Monitoring 12V Battery Current

[SnowStorm]

The 12V battery problem hasn't gone away so I want to monitor actual current into/out of the battery.  Measuring voltage, although helpful, has problems.  It can be OK one evening and dead the next morning if there is too much current flowing all night.  Here's what I've done so far.

1. Removed the big plastic cover/tray that goes across the back.  There are 3 screws along the front edge and one 10mm(?) nut in a deep pocket near where the ground cable bolts to the frame.  The two outside rear corners are held in place with spring clips - you pull the whole thing straight up once the screws and nut are removed.
2. Connected a two conductor wire to each end of the big ground cable that runs from the negative post to the car's frame.  The ground cable now acts as a current "shunt" producing a very small voltage that is proportional to current per good ol' ohms law.  Ran the wire under the rear compartment "flooring" into the rear seat area where it could be connected to a digital multimeter set for DC volts.  You need one that can indicate down to 0.1 millivolts.  DO NOT CONNECT ANY WIRES TO THE POSITIVE TERMINAL!
3. Calculated the resistance of the ground cable by measuring the current through a light bulb connected across the battery and the voltage drop across the "shunt".  Two different bulbs gave the same value of 0.54 milliohms (0.00054 ohms).  Now you have the value so "don't try this at home"!  Current can now be calculated as:
   
        12V Battery Current (amps) = Voltage Reading (millivolts) / 0.54
   
   Or in "round numbers", just take the DC millivolts and multiply by 2 to get amps.
   
   If the car's frame is positive (relative to negative post), the battery is discharging.
   If the battery post is positive (relative to car's frame), the battery is being charged.
    
4. With car OFF, opening/shutting the door sends the current to about 8 amps.  After a minute or so it steps its way down to about 0.4 amps.  Then after maybe 30 minutes or more it finally drops to under 0.01 amps.  (Used a very high resolution meter here as you are down in the microvolts.)  The rear cargo lights appear to draw a bit over 1 amp.  They eventually turn off if hatch is left open (at least it happened once while testing).
5. Next step should be to build a little micro-power monitor I can run from the accessory jack that can stay in the car all the time.  Just look through the window and see how it is doing.
6. But we shouldn't have to do any of this!!!

Here are the monitor connections.

 

For the curious, here is the sticker on my new battery.  The bar code number you can't read is 31508 53155.  Note the 100 month warranty (36 months free)!  Since it doesn't start the ICE it should almost last forever.  Quite a pity we're having ANY dead battery problems.

 

The left side of the battery compartment looks like this with the HV battery fan (Made In USA!).  It should point straight out the back for a bit of jet assist! :lol:

[plus 3 golfer]

Great job.  

 

How do you plan on recording the data?  Have you thought about finding a data logger like this or this one. This would be great evidence to document the issue especially for arbitration / law suit if lemon period has expired. Manually recording the data every few hours for several weeks or so of no abnormal activity may become too tedious to continue as the frequency of the battery issue seems to be months between events.

 

 

[ptjones]

I have had problems with the HVB fan running after I shut the car off and only had the car on for 30-60sec. I got it to stop after turning the car on and off again. The ICE never came on. Nice job SnowStorm :)

 

Paul

[Edsel]

ptjones,

 

Same here.  It's odd that the fan would turn on when backing the car out of the garage three feet to get in the attic.  I was worried it would run down my battery so I turned the car off and on and it stopped.  I didn't watch it, so it might have turned back on.  Who knows? :headscratch:

[SnowStorm]

Plus 3:  Hadn't really planned on logging but thought about it - the data would be quite interesting.  Main plan was just to be able to walk by the car, say an hour after parking, and see that no current is being drawn.

 

Paul:  That seems odd for sure.  There might be some shutdown sequence that leaves something on.  daveyork suggests turning everything off before switching off the key and I think that is a good precaution.  But the fan coming on like that makes no sense.

[wab]

The 12V battery problem hasn't gone away so I want to monitor actual current into/out of the battery.  Measuring voltage, although helpful, has problems.  It can be OK one evening and dead the next morning if there is too much current flowing all night.  Here's what I've done so far.

