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3.6 mA Parasitic Drain - starter/alternator line

Help, Advice and DIY Tutorials on Volvo's extremely popular car line -- Volvo's 1990s "bread and butter" cars -- powered by the ubiquitous and durable Volvo inline 5-cylinder engine.

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Re: 3.6 mA Parasitic Drain - starter/alternator line

Post by Eddystone » Sun Aug 11, 2019 11:55 am

abscate wrote:
Sun Aug 11, 2019 8:08 am
Most voltmeters won’t reliably measure mA of current so don’t base those measurements as being too reliable. You can get surface leakage in the battery from moisture at this level too, so you can really chase your tail on this
"Most voltmeters"

If you buy a piece of crap ratchet, it will break. If you buy a piece of crap multimeter, it won't measure accurately.

I worked in electronics for 20 years with various kinds of high tech and low tech electromechanical devices some of which were highly precise and could not function with a few millivolts of ripple on a DC supply voltage.

I will tell you that 3.6 mA is not an insignificant flow of current, that it is easily measured by a decent modern meter with good specification, and that 3.6 mA at 12 VDC is not going to somehow leap across the battery from post to post, moisture or not.

As I have said, I am not concerned about the 3.6 mA and won't be chasing it down, but any kind of constant current draw between either the starter or the alternator and ground indicates that there is a defect somewhere.


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Re: 3.6 mA Parasitic Drain - starter/alternator line

Post by abscate » Sun Aug 11, 2019 1:56 pm

Your battery will be self discharging at about 1/2% per day , equivalent to a discharge of 40 mA per hour.

I’m guessing then 4mA is the diode leakage


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Re: 3.6 mA Parasitic Drain - starter/alternator line

Post by Eddystone » Sun Aug 11, 2019 2:21 pm

abscate wrote:
Sun Aug 11, 2019 1:56 pm
Your battery will be self discharging at about 1/2% per day , equivalent to a discharge of 40 mA per hour.

I’m guessing then 4mA is the diode leakage
I don't know what it could be except the diodes, but they are not supposed to be passing current. 3.6 mA leakage across a silicon diode is huge.

A continuous 3.6 mA over the course of an hour is probably a lot more than than 40 mA.

If the battery is discharging, I am sure that is inside the battery, not a current you can measure from post to post as a flow of current.

As long as it doesn't pull my battery down enough to not start the car, it doesn't matter. The old battery with the bad cell was low to start, and would not have recovered to a full standard charge.


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Re: 3.6 mA Parasitic Drain - starter/alternator line

Post by Eddystone » Mon Aug 12, 2019 7:12 am

If you use a meter that can't reliably measure mA, even at low current levels, it isn't a useful tool, and you might as well use one of those light bulb continuity testers. There's no point in using a tool that is incapable of doing what it is intended to do, and you need to understand how multimeters work and what their specifications tell before you purchase one.


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Re: 3.6 mA Parasitic Drain - starter/alternator line

Post by erikv11 » Mon Aug 12, 2019 12:56 pm

Since there are several unknowns / assumptions at play here ... what reading do you get if you disconnect the cable to the starter/alternator and measure draw through only the fuse box cable? I know you measured the same draw with and without the fuse box cable, I saw that, just curious if you actually then see zero through it as would be expected.


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Re: 3.6 mA Parasitic Drain - starter/alternator line

Post by PeteB » Tue Oct 08, 2019 3:07 pm

It is very true that there are many junk meters out there, I use a Fluke which is very reliable.
I'm an EE and the reverse leakage in a Silicon diode should be far less than micro amps,
really it should be no more than pico amps. Si junctions show increased leakage current
when they are damaged or failing.
The battery in our 850 is also going dead overnight but I'm not able to measure much of
any leakage current. I believe that it is intermittent.
Edit: fuse was blown in the meter now reading 500+ mA THAT should be easy to find pulling fuses!



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