Parasitic drain - is 55 milliamps ok?

Yes he was clearly using both of the meters in series but the second meter should be using the 10a terminal. I have the 210e with dc clamp on but find it is also inaccurate at low ma. Believe it or not the harbor freight free meters work fine in series for low ma. As do many others.

 

I wonder if even non-cheap-o, moderately priced DVMs are not going to work with this method. My DVM, a Radio Shack 22-813, was (if memory serves) roughly $30-$40 about 15 or 20 years ago, and it measured "0.0 mA" (precision of one-tenth of a mA) at every fuse and fusible-link, including one (DOME) that turns out to have a current loss of 12-28 mA. The specs for this DVM say it has an input impedance of 10 Mohm, and the accuracy in the 400 mV range is +/- 4 in the last digit, which I believe (please correct me if I'm wrong!) means that this meter can be off by up to 4 mV. From a webpage with more info on determining current from mV measurement across a fuse (link below), I gather that this is nowhere near enough accuracy:\
https://www.meeknet.co.uk/E64/Fuse%20Voltage%20Readings%20To%20Give%20Current%20Readings.pdf

My point: don't think that anything better than a $4 cheapy is going to be good enough. (I know that rjparker didn't exactly say that.)

-Sean

 

The fuse test is using mv not ma with the fuse in. You measure across the fuse. It depends on the very low resistance of a fuse to create a very small millivolt drop.

 

I know, but thanks. I measured mA across the DOME fuse contacts (with that fuse out) only because measuring mV at all fuses (fuses NOT removed) showed nothing. This was after I also tried, out of desperation, pulling the DOME fuse to see if the total parasitic loss (measured through battery neg. connection to car body) would drop, and it did.

 

Sounds like the fuse inside the meter is blown. I usually check mine every time I use it by seeing that the meter can pass enough current to illuminate a small test light in the Amp range.

The meter's mA range (dedicated jack) is useless for automotive stuff, that tiny fuse will pop if you blink at it. "Most" meters have a milliamp reading on the amp range jack- that's good enough resolution for testing key off draw.

Also, if you want to read mV drop across a fuse (for small draws), your meter has to be capable of of 0.1mV resolution.

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While I can believe that the drop across a fuse in a normally active circuit, moving several amps, would be in the mV range, that seems really unlikely for a parasitic draw which is itself only 55 mA. (Really it is more like 35 since there is always a parasitic current of 20 mA on a Prius which is working normally.) Making up some numbers, let's say we measure 5 mV across a fuse on a 5A circuit, rounding the parasitic draw down slightly to 50mA that is 1/100th as much current, so it would be .05 mV, or 50 microvolts, across the fuse. It would take a very, very good meter to measure that.

On the other hand, it isn't hard to measure 50 mA with an inline ammeter. I suppose one could build a tester for this by pulling the fuse and plugging a testing jig into that socket which has a circuit path between the plug prongs consisting of: fuse holder, (in parallel: ammeter, shunt switch). Put the fuse in the fuse holder and close the shunt switch. Turn the car on, turn the car off. Wait until car is "quiet", open shunt, read ammeter.

Seems like a lot more work than just pulling fuses one at a time though!

 

This thread seems to have become an academic exercise since the Op used a new meter and read 17ma. Probably was chasing a ghost.

No need to make up numbers, since a 5 amp fuse is 17.5 milliohms. Each fuse size has a milliohm value.



In reality, you measure the voltage first and apply either ohms law I=E/R to calculate the milliamps or use a chart. As an example, a 0.5 millivolt reading across a 17.5 milliohm (5 amp) fuse is 28.57 milliamps.

This chart rounds it to 30 ma.



Of course a 30-50 ma parasitic draw is not a big concern; it is the 200-1000 ma or more drains that kill a battery in a day or two. Using a mv fuse check is quick, does not cause ecus to reboot due to power interruptions and prevents fuses from being returned to the wrong spots. Once it is narrowed down to one fuse, subcircuits may need to be disconnected while monitoring millivolts or milliamps.

Many under $40 meters are capable of reading tenths of millivolts. Accuracy within a couple of tenths is enough as well.

South Main Auto finding a 591 ma drain

 

Those fuses are higher resistance than I expected and the extra 10X raises the voltage to where a low current (10s of mA) is detectable with a multimeter. Just. Live and learn.

 

A somewhat comprehensive mv fuse chart which includes several fuse types