1. Removed the big plastic cover/tray that goes across the back.  There are 3 screws along the front edge and one 10mm(?) nut in a deep pocket near where the ground cable bolts to the frame.  The two outside rear corners are held in place with spring clips - you pull the whole thing straight up once the screws and nut are removed.
2. Connected a two conductor wire to each end of the big ground cable that runs from the negative post to the car's frame.  The ground cable now acts as a current "shunt" producing a very small voltage that is proportional to current per good ol' ohms law.  Ran the wire under the rear compartment "flooring" into the rear seat area where it could be connected to a digital multimeter set for DC volts.  You need one that can indicate down to 0.1 millivolts.  DO NOT CONNECT ANY WIRES TO THE POSITIVE TERMINAL!
3. Calculated the resistance of the ground cable by measuring the current through a light bulb connected across the battery and the voltage drop across the "shunt".  Two different bulbs gave the same value of 0.54 milliohms (0.00054 ohms).  Now you have the value so "don't try this at home"!  Current can now be calculated as:

    

        12V Battery Current (amps) = Voltage Reading (millivolts) / 0.54

    

   Or in "round numbers", just take the DC millivolts and multiply by 2 to get amps.

    

   If the car's frame is positive (relative to negative post), the battery is discharging.

   If the battery post is positive (relative to car's frame), the battery is being charged.

    

4. With car OFF, opening/shutting the door sends the current to about 8 amps.  After a minute or so it steps its way down to about 0.4 amps.  Then after maybe 30 minutes or more it finally drops to under 0.01 amps.  (Used a very high resolution meter here as you are down in the microvolts.)  The rear cargo lights appear to draw a bit over 1 amp.  They eventually turn off if hatch is left open (at least it happened once while testing).
5. Next step should be to build a little micro-power monitor I can run from the accessory jack that can stay in the car all the time.  Just look through the window and see how it is doing.
6. But we shouldn't have to do any of this!!!

Here are the monitor connections.

 

Wasn't that ground connection one of the ones Ford suggested checking/removing the paint?

In telecommunications, (a lot of the power is less than 12v DC), a missing or high resistance ground IS ALWAYS the hardest to find.

 

I have had problems with the HVB fan running after I shut the car off and only had the car on for 30-60sec. I got it to stop after turning the car on and off again. The ICE never came on. Nice job SnowStorm :)

 

Paul

I leave ours running (only happened a couple of times), if the battery ever fails during one of these short/no ICE runs then maybe more evidence for Ford.

[ptjones]

ptjones,

 

Same here.  It's odd that the fan would turn on when backing the car out of the garage three feet to get in the attic.  I was worried it would run down my battery so I turned the car off and on and it stopped.  I didn't watch it, so it might have turned back on.  Who knows? :headscratch:

 

Plus 3:  Hadn't really planned on logging but thought about it - the data would be quite interesting.  Main plan was just to be able to walk by the car, say an hour after parking, and see that no current is being drawn.

 

Paul:  That seems odd for sure.  There might be some shutdown sequence that leaves something on.  daveyork suggests turning everything off before switching off the key and I think that is a good precaution.  But the fan coming on like that makes no sense.

The Fan didn't come back on, now that I think about this it has happened three times. What voltage are you reading before you get in the car? Again I'm seeing 11.5v at power point when I open the door. Maybe I need to check my ground.  Wonder if I checked the resistance if I'd see any? :)

 

Paul

[SnowStorm]

Hadn't heard about the ground connection problem.  Just checked mine and got 0.55 millivolt drop with 3.12 millivolts across the ground cable.  Looks fine to me.

 

Paul,  Just now my battery read 12.00 to 12.08 at front with all lights out.  Car had been out in 31F weather for several hours so battery is colder than usual (may affect voltage).  Opening rear hatch to turn all the little lights on gave 11.71 in front and 11.74 in rear at battery.  After closing hatch, voltage at front went to 11.97.  At other times I had read about 12.4 to 12.6 with lights out.   ..."I don't like the look of it"....

[webcontrol]

To monitor the current flow, you will need an op-amp to amplify the voltage, and log it somehow.  One chip to recommend is DS2438, that chip has a function to log in and out current, store it in an accumulator, backed by EEPROM.  After driving few days, you can read to see if IN current is at least 15% more than OUT current, because the battery charging only about 85% efficiency.

 

From what your voltage measurement, I think the problem is this battery is constantly under charged, so that battery is sulfation due to low charge too long.  Each cell fully charged should be 2.1V and fully discharged should be 1.95V.  Your 11.97V is almost fully discharged.  Lead acid battery like to be charged little more than discharged.  I think adding a little charger will help battery life for a long way. 

