chrisp
Well-Known Member
- Joined
- Dec 9, 2009
- Messages
- 1,877
- Reaction score
- 5,008
- Points
- 113
- Location
- Melbourne Victoria
- Members Ride
- VF2 MY16 SS Redline Sportwagon
I have noticed a few recent threads where batteries were going flat over a period of several days. The usual problem is either a faulty battery or something in the electrical system causing battery to go flat while the car is parked (aka a ‘parasitic load’). Parasitic loads can be difficult to find in modern cars as there are many different circuits and modules in the car, that tracing a small fault current can be like finding a needle in a haystack.
I thought I’d share an alternative current measuring technique that is both quick and easy to help identify the circuit that has the parasitic current.
But before diving in, I thought it would be helpful to cover a couple of traditional current measurement techniques, and then introduce the simple method. I think it is best to illustrate rather than write volumes to explain.
So, I rigged up a very simple circuit using a bench power supply, some clip leads, and a load (a map light). I also placed two current measuring meters in to the circuit: (a) a ‘clamp on’ ammeter and (b) a multimeter in ‘current‘ mode.
The circuit is…
And the actual physical set up is…
I’ve tried to keep the wiring (clip leads) reasonably tidy so that the details of the circuit are fairly apparent. The power supply is reporting 0.75A (@12.0V), the clamp-on meter is reading 0.78A, and the multimeter is reading 0.756A. There all in reasonable agreement on the current (give or take).
The problem with using either of these current measuring techniques in practice (and in a car) is that it is:
(a) not always easy to break in to a circuit to use a multimeter - a multimeter needs to be in series with the circuit so the circuit needs to be ‘broken’ in order to measure the current. However, it is possible to pull a fuse and use the multimeter (and a series fuse) to bridge the fuse and measure the current.
(b) the clamp-on ammeters are quite bulky are can be difficult to physically get on to a particular circuit or wire in the tight confines of a car.
I’m definitely not suggesting not to use the above techniques - they are both good and reliable methods and approaches to measuring currents, and both techniques are worthwhile understanding and using.
However, what I really wanted to share is an alternative technique that is very simple and only requires a multimeter and a little bit of calculating …
Notice, that in my rigged up circuit I placed a fuse in it. This was intentional - and it is also representative of a typical automotive circuit - they all have fuses. It’s the fuse that we can use to quickly and easily measure the current.
If you look carefully at the back of a fuse, they usually have a couple of openings to the terminals of the fuse. These can be used to measure the voltage drop across the fuse…
So, if we know the resistance of the fuse, we can use Ohm’s law to work out the current.
Going back to the bench circuit I rigged up (which has a current of 0.75A), I measure the voltage across the fuse…
The multimeter (in the millivolt range) reads 5.5 mV. This corresponds to 0.75A flowing through the circuit. Ohms law gives the resistance of the fuse as R = V/I = 5.5mV/0.75A = 7.3 milliohm.
Just to confirm, I did a simpler test where I placed the fuse directly across the output of the power supply, but I limited the power supply to 1A. The 1A (actually 0.99A) is giving a 7.3 mV reading across the back of the fuse.
This now allows me to determine how much current is flowing through a 10A fuse without (a) having to remove the fuse, or (b) having to get a clamp on meter on to a wire to that fuse. Hell, all I have to do is remove the cover of the fuse panel, and back-probe the fuses with a multimeter - easy. The hardest part is that I have to scale the millivolt reading back to amps. In this particular case (a 10A fuse), each 7.3 mV corresponds to 1 A. So, 5A should result in 36.5mV.
But what if the circuit a has a different fuse rating? Essentially, you’d need to determine the resistance of that fuse. As I had the circuit set up, I redid the above measurements with some other fuse ratings.
fuse : mV/A
5A — > 16.5 mV/A
10A —> 7.3 mV/A
15A —> 4.6 mV/A
20A —> 3.3 mV/A
The above should be treated as ‘ball park’ rather than absolute. But with a little bit of experience, it becomes very easy to check the (ball park) current flowing through a fuse by simply measuring the voltage across it fuse. This technique will struggle to measure down to the 10s of mA, but it certainly can be used to find 1/2A-plus parasitic loads. The example above was using a map light, and it was very easy to read the 7.3mV.
