Voltage drop on car battery. 1

[pauljohn]

Can anyone tell me what would be the acceptable voltage drop per day or week on a battery in a car that wasn't being used? If car is new, has factory radio/alarm/etc.


Is there a standard that companies use on how long their car can sit unused before it fails to start?

 

[jbthiel]

One tech bulletin I found stated that a fully loaded GM vehicle could have an expected parasitic drain of 25-35mA. Seems like a lot, I think that we try to stay in the 10-15mA range. (off road vehicle)

 

[IRstuff]

Sorry to be so late. The $10 meters that I've bought recently all come with a 10A scale, so there should be no problem with measuring large static currents from a car battery. If the battery is draining down in a week, the current draw would be on the order of 1 to 2 amps.

TTFN

 

[pauljohn]

You could be right. However what I wanted to do was measure volt drop on the battery every day to see if the fault was there not measure amps. As I mentioned in a previous post disconnecting the battery would reset the computer and possibly remove the problem as it was erratic and I did not want to do the mechanics job for him only tell him the fault was still on the car.

I finally got the answer I was looking for from a battery tech. who told me that a 0.02-0.03 volt drop was about average for lead acid batteries although it varied quite a bit with humidity/temp type of vehicle etc. This would roughly agree with greglococks remark that 6 weeks would be a minimum for cars left unused.

Sure enough when I checked volt drop on the battery it was dropping 10 times that fast at about 0.2 volt per day.

Another trip to the dealer and they tell me that the radio was pulling 1 amp when everything was off so they think they finally caught the problem.

I'm still checking the volt drop but so far everything looks good. Thanks for all the replies.

 

[engtwo]

ON A 4CYLINDER CAR, WHAT IS THE VOLTAGE READING WITH THE ENGINE TURNED OFF.
WITH THE SIDE LIGHTS ON
CRANKING THE ENGINE SIDE LIGHTS OFF.

 

[GregLocock]

IT SHOULD BE MORE THAN EIGHT VOLTS

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[patprimmer]

My old Beetle dropped to 4 volts ;-)

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Warpspeed]

Yes Pat, but they only ever had half a battery to begin with.

 

[patprimmer]

Ah, but they are 4 cylinder and they do have side lights, so they match the "VERY BROARD" question.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[statorwinder]

If one dosen't have a clamp on low amp meter
Here is a procedure that does the job for measureing
drains
1. parallel another battery across the vehicle's battery
2. disconnect the vehicles battery
3. install a 1 ohm 10 watt resistor in series with the vehicles battery
4. disconnect the other battery...with the 1 ohm resistor installed enough current will flow keeping all memory circuits active
5. with a volt meter measure the voltage drop across the 1 ohm resistor
6. the voltage drop reading represents the current draw

 

[gaber2]

thats a good topic and let me share u it.
i just read the all posts,and just have a comment on what statorwinder said.
i just don't understand why we had to use another battery which is connected parallel to the vehicle battery,what if we connected the resistor in series as u said,and measured the volt drop,and then we would get how much current flow through the resistor?
why should we use the other battery?

 

[MacGyverS2000]

The parallel battery will prevent all of your circuits from resetting while you place a resistor in series with the original battery.


Dan - Owner

http://www.Hi-TecDesigns.com