[hybridbear]

Fascinating info. Lots of FFH owners have had battery issues. Few have come out to their car to find the battery dead, but very many have frequent "battery saver" messages (nicknamed BS msgs) on the MFT screen. It seems to be a design flaw more than anything else. A few owners have replaced their 12V with a higher capacity battery and the BS msgs have gone away. Some owners have gotten Ford to replace their 12V battery under warranty but their BS msgs have reappeared.

 

The data you're gathering will certainly help prove the design flaw - the 12V battery provided is too small.

[webcontrol]

Non -hybrid car charges its battery as long as engine running.  C-MAX only charge its 12V battery when software decided to do so when car is on.  Lead Acid battery needs "top-off" charge often to have a long life. I don't think hybrid car can afford that constant current drain to the EV battery.   Toyota in its later model hybrid cars have a solar panel to "help cool the cabin", I suspect that solar panel is also to help maintaining the 12V battery.  Also, please notice Toyota uses much smaller 12V battery, so that it will not take much to top-off charge it.  The bigger capacity battery, the more charge current required to top-off charge. Unless you found the battery being drained a lot after key is removed, change to a smaller battery maybe a better choice than a larger one.

 

If you car has chance park outside with some sun, adding a little solar charger will greatly help your battery last longer.

[ptjones]

The HVB charges the 12v battery every time I get in my car, I monitor that with power point digital volt meter. A Lithium ion 12v battery would save some weight, don't know it they even make them yet. :)

 

Paul

[C-MaxSea]

Great sleuthing A-Team (Snow, Web, Hybr., PT ... ) on battery stuff - keep up this work & thanks.  :hat_tip:   Nick

Edited March 27, 2014 by C-MaxSea

[mxanotcar]

Yes, I agree we all should not need to do all of this findings, but unfortunately it seems Fords cannot fix the problem/s with their cars, especially on this 12V battery issue continue to be a problem even after the update (13B12 to correct the problem of electronic module fails to transition to sleep state).

 

That update, however, made my radio able to run longer than 10mins, roughly around 20mins to 50mins.  Before that update, I sometimes could play radio longer than 10mins, and some other times around 10mins to 12mins runs, I recieved a message "Low battery" saver and then the radio shut off.  I had to turn the key to ignition 3 to get the radio turn on again and left that for 15mins and hopefully the 12V could be recharged.  The ignition key was in the 1 position during these radio playing times (not 0 2, or 3).

 

This coming Saturday I might be able to bring the car to my dealership service manager for, I think, a look at (from Ford Service outreach) after I can confirm the Saturday date on this Friday for the issue of road-salt-water-spot collected badly in the engine compartment.

 

http://fordcmaxhybridforum.com/topic/3599-road-salt-badly-collected-into-engine-compartment/

 

I am not sure if the dealership can fix the 12V battery failure that I also posted in this thread below

http://fordcmaxhybridforum.com/topic/1294-for-those-who-have-had-12v-battery-problems/page-9 at #178

[ptjones]

If I was having problems with the 12v batt I would carry a portable jumper battery just in case. Then I wouldn't have to stress about it. Then I could take it to Dealer when it was convenient for me.  IMHO :)

 

Paul

[webcontrol]

I added a 5W solar charger to the car, so that whenever the car is parked under the sun, the battery is charged by the sun.  I noticed HV battery charging 12V much less frequently.  That actually helps getting better MPG.  For the cost less than a tank gasoline, also keep battery strong,  I think that is worth to be added.

[mxanotcar]

I see that the 12V battery is located in the back of the car.  If the 12V battery were located in the engine compartment and with the problem of hood not properly seal engine compartment, how long those exposed battery terminals, connectors, wires would last against with water and salt mix that get spray through the gap openings? 

 

When driving in the rain or salt mix during weather inclement, there is always spray at high-speed from other vehicles in front of and left and right side of your car.  Wondering if Ford has somekind of water spray tunnels that they can run tests on their vehicles besides wind tunnels.