I thought I’d share an alternative current measuring technique that is both quick and easy to help identify the circuit that has the parasitic current.
But before diving in, I thought it would be helpful to cover a couple of traditional current measurement techniques, and then introduce the simple method. I think it is best to illustrate rather than write volumes to explain.
So, I rigged up a very simple circuit using a bench power supply, some clip leads, and a load (a map light). I also placed two current measuring meters in to the circuit: (a) a ‘clamp on’ ammeter and (b) a multimeter in ‘current‘ mode.
The circuit is…
And the actual physical set up is…
I’ve tried to keep the wiring (clip leads) reasonably tidy so that the details of the circuit are fairly apparent. The power supply is reporting 0.75A (@12.0V), the clamp-on meter is reading 0.78A, and the multimeter is reading 0.756A. There all in reasonable agreement on the current (give or take).
The problem with using either of these current measuring techniques in practice (and in a car) is that it is:
(a) not always easy to break in to a circuit to use a multimeter - a multimeter needs to be in series with the circuit so the circuit needs to be ‘broken’ in order to measure the current. However, it is possible to pull a fuse and use the multimeter (and a series fuse) to bridge the fuse and measure the current.
(b) the clamp-on ammeters are quite bulky are can be difficult to physically get on to a particular circuit or wire in the tight confines of a car.
I’m definitely not suggesting not to use the above techniques - they are both good and reliable methods and approaches to measuring currents, and both techniques are worthwhile understanding and using.
However, what I really wanted to share is an alternative technique that is very simple and only requires a multimeter and a little bit of calculating …
Notice, that in my rigged up circuit I placed a fuse in it. This was intentional - and it is also representative of a typical automotive circuit - they all have fuses. It’s the fuse that we can use to quickly and easily measure the current.
If you look carefully at the back of a fuse, they usually have a couple of openings to the terminals of the fuse. These can be used to measure the voltage drop across the fuse…
So, if we know the resistance of the fuse, we can use Ohm’s law to work out the current.
Going back to the bench circuit I rigged up (which has a current of 0.75A), I measure the voltage across the fuse…
The multimeter (in the millivolt range) reads 5.5 mV. This corresponds to 0.75A flowing through the circuit. Ohms law gives the resistance of the fuse as R = V/I = 5.5mV/0.75A = 7.3 milliohm.
Just to confirm, I did a simpler test where I placed the fuse directly across the output of the power supply, but I limited the power supply to 1A. The 1A (actually 0.99A) is giving a 7.3 mV reading across the back of the fuse.
This now allows me to determine how much current is flowing through a 10A fuse without (a) having to remove the fuse, or (b) having to get a clamp on meter on to a wire to that fuse. Hell, all I have to do is remove the cover of the fuse panel, and back-probe the fuses with a multimeter - easy. The hardest part is that I have to scale the millivolt reading back to amps. In this particular case (a 10A fuse), each 7.3 mV corresponds to 1 A. So, 5A should result in 36.5mV.
But what if the circuit a has a different fuse rating? Essentially, you’d need to determine the resistance of that fuse. As I had the circuit set up, I redid the above measurements with some other fuse ratings.
fuse : mV/A
5A — > 16.5 mV/A
10A —> 7.3 mV/A
15A —> 4.6 mV/A
20A —> 3.3 mV/A
The above should be treated as ‘ball park’ rather than absolute. But with a little bit of experience, it becomes very easy to check the (ball park) current flowing through a fuse by simply measuring the voltage across it fuse. This technique will struggle to measure down to the 10s of mA, but it certainly can be used to find 1/2A-plus parasitic loads. The example above was using a map light, and it was very easy to read the 7.3mV.
