How long does our battery last???? - Page 2

Aug 17, 2013 | 06:24 PM

#26

mleskovar

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Joined: Nov 2001

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From: Huntington Beach, Ca.

'17 Jaguar XF

Quote:

Originally Posted by pdjv

..."When measuring SoC by open circuit voltage, the battery voltage must be truly 'floating' with no load present. Installed in a car, the parasitic load present makes this a closed circuit voltage (CCV) condition that will falsify the readings. Adjustments must be made when measuring SoC in the CCV state by including the load current in the calculation. In spite of the notorious inaccuracies, most SoC measurements rely on the voltage method because it’s simple."...

That kind of accuracy isn't necessary and that's why it's usually measured this way. We're not talking transistor bias voltages. The only draw on a battery with the ignition/radio/all lights off should be the minimal amount required to maintain the memory functions and analog clock if present....mice nuts to a car battery. If you really want/need that kind of accuracy then you also have to take into account the time from the last charge cycle, time at rest, and temperature. Not practical in this application. I've measured both ways and saw no difference with my DVOM. Just hook it up to the +charge connector under the hood and ground and you're accurate enough. No, I'm not an EE

Aug 17, 2013 | 07:34 PM

#27

thenew3

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Joined: Dec 2003

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From: Central Coast, CA

2020 X7 xDrive40i, 2016 X3 sDrive28i

Quote:

Originally Posted by mleskovar

No, I'm not an EE

I am an EE and I agree 100% with what you've said.

Also, most consumer Voltmeters (i.e. those under $100) don't have the accuracy necessary to measure any difference with the batter loaded by the car computer/parasitic drain vs no drain.

Aug 18, 2013 | 11:33 AM

#28

Arrie

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Joined: Oct 2010

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From: Southern US

2010 E550, 273 Engine: 2012 S550, 278 Engine

Quote:

Originally Posted by thenew3

I have tested the "parasitic" drain on two of the cars I used to have. These cars did not have all the "gizmos" that this E has so there was no possible harm done by disconnecting the battery.

I simply took the cable of the negative battery terminal off and connected my digital Fluke between the terminal and the cable and measured the amps the car draws when everything is off. After figuring that the automatic hood light was messing with my results and disabling it I got 0.05A pull on both of these cars.

How much would that lower the "rest" voltage of the battery if it is connected in the car?

I know these moders cars have more running in the background but this is all modern computers and I would not be surprised if they would pull even less "parasitic" load than the older cars I measured.

Aug 18, 2013 | 09:37 PM

#29

mleskovar

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Joined: Nov 2001

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From: Huntington Beach, Ca.

'17 Jaguar XF

Quote:

Originally Posted by Arrie

... it I got 0.05A pull ...How much would that lower the "rest" voltage of the battery if it is connected in the car?....

I'm too lazy to do the math (it's not linear) but to put it in perspective.....our batteries = 85 amps, or 85000 milliAmps. A 50 milliAmp draw = 6/100 of one per cent. That's why the voltage method is used as a quick check. It doesn't account for breakdowns in cells when a lot of current is needed....like starting...but just gives you a battery health check.

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