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Amp Draw on Car Battery

Amp Draw on Car Battery

(OP)
Hi everyone,

I have been trying to measure the amp draw on my car battery when the vehicle is off. When I unhook the negative lead and measure the amp draw from this, I initially get a reading of 1.9amps with my multimeter. However, after about 10-15 seconds this amp draw quickly drops to approximately 0.04 amps. Not being very electrically inclined I am not sure what to think of this, but I am trying to determine whether there is a parasitic amperage draw on my battery so the difference between these two readings is critical. Does anyone have any thoughts?

Renee

RE: Amp Draw on Car Battery

Probably capacitors. Put a voltmeter across the leads and see if the voltage is maintained for a while after the terminal is disconnected from the battery. It may not show up if the caps are charged via diodes. In that case, pull the fuses from any control modules and see if the same thing happens.

The later explanations make sense, but you can still pull fuses to see which components are active.

RE: Amp Draw on Car Battery

Perhaps a computer is waking up, when you make the connection, then going back to sleep.

RE: Amp Draw on Car Battery

I go along with VE1Bll, with the addition that today's cars have *lots* of processors, from engine ECU(s) to radio, brakes, sensors, air bags, and they're probably all waking-up, doing basic checks, probably checking with each other over the CAN bus, and then sleeping. And, if you have a embedded cellular (OnStar, security alarm, etc) they're probably waking-up handshaking with the local cell tower, and going into a standby mode.

It's a wonder a car battery can stay charged anymore.

RE: Amp Draw on Car Battery

The behaviour you're seeing is absolutely normal, and the 40 mA quiescent current at the end is also OK. No need to worry.

RE: Amp Draw on Car Battery

I've actually put a Fluke 8845A benchtop multi-meter in series with the battery on select Japanese (made and sold in Japan) models about a year ago (all 2015 or 2016 model cars). On all the cars the lowest standby current draw was measured to be 10mA to 12mA. But it takes a while to see the low values. Initially, I saw several amps, then it dropped to about 0.5A, then slowly went down to 100mA, but it took about 20 minutes for me to see the 10-12mA level (the time it takes for all the MCUs in the car to finally sleep). I also put the meter in averaging mode so I could confirm that the average current draw was indeed between 10-12mA. But again, that current consumption was measured on select Japanese makes. American and European models could be higher. But I suggest you leave your meter in the circuit for 20 min. or longer to see if it drops below the 40mA level you reported. 40mA seems a bit high, although nothing that would drain your battery in a matter of a few days. If you test for 20 to 30 minutes and still see 40mA, assuming your ammeter is a high quality device, perhaps there is a car alarm or other 3rd party electronics attached which contributes to that current draw. If not, then I would simply conclude that the computers in the car you are testing are simply not very efficient when in their sleep mode. Not a big deal UNLESS you have a lot of third party electronics you intend to attach, which would add to the overall current consumption.

RE: Amp Draw on Car Battery

Typical automobile has about 60 computers so several minutes until they all go idle is pretty reasonable.

Keith Cress
kcress - http://www.flaminsystems.com

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