12V current draw when car is off?

Has anyone measured the current draw on the 12V battery when the car is off? Since late this spring I've been having 12V problems with my 2.5 year old FFE. This past Wednesday after I parked the car at 1pm, by 5pm my 12V battery went down to 8.1V. I jumpstarted it to drive home and see what troubleshooting I could do. I accidentally cleared the DTCs when I hooked up my PC to see what codes had been set. When first shut off the car was pulling over 3A from the 12V and dropped from there as things shut down. I waited 45 minutes to make sure everything was shut down and the car was still pulling 0.4A I would have expected about 0.1A or less. Am I the only one that thinks 0.4A load is high for a shut down car? FYI I was measuring with a Fluke 381 meter which I admit isn't really intended to measure low current.

UPDATE 7-17: Anyone reading this for the first time not wanting to read thru the thread for the answer; 50mA (0.050A) is the maximum current draw when the car is off. You need to wait 40 minutes for everything to go to sleep and if you unhook the battery to put a meter in that resets the 40 min clock.

0.4 Amps seems high. What do ICE's draw when off?

I guess you could also do a calculation, Ah rating of the battery and how long it would take to get the battery to zero.

I have no idea what the draw is. However, I often leave my FFE sitting at the airport for a week or more. I have had a dead 12 volt battery twice in a little over two years. Both times it happened at home and the car had been driven less than 12 hours before the battery drained. I actually avoided installing the MFM app on my phone since I had a theory about the cause of the drain. I still have my original 12 volt battery that has drained twice and it still works after sitting for up to 10 days unplugged. Both of the dead 12 volt battery experiences happened before I had it reprogrammed to prevent the problem on my 2015. Haven't had it happen since.

So it was dead on me again Friday morning. I'll chalk that one up to user error since I think I left the auxiliary lights on. I also was using a clamp on meter that's intended for measuring hundreds of amps not trickle current drain so that 0.4A I measured was probably wasn't real. I found out the Ford spec for maximum current drain is 50mA or 0.050A. You need to wait 40 minutes for everything to go to sleep to get down below this current level. Unhooking the battery and then connecting a meter in series to measure the current resets the 40 minute timer as that will wake things up. My FFE pulls a little over 3A at first shut down and then it slowly drops as modules go into sleep mode. I came back an Hour later and the current draw was 20mA well within the 50ma max allowed. I checked the battery to see if it was damaged and it was good measuring 5 mOhm internal resistance. with an estimated cranking amps of 534A. I also checked all the DTCs (diagnostic trouble codes) and I didn't see anything other than codes set due to the battery being low and causing communications errors as modules lost enough juice to function. So who knows why it randomly went dead on me. Maybe I left the parking lights on last Wed. after lunch. I don't remember turning them on but who knows. I'll just keep an eye on things.

If anyone else is having 12V problems you can tell the dealer about TSB 16-0024 which hopefully the dealer should be smart enuff to find on their own without you telling them about it. This TSB details troubleshooting dead 12V battery issues with the FFE where 'off' 12V current draw is greater than 50mA. I am not certain but I believe my dealer did this procedure back in May/June when the dead battery caused my traction control to freak out. Part of the procedure is reprogramming the computer which I know they did.

:evil: 7-24-18 again when driving around dinner time my FFE 12v was dead after driving at lunch. Of course the dealer says there is nothing wrong with it. They have started it and driven it every day or so and can't seem to duplicate the problem.

Perhaps one of these little guys might help with diagnosis:
https://moosh.im/mooshimeter/

It's a very compact bluetooth multimeter that could be mounted in the car for data logging (also has a microSD slot IIRC)

For current measurements, you could plug in an external transducer ( e.g. https://www.amazon.com/Mastech-CA100-Digital-Clamp-Transducer/dp/B01CV7WH0A/ref=sr_1_3?ie=UTF8&qid=1480289990&sr=8-3&keywords=mastech+DC+current+clamp ) or just estimate from the small voltage drop across the battery's ground cable (example set up here:
https://moosh.im/mooshimeter/example-videos/ ).

Mine is at the dealer right now for this problem. Will be interesting to hear what they find out.

campfamily said:

Mine is at the dealer right now for this problem. Will be interesting to hear what they find out.

Click to expand...

