Multimeter DC Power Measurement

John.G

Multimeter.jpg

I can measure the above by inserting the red lead in the "10A" socket, you then turn the selector switch to 10 and measure the required circuit. you the turn the switch to get the amp reading, you may get 50ma on the 200ma setting. Is the meter calibrated to read, 50/200, 25% of range on this setting or is it only 0.05/10, 0.5% of range. The reason I ask is that there is a fused 200ma max measuring point with the red lead in its normal position. Should a 50ma load read the same on this and on the 10A range if the selector switch is switched to the 200ma setting with the red lead in the 10A terminal?.

I have never used the fused one when checking parasitic losses in cars with the battery negative strap disconnected.

Forgot to add the multimeter image.😀

10-10-20

It's normally just using factorisation due to the limited count of digits on the DVM.

Some meters will differ, but on a 4-digit meter here the 10A setting places the decimal-place as follows: 0.000A. For the mA setting it shows 00.00mA, but the decimal place will move one to the right when reading greater than 99.99mA.

So you're reading the value in the notation of the measurement - 50.00 in the 200mA range would be 50mA and this should show as 0.050A in the 10A range.

But just one thing to note when measuring larger power-sources than the meter's main fuse can take - you need to be careful of capacitive draws when the meter is inserted to measure current. Often the device under test will drain down the residual current and when it's reenergised it may create a brief surge as capacitors and devices are re-energised. This can often be above 10A for a very short duration and may short the meter's fuse. To avoid this, touch the negative cable off the battery briefly and then break the connection and attach the meter-leads.

Wearb

Another way to check a car for parasitic leaks is to measure voltage across the tops of each fuse.

If e.g. you identify the interior light fuse and measure across it with door opened and closed you will see what I mean.

John.G

Yes, there is a post on that in motors diy with fuse ratings and millivolt drop for each rated fuse, I find it difficult to see how you can get a accurate summation as I don't know how many fuses to check say in a 2019 VW Polo, every one of them??.

Bus’ battery doesn’t survive light use — boards.ie - Now Ye're Talkin'

Wearb

You don't need to know the voltage drop amount, just note if there is a drop. There won't be a notable drop unless some current is flowing. The old way of doing it is as you described, disconnecting a battery lead and putting mm in it while pulling each fuse. The other way is easier.

On another note, a failing diode in the alternator is a common cause of battery drain.

John.G

But we are taking about a parasitic loss which I consider to be < 100ma, I would consider anything greater than say 200ma to be a problem, all my VWs had a parasitic loss of 40 to 60ma, bar one, a 2005 Polo still in the family which pulled 95ma from day zero and still does but not a problem as the original battery lasted approx 12 years.

Up to 200ma really should not be a problem for normal daily driving as its only 2.4AH overnight or 4.8AH if stopped for 24 hours and if left at the Airport or whatever for a week or so it will deplete the battery by 34AH. I used to leave that old Polo for 2 weeks at the Airport and for at least 7/8 years it would still start no problem after a battery depletion of ~ 32kwh (95ma parasitic loss apparently). I used to disconnect the battery negative terminal for the last 3/4 years of the battery before replacing it purely as a precaution. I have a 2019 Polo as well but it seems to be a absolute PITA to remove any battery terminal but I have VCDS which gives some very interesting data re the battery which I will post sometime soon.

Wearb

It's over 40 years since I worked as an automotive electrician and between changes to electronics and my memory I could only give a basic overview. Things have changed a lot with onboard computers staying awake for periods or wakening up when the key even comes close to the car etc. It's all plug in diagnostics now and not as straightforward as yesteryear.

10-10-20

That's a good point Wearb, the parasitic loads these days are often timed or work off interrupts.

In the graph below I'm monitoring a Kona electric EV's 12V battery voltage over a period. You can actively see the battery reduce from 13v to 12.7v over 3 hours. The high peaks of 13.7 are actually the ECU waking up every 4hrs 20mins to do some tasks and check the state of the 12v battery and apply a charge to it from the main battery. From this you can infer that there is a load which is bringing the voltage down from 13v to 12.7v, but you would have to do more probing/measuring to understand that better.

Anyway, the point here is that loads aren't linear these days, you sometimes really need to take the long-view.