I'm using a friend's car battery charger on my 2016 Mazda 3, and the amperes meter alternates between max (8) and zero every 5 or 6 seconds. Normally it goes to about half way (3ish) and slowly decreases to zero. I disconnected the leads on the Mazda's battery from the car and the same thing happens. Why is this happening?

  • Sounds like a loose wire. It either draws a bunch when it connects because the battery is kind of dead, or nothing at all. Apr 29, 2019 at 22:45
  • I think it's too regular for that, and it works normally on another car. Apr 29, 2019 at 22:52
  • 2
    Just in case disconnect the battery from the car and test again, but I think the battery is faulty.
    – Solar Mike
    Apr 30, 2019 at 4:08
  • 1
    I wonder if the charger is hitting full power and has some kind of safety limiter that is kicking in? You could try connecting it to 2 batteries simultaneously using jumper cables to see if it behaves differently. Apr 30, 2019 at 6:56
  • @SirSwears-a-lot so decrease the effective resistance of the load to increase the current from the charger - is that wise?
    – Solar Mike
    Apr 30, 2019 at 7:09

1 Answer 1


A dead (but not necessarily defective) battery will draw a lot of current from a charger. The charger may not be able to supply that current for very long (a few seconds) before it will go into a self-protection mode (will stop charging the load by some mechanism). The charger will then "recover" and once again start charging. The visible result of this is the ammeter going from full-scale to zero over and over. Normally, once the battery recovers some internal charge, the charger will settle down.

  • I think this is it. I jumped the car and it ran but was dead the next day, but now the trickle charger works fine. May 2, 2019 at 2:55

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