5

How much current in amperes is flowing while a car is completely turned off? How long can it sit before battery is drained enough to be a problem?

5

On most (all) modern cars there will be a small drain while the car is off. Some of the causes are:

  • Clock
  • Alarm
  • Keyless entry system
  • Perhaps the radio (anti-theft)

The drain should be small, the maker designed the electrical system to accommodate a small draw and the car should be able to handle it for at least a week, maybe even longer. The owner's manual may give you guidance. But if you find that the car won't reliably start after a few days or even a week, odd are good that your battery is reaching the end of its life or that you've got an "unplanned" draw.

If you think something is drawing more than it should the first thing to do is to look for lights that shouldn't be on – say a trunk light or maybe a vanity light on the visor mirror. Or what happened to me, an inverter without a power switch. After that you can check by watching an Amp meter (pull one of the battery terminals and connect it between the terminal and the battery post) as you pull fuses. That will tell you where the load is.

  • 2
    I'd advise against disconnecting the battery to measure draw. Either voltage drop fuses or use an inductive probe on one of the battery cables. Disconnecting the battery can lead to a loss of the source of the draw on newer cars. As well as reset monitors and stored values in the ECM or other computers. – Ben Jul 14 '16 at 20:47
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It depends on the car and battery. Look at the information label on the battery. The amount of acceptable draw is Reserve Capacity divided by 4 = x mA. So if a battery has a 100 minute reserve capacity, 100/4 = 0.025A or 25mA.

Typically (again depending on car and battery) you shouldn't see > 50mA. Anything more may indicate a parasitic drain or a computer hasn't gone to sleep yet.

Reserve Capacity is the amount of minutes a battery can discharge 25A at 80°F before dropping below 10.5V.

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