cigarette lighter outlet that is always on (instead of just being on when the Prime is on like the OEM outlet).

    Results of Parasitic Draw Tests:
    Normal parasitic draw (with car alarm): 60-80 milliamps (mostly right around 65 milliamps)
    Parasitic draw with lighted USB cigarette plug adapter: 80-95 milliamps
    Parasitic draw charging iPad Pro: 900-1,200 milliamps (reducing as the iPad's charge nears 100%)
    Parasitic draw using CPAP machine: 350-3,500 milliamps (varying greatly as one breathes in and out)

    In the past, 50 milliamps or less was considered a good, normal parasitic draw. This has changed in recent years with the addition of so many systems to cars (like OnStar, Toyota Sense and proximity key sensors). Now a parasitic draw might be normal at up to 100 milliamps. And then there are things you might add to a car that add to its parasitic draws, like car alarms and dash cams with parking modes. Of course, the larger your parasitic load, the faster your car's 12v battery will drain when it is off and parked. If you drive your car frequently and for a long time, even a larger parasitic load may be no problem.

    The normal parasitic draw of my Prime Advanced hovers right around 65 milliamps. This does include the factory car alarm that the dealership installed before I ever laid eyes on my Prime. It is a "Karr" brand alarm, and I had it in valet mode during these tests (which causes the very bright blue LED for the alarm to remain constantly on instead of flashing). It is a very basic alarm, and I believe it will draw less than 15 milliamps, but I can't test that as I can't disconnect the alarm.

    The Prime owners manual says to not leave the Prime plugged into its charger (the traction battery charger) after charging is completed because various systems stay on which increase the parasitic draw. While the Prime is actually charging, I have verified that it does charge the 12v battery in addition to the traction battery. However, I tested the Prime with it plugged into the wall after the charging of the traction batter was done, and I could not see the additional parasitic draw that the owners manual warns of. Perhaps the warned of additional draw only occurs at certain intervals or after a certain amount of time, but I could not see any parasitic draw over a period of 30 minute. I will test this more at some point and post the results in this thread.

    First, using the always-on 12v outlet that I had installed, I tested the draw of a dual usb cigarette lighter charging plug. Such a plug would probably draw nothing, except this one has an LED light or lights that illuminate the USB outlets the entire time it is plugged in. As a result, it uses 20 milliamps (almost 1/3 of the normal parasitic draw) on its own, which means I will never leave it plugged into the always-on outlet when I'm not using it.

    Next, I tried charging a couple of iPad Pros. The first was a completely-discharged first-generation iPad Pro 12.9" that still has good battery life (it is about 2.5 years old, but the battery is good). It immediately started charging at almost 1200 milliamps (1.2 amps), which means it added over 1100 milliamps to the parasitic draw, which is very significant. Next, I tried charging a 10.5" iPad Pro (about 6 months old with a good battery) that was 76% charged when I plugged it in. It began charging at about 960 milliamps. I know that iPads themselves control the rate at which they charge, and testing I've done indicates that they draw less and less milliamps as they get closer to 100% charged.

    Finally, I tested my Resmed Airsense 10 Autoset CPAP machine (this is a medical device that helps you breathe when you are sleeping) using a 12v power adapter/brick and cord made by the manufacturer of the CPAP machine. The CPAP has a blower motor in it, so it has a large draw, and that draw is particularly high when you have a high setting (causing it to blow more air), which I do. It also has a heated humidifier and heated air tube that place a substantial extra power draw when they are used. The manufacturer says to not use them when you are running the CPAP machine on a 12v battery, so I turned them off for this test.

    With the heated air tube and humidifier turned off, the CPAP machine had a draw that ranged between about 600 milliamps to 3.5 amps (3500 milliamps). It ranges widely because when you breathe in, the motor automatically and instantly blows harder, and, when you breathe out, the motor blows softer. So, with each breath in and out, the power draw spikes and drops. Obviously this is a huge draw, and you would not want to use a CPAP machine for any amount of time unless the Prime was in "READY" mode so that the traction battery would power the 12v battery and the Prime would automatically turn on the ICE to charge the traction battery when it is low. You do not want to discharge your 12v battery or your traction battery (especially not your traction battery), so it is imperative that your prime be in "READY" mode if you are going to run a CPAP machine off the Prime's 12v battery.
     
    #1 Insighter, Aug 2, 2018
    Last edited: Aug 2, 2018
    Andyprius1 likes this.
  2. Insighter

    Insighter Active Member

    Joined:
    Feb 1, 2018
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    274
    0
    Location:
    SoCal
    Vehicle:
    2017 Prius Prime
    Model:
    Plug-in Advanced
    I just checked one other thing. One of the problems I had that discharged my 12v battery was that I had left my Carista* plugged into my OBD2 port all the time. It turns out that the OBD2 port is on even when the Prime is off, and the Carista more than doubles the parasitic drain of the Prime. On my Prime Advanced (that has a car alarm installed), the parasitic draw went up from about 65 milliamps (normal for my Prime with its alarm) to around 130 milliamps. That is, of course, a huge difference that will drain your 12v battery twice as fast when your car is parked and off. So you really don't want to leave anything plugged into your OBD2 port when your Prime is off.

    Result:
    Carista* Parasitic draw: 60-70 milliamps

    *The Carista is a device that plugs into your OBD2 port that lets you change some of the settings on your Prime (and likely other Prius models) like whether the Prime beeps the entire time it is in reverse, how many flashes the turn signal gives when you just lightly press it down without locking it into place.
     
    #2 Insighter, Aug 2, 2018
    Last edited: Aug 2, 2018
    Njner and Andyprius1 like this.
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