IainM

I came out to a completely dead battery the other day after only 4 days of non-use. This after an engine out clutch job & while yer in there stuff all put back together with a new Interstate battery and confirmed alternator function. I must have a parasitic drain. On futher investigation I found that I had failed to completely close the engine lid when I put her away but I still couldn't believe that 200mA x 96 hours = 19.2 AHr could kill a nominally fully charged 80AHr battery. As an EE I went into full geek mode but thought that my process might be of help to other, less geeky folk with similar problems.

I set up my trusty Fluke multimeter to measure amps and ran a cable through the hole in the spare tire panel into the frunk so I could close all doors and get to the lowest power "locked" state of the immobilizer. It looked like this:



My meter has a 10A fuse and so to stop this blowing when I a) initially connect the battery and b) operate the door locks, I used a short circuit to bypass the meter and protect it. Here you see the top banana plug with a short circuit plugged into the wire that connects the battery +ve terminal to the car 12V supply (circuit 31 unswitched +12V as I read it). I pull this banana when I want to make a current measurement.
Here's the other end:



Now I'm comfortable with live wires with 100's and 1000's of volts on them so 12V with an infinite current capacity across bare conductors in close proximity is not a problem for me. But be warned, if these metal parts contact, you will need fresh underwear and possibly some new tools. A fully charged 12V battery has an impressive energy density. If you're not comfortable handling this, take the time to make insulated cables with stable connectors. Electrical tape and heatshrink are great tools for electrical isolation. Yes, using a spare crock clip cable to make these connections is real hokey but I'm not responsible if you set fire to your fuel tank. Have a fire extinguisher handy at least

Anyway, onto the results which I found quite illuminating.

I started with all doors closed except the frunk which was lowered such that the safety latch was engaged but the hood was not closed. The car was unlocked and unpowered. I connected the battery and removed the meter short. Current = 740mA.

I closed the frunk, replaced the meter short and locked the doors with the fob. By the time I had removed the meter short, the current was 35mA. This is, I believe, the best you can do for a locked car in "standby" condition. This is the current required to power the sensors and the immobilizer and keep the radio receiver running to detect the fob button press when you come to your car. The conclusion here was, no, I don't have a parasitic leak. But I wanted to know why my new, fully charged battery was dead after 4 days.

Seeing as I had instrumented this whole shebang, I got a beer and ran some tests:

watching the meter for a few minutes as I sipped on a fine IPA, I noticed that every now and then, the current would jump up to 48-50mA for a sec and then return to the 35mA baseline. It did this in bursts of 1 second in every 3 for cadences of 3-8 blips and then go quiet for maybe 4-8 secs. then repeat.
Don't know whats happening here, maybe some circuit waking up every now and then to check something, who knows. Bottom line, the average quiescent current is likely more than the baseline stable reading you make. Just a casual observation.

I replaced the meter short, pressed the fob button to unlock the doors, removed the short and current went up to 820mA for 15-20 secs then fell back to 35mA.

I opened the drivers door, I have interior lights set to off so no lights came on, current = 820mA but didn't drop to 35mA. 820mA is a constant for door open.I closed the door and current dropped to 35mA. Good stuff.

Opened driver door again, current = 810mA (essentially the same I would say)
I popped the frunk and the current was unchanged at 810mA. surprise, I expected a rise of 100-200mA for the frunk light coming on.
I closed the drivers door and the current was 808mA, 2mA is like the current needed for the door open/close sensor, not enough for incandescent bulb, but not lights on inside anyway so OK.
I fully opened the frunk and saw the frunk light on, current = 808mA. surprise, this says the frunk light was on all along after the drivers door opened. Weird, if I'm motivated maybe Ill video this with my iphone to find out, kinda like does the fridge light go off when you close the door, Not sure I'm motivated.

I opened the drivers door again, 808mA
Turned on drivers side interior light, 1493mA
Opened glove box, 1.748mA
turned on passenger light, 2417mA
turned off all interior lights, 807mA
popped engine lid, 1510mA
opened engine lid, 1507mA

So what did I learn?
As long as all doors are closed, regardless of whether car is locked, battery drain = 35mA+ whatever that pulsing thing does,
Frunk light comes on as soon as car is unlocked, turns off after 15-20 secs.
Frunk light consumes 795mA (what else is on?)
glovebox lamp consumes 120mA
Drivers pillar lamp consumes 685mA
Passenger pillar lamp consumes 670mA
Engine compartment lamp consumes 700mA

Of course if you have fancy stereo or other aftermarket gadgets (lojack for instance) attached, your quiescent (and all readings in fact) will be higher.

I found it quite educational and now understand that leaving my engine lid ajar for 4 days was indeed enough to discharge my new battery. Hope this helps some folk too.

EckFe1

Good to know those current numbers.
l recently tested for current draw too and came up with 80mA, not locked, doors closed.
I have a tracking device with GPS and SIM hooked up so looking at your numbers, I most likely don't have any other open circuits, otherwise I'd show higher amps.
Thank you for your write up.
Ed

pp000830

Hi IainM,
Thanks for the info!
Yes, I noticed the frunk light illuminates with the dome lights, frunk open or closed, go figure.
Glove box and engine lid on and off via their respective switches unrelated to the state of the dome lights in the cabin.
My thoughts are

• Yes, a frunk or engine light can run a battery down below cranking capacity,
• so can a turn signal being left on. In both cases 4 hours later on a 1.5 Amp charger things will fire right up.
• a brand new battery can have diminished capacity simply from being older stock or being banged around during distribution.
  

Andy

Lorenfb

Concerned about your battery voltage while not driving, install one of these; https://antigravitybatteries.com/pro...ker-lead-acid/

Jay777

Nice!

Novice

I use this DC capable Clamp amp meter. Many clamp meters only work with AC, but this one works and it is only for $45.