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Thread: battery issues

  1. #1
    Registered Member

    Join Date
    Apr 2019

    Exclamation battery issues

    hello, I bought a P5 and it had an after market amp installed but it wasnt working in it. long story short there was a fuse on top of the fuse box that was installed and an amp that has a ground wire also installed. it seemed to work for 2 days but when i tried to start the car it wouldnt turn on because the battery was dead. i then removed the after market radio and waited a few hours. after i came back, when i put the key into the car lights came on but the engine was still unable to turn on.

    im assuming that i have a parasitic draw from the amp, but im not sure how to test this. can someone please help me find the battery draw or maybe my battery is just old. if the latter case is true how would i go about testing a battery... does the auto store test my battery for me with their own meter?

    thanks for the help in advance

  2. #2
    Registered Member


    Join Date
    May 2016
    You need a load tester to test the battery. Any auto parts store will test the battery for free.

    As for the parasitic draw, you need a digital multi-meter capable of reading current (in either Amps or mA). if it is a fluke use the 10A setting. close all the doors, and let the lights and stuff go off. Then disconnect the negative battery terminal. Connect the negative probe of your meter to the negative battery post, and the positive lead of your meter to the terminal you removed and set the meter to the 10A setting. I don't know about these cars specifically, because I never tried. But sometimes they will start out high, and it takes 20 minutes or so for all the computers and stuff in the car to go to sleep mode, before you can get your true idle current reading. generally speaking, you want to see less than 50mA (0.050 Amps) of current draw when the car is off. IF it's over 50mA you've got something drawing. If it's dying overnight, then I'd expect to see something in the range of 200-300mA (0.200-0.300 amps).

    Get your reading. If it's high and you suspect the amplifier, then disconnect the amplifier and re-test.

    I say 50mA as a guideline, but that's max on the high end. realistically, most cars when they are asleep are around 10-15 mA (0.010-0.015 Amps).

    Radios draw very little unless something is wrong, maybe 1-2mA.
    If you car has an alarm system, it may add around 10mA.
    phone chargers and stuff can draw a few mA if connected to battery source with no phone plugged in.
    phone chargers can draw 1-2 amps (depending on the wattage of the charger) with a phone plugged in and charging.
    Dome lights in our cars suck, but can draw up 300+mA if left on
    If you replaced the dome light with a LED dome light, it will typically draw around 20mA if left on

    IF it is not the amplifier, then you want to go through the fuse block, and remove 1 fuse at a time until the draw goes away.

  3. #3
    The Diagram Dude pcb's Avatar

    2002 mazda protege 5

    Join Date
    Jun 2011
    ontario, canada
    Our car is supposed to have 20 mA or less of back-up current.

    Check the light in your trunk too. That's an easy one to forget and leave on.
    Last edited by pcb; 05-09-2019 at 08:47 AM.
    The Diagram Dude

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