UPDATE: Dealer diagnosed a bad cell in the 12V battery. They replaced the battery at no charge under the 3/36 warranty. I asked about the TSB and they said that they verified all software updates had been applied to my vehicle.

Keith

amped said:

Perhaps one of these little guys might help with diagnosis:
https://moosh.im/mooshimeter/

It's a very compact bluetooth multimeter that could be mounted in the car for data logging (also has a microSD slot IIRC)

For current measurements, you could plug in an external transducer ( e.g. https://www.amazon.com/Mastech-CA100-Digital-Clamp-Transducer/dp/B01CV7WH0A/ref=sr_1_3?ie=UTF8&qid=1480289990&sr=8-3&keywords=mastech+DC+current+clamp ) or just estimate from the small voltage drop across the battery's ground cable (example set up here:
https://moosh.im/mooshimeter/example-videos/ ).

Click to expand...

I like this idea. Unfortunately clamp on meters aren't accurate down in the mA range we'd like to measure. If only there were a way to make it start recording when the car is shut off (and/or have a time/date stamp in the datalog). Also there would have to be a way to make the current measurement more accurate since we need to measure in the 0-50mA range and be tolerant of a little over 3A say 0-4A range with accuracy down to 1mA in the 0-50mA range.

I've measured current approaching 50A going into the battery after jumpstarting a dead battery so trying to rig a 10A max rated meter in series with the battery won't work.

I may not be reading all the specs correctly ( https://cdn.sparkfun.com/datasheets/Tools/MooshimeterManualRev1.pdf ) but it looks like it MIGHT be able to indirectly measure current down in that range with the type of set up they used in the video (ground cable as external shunt). They claim a noise floor "as low as 1 microvolt." The manual suggests accuracy of +/- (0.5% + 0.5 microvolt) in the lowest range. IIRC, for their demonstration they had an estimated 3 milliOhm resistance ground cable, which suggests 1 mA could be measured with reasonable accuracy (subject to error in resistance estimate):

3uV/3milliOhm=1mA ; 3uV*0.005 + 0.5uV=0.515uV ...assuming I didn't misinterpret the tables...

I suppose you could also increase sensitivity/accuracy quite a bit by temporarily replacing the ground cable with smaller gauge wire of known resistance. I imagine that would be OK as long as the car stays off.

It looks like data logging can be started and stopped via the app and the log files are named with a time/date stamp (presumably when log starts?) so I guess you could figure out time of day etc from that plus interval X # of data points.

Thinking what it would take to produce a gadget that could handle live loads from the car's battery, but measure down to the milliamp range and be left under the hood...

A 100-microohm current shunt resistor - http://www.mouser.com/ProductDetail/Vishay/WSBS8518L1000JK

This can dissipate up to 36W (with appropriate heatsinking), which yields a maximum current of 600A. I think that's plenty for our little EV batteries. At 100A the current shunt will dissipate 1-watt, easily dealt with.

Using an off-the-shelf microcontroller I'm familiar with, the TI MSP432, it has a 14-bit ADC with programmable voltage references of 1.2, 1.45 and 2.5V.

At 1.2V, that's 16384 steps (14 bits) or (1.2/16384) = .000073 volts, i.e. 73uV.

With 100uOhm the minimum current sensed would be 730mA (1 LSB of the ADC value).
However, a current sense amplifier can help us out, say dividing it by 1000 - the INA285 ( http://www.ti.com/product/INA285 )

Assuming we hold the output of the amplifier at the midway point of 0.6V (so we can measure current going either way and get it off the amplifier's lower rail voltage), our resolution is still 73uV, divided by 1000 it's 73nV, this gives us a 1-LSB resolution of 0.73mA!

At saturation (voltage drop of 600uV times 1000 plus 0.6V equals 1.2V), the reading would be 6A. Switching the MCU's voltage reference to 2.5V would yield a saturation of 19A. That's probably good enough for our purposes here. The current shunt would still conduct, we just wouldn't know how far above 19A it is.

I know just enough about electronics that most of this makes sense. I'll have to think about this some more and try to figure something out when I have time. I've only tried to use a shunt resistor to measure current on a dc circuit once and that was with a simple analog gauge. Not sure if the gauge was bad or what but I never could get it to work right.