[ptjones]

Checked two FFH yesterday at Dealer and one read 11.9v with acc switch on and other one didn't come on until you started the ICE. Weird! :)

 

Paul

[SnowStorm]

Here is a plot of 12V battery current just from opening the driver's door and closing it again.  (Interior lights were set to stay OFF.)  I ordered an inexpensive multimeter with 0.00 millivolt resolution and an optically isolated RS232 output for data logging.  As you can see, plenty of points along the way that, if things stopped shutting off, battery would get drained quite handily.  BTW, just inserting the key (not turning at all!) will cause the current to go to 5 amps for about 10 seconds.  Removing the key causes about 3 amps.  Nothing happens on the dash.  After 30 minutes or so, current drops to essentially zero (0.00 mv reading on my meter).

 

X Axis = DC Amps  -  Y Axis = Samples (almost 2 per second)

[SnowStorm]

After posting above graph I go out to unhook the meter and see that current is back to 1/3 amp!  As I watch, it suddenly jumps to about 6 amps for a few seconds then steps down and soon goes back to zero (probably within 30 seconds).  Good grief.  This means I can't just check that everything has "gone to sleep" and then forget about it.  If things keep turning ON and OFF all night, who knows when something might stay ON.

[plus 3 golfer]

Interesting, maybe there's a scan / check of modules every so often for another few hours.  If I understand the graph, the X axis goes from 0-280 scans and each scan is about 2 seconds so the total time is about 560 seconds or 9 minutes 20 seconds.    IIRC, on the VW procedure for checking for current drain via measuring voltage drop across fuses, the procedure has one begin the check after 2 hours so all modules are asleep.  Can you log all night? 

[SnowStorm]

+3,  I wasn't very clear, the logging was going at its maximum rate which was almost 2 scans each second so the total time was more like 2.5 minutes.  I'll get that axis straightened out next time.  Yes, I want to log for an extended period but I might have to settle for fewer scans/minute - I think the software limits at 10,000.  Also the meter's 9V battery will need to last all night.  If not, I guess "one dead battery deserves another"! :lol:

[webcontrol]

Great measurement result. This may help isolate where the problem is.  If possible, get the wire harness layout, then measure a few places at the same time, to see where this current going to.

[plus 3 golfer]

+3,  I wasn't very clear, the logging was going at its maximum rate which was almost 2 scans each second so the total time was more like 2.5 minutes.  I'll get that axis straightened out next time.  Yes, I want to log for an extended period but I might have to settle for fewer scans/minute - I think the software limits at 10,000.  Also the meter's 9V battery will need to last all night.  If not, I guess "one dead battery deserves another"! :lol:

That's why I ask because I would have thought that the car might not reach sleep mode in just 2.5 minutes (and it may not have given that you saw higher readings later).

 

I think if you set the scan to once every 4 seconds, it would be sufficient to capture any changes and thus allow you to record 11+ hours. And then if there is a pattern of blips seen, you could decrease the time between scans appropriately to see what the blips look like. Or you could do it in reverse and start with a small change in time between scans and capture data for a few hours first and then extend to more hours.

 

Either way this will be very interesting to see what happens over time.   

[SnowStorm]

Here are results from "all night" monitoring.  Graph shows first 250 seconds of 12V battery current, positive is discharging, negative is charging.  Events shown are opening door and getting in, turning key "ON" (battery starts charging), "starting" car (Ready To Drive but ICE is OFF), turning key OFF, getting out and closing door, waiting for cuurent to drop to a low value.  (Note: Software was only logging changes in value so the exact shape would be slightly different than shown but peak values should be correct.)

 

 

Overall sequence follows:

 

10:04:31 PM  Start of graph shown above

10:08:41 PM  End of graph

10:36:51 PM  Current jumps to about 3 amps for 6 seconds then back to 0.3

10:38:28 PM  Current drops to 0

1:31:16 AM    Current goes to 0.2 amps

1:32:10 AM    Back to 0

2:05:06 AM    Jumps to 0.2 again

2:06:01 AM    Back to 0

2:24:40 AM    0.2 amps

2:25:34 AM    0 amps

2:58:34 AM    0.2 amps

2:59:27 AM    0 amps

6:39:32 AM    Still 0 - End of test

 

Of course I am not suggesting that this data shows anything wrong - its a record of what happened.  It would be nice to know what "wakes up" periodically to draw 0.2 amps for about a minute, but it represents a trival drain on the battery.  The main point here is to characterize "normal" operation and then look for deviation from that norm.  We don't know if something fails to shut down after parking the car or if something wakes up at a later time and drains the battery.