Parasitic battery drain advice please

[john-e89]

I’m sure this has been covered but I can’t remember when chaps so can anyone point me in the right area?

Dropped a new Bosch battery in two weeks ago, it’s now so flat it barely lights the interior light, I charged it up 4 days ago with a Ctek to full charge and it’s as flat again after no use. I haven’t as yet tested the alternator as I can’t start it until it’s charged again tomorrow but there must be another drain somewhere as it hasn’t been used.

Cheers in advance.

[Mangozac]

It's a painful process to do. When I was having parasitic drain issues I would hook an ammeter in line with the battery and wait the several minutes it took for the car to go to sleep. At sleep the draw should be somewhere around 45mA. Mine was intermittently drawing a lot more than that and I then pulled fuses until it went away.

In the end it turned out to be that my gaptech module was faulty and kept waking up other modules!

[john-e89]

It's a painful process to do. When I was having parasitic drain issues I would hook an ammeter in line with the battery and wait the several minutes it took for the car to go to sleep. At sleep the draw should be somewhere around 45mA. Mine was intermittently drawing a lot more than that and I then pulled fuses until it went away.

In the end it turned out to be that my gaptech module was faulty and kept waking up other modules!

Oh joy...

Thanks very much Mangozac, I was afraid it’s trial and error...at least it’s a toy so time off the road doesn’t matter, apart from no smiles....

[ph001]

To check for parasitic current draw, get a basic voltmeter and measure the voltage drop across all the fuses in the fusebox behind the glovebox (each fuse has a couple of test points on the top so you can measure across them in situ). Any volt drop over the below indicates a fault current flowing somewhere:

5A fuses - 2mV (indicates approx 100mA flowing through it)
10A fuses - 1mV (indicates approx 130mA flowing through it)
20A fuses - 1mV (indicates approx 300mA flowing through it)
30A fuses - 1mv (indicates approx 500mA flowing through it).

This should be done with the car in sleep mode, i.e. all accessories off, car locked with you inside (so press lock on keyfob twice to disarm interior microwave sensors) and wait 10 mins.

Also don't forget there are some other larger fuses under the bonnet next to the ECU bucket...

[derin100]

To check for parasitic current draw, get a basic voltmeter and measure the voltage drop across all the fuses in the fusebox behind the glovebox (each fuse has a couple of test points on the top so you can measure across them in situ). Any volt drop over the below indicates a fault current flowing somewhere:

5A fuses - 2mV (indicates approx 100mA flowing through it)
10A fuses - 1mV (indicates approx 130mA flowing through it)
20A fuses - 1mV (indicates approx 300mA flowing through it)
30A fuses - 1mv (indicates approx 500mA flowing through it).

This should be done with the car in sleep mode, i.e. all accessories off, car locked with you inside (so press lock on keyfob twice to disarm interior microwave sensors) and wait 10 mins.

Also don't forget there are some other larger fuses under the bonnet next to the ECU bucket...

That's a good bit of info to know for any future references!

[john-e89]

To check for parasitic current draw, get a basic voltmeter and measure the voltage drop across all the fuses in the fusebox behind the glovebox (each fuse has a couple of test points on the top so you can measure across them in situ). Any volt drop over the below indicates a fault current flowing somewhere:

5A fuses - 2mV (indicates approx 100mA flowing through it)
10A fuses - 1mV (indicates approx 130mA flowing through it)
20A fuses - 1mV (indicates approx 300mA flowing through it)
30A fuses - 1mv (indicates approx 500mA flowing through it).

This should be done with the car in sleep mode, i.e. all accessories off, car locked with you inside (so press lock on keyfob twice to disarm interior microwave sensors) and wait 10 mins.

Also don't forget there are some other larger fuses under the bonnet next to the ECU bucket...

Thank you very much for that, very helpful and much appreciated.

[Smartbear]

To check for parasitic current draw, get a basic voltmeter and measure the voltage drop across all the fuses in the fusebox behind the glovebox (each fuse has a couple of test points on the top so you can measure across them in situ). Any volt drop over the below indicates a fault current flowing somewhere:

5A fuses - 2mV (indicates approx 100mA flowing through it)
10A fuses - 1mV (indicates approx 130mA flowing through it)
20A fuses - 1mV (indicates approx 300mA flowing through it)
30A fuses - 1mv (indicates approx 500mA flowing through it).

This should be done with the car in sleep mode, i.e. all accessories off, car locked with you inside (so press lock on keyfob twice to disarm interior microwave sensors) and wait 10 mins.

Also don't forget there are some other larger fuses under the bonnet next to the ECU bucket...

Are you sure about that? If the fuse hasn’t blown then in effect your measuring across an unbroken piece of wire and there won’t be a potential difference?
You need to be measuring across the load (or fault) to measure a volt drop I think
Rob

[Mangozac]

Smartbear, the technical reasoning is sound, but in practice I'm not sure now effective that technique would be. Yeah, fuses are essentially a piece of wire but as with all wires, depending on the characteristics, there will be a minute voltage drop across it.

You would want a quality multimeter to be giving reliable readings down below the 1mV scale. And very sharp probes, as a bit of resistance in the probe connection can easily add a few millivolts and thus make you think an active circuit is inactive.

[ph001]

Fuses by their nature contain quite thin conductors so ANY current through them will cause a volt drop across them, which is what the table is based on. The voltages are quite low because the resistance of the fuse is quite low. Even budget voltmeters tend to be quite good at reading voltage though, even mV.

And very sharp probes, as a bit of resistance in the probe connection can easily add a few millivolts and thus make you think an active circuit is inactive.

You have to remember that you only get a volt drop where you have current flowing. As there is no current flowing into the meter then the resistance of the probe connections are irrelevant - even if you had hundreds of ohms on your probe connection, the voltage reading across the fuse would be the same as if you had zero ohms. That's why it's such a good way of measuring things - it's fairly immune to measurement error.

[Mangozac]

You have to remember that you only get a volt drop where you have current flowing. As there is no current flowing into the meter then the resistance of the probe connections are irrelevant - even if you had hundreds of ohms on your probe connection, the voltage reading across the fuse would be the same as if you had zero ohms. That's why it's such a good way of measuring things - it's fairly immune to measurement error.

You're right, I was confused with the significant voltage drop that can be generated if the contact is a high resistance and thus dividing with the internal resistance of the multimeter. I had such an issue recently with some crappy multimeter probes and corroded contacts but of course the fuses should not have that degree of oxidation.

Still, I'm sceptical about the ability of a cheap meter to be able to reliably resolve the sub mV difference caused by 200mA flowing through a 20A fuse, as was the case with the faulty Gaptech module. I'll certainly give it a go with my Fluke meter if I ever find myself with this problem again though!

[ph001]

You got it - if the connection is in series with the current then it causes problems just like you describe. Fortunately with the meter on volts you are making a high impedance parallel connection so no such issues.

Even my crappy £6 DVM from Amazon has a 200mV range that can resolve down to 0.1mV on the display. Probably not that accurate down there but doesn't need to be as we are only looking for an indication really.

Of course, finding which fuse the parasitic current draw is going through is only half the battle. The real fun then begins trying to isolate the exact cause as individual fuses often feed several components. It's nearly always the CD changer / Stereo / Fag lighter / internal illumination ones when you are looking at leakages in to 100mA - 1A region. Most people also plumb their ancillary gadgets into that circuit as well.

[john-e89]

Brilliant posts, thanks very much chaps.