Impossible Battery Drain/Load-over-time problem?

[Doug B.]

Something causes the battery to run down if the car isn’t driven for 5 or 6 days, but I have measured 10.7 mA when the car is in “sleep” mode. I assumed there was going to be an obvious parasitic load, but can’t see how 10.5 mA could drain a battery to the point the electric liftgate stops working (the first item to experience problems when the battery charge is low). If the car is driven every day for 45 minutes at a time or more, I don’t have a problem with the battery running down.

This is a Porsche Cayenne SUV, 2008 model, V8. After 5 to 7 days of not driving the vehicle, the battery charge is low enough that the motorized liftgate will not work. Owner’s manual says the first thing that stops working when battery voltage is low is the lift gate motor. It takes a standard Battery Tender quite a while to fully charge the battery after it gets that low… more than 24 hours… probably in the 16 hour range, so the battery charge is definitely depleted.

This car came to us with a battery that was a little too small (H8) for the battery box in the car… apparently every battery size database in the world says it takes an H8 battery, but it doesn’t it needs an H9 that’s the same height and width, but 2-inches longer than the H8. The H9 fits the battery box (under the drivers seat) perfectly. So this car has a brand new AGM H9 battery (which is a HUGE battery, BTW, 950 CCA). The H8 battery in it had a December 2015 manufacture date so it wasn’t old, it just didn’t fit right.

To measure the draw on the batter, I put 2 ammeters on the battery…. a Fluke series Ammeter with the current flowing through the test meter leads… connected to negative battery terminal and to the removed negative battery cable. Second ammeter is a clamp-on meter, clamped around the positive battery cable. Clamp-on meter reads higher than the Fluke meter by maybe 15% to 20% but it’s there as a sanity check because what is happening CAN’T be happening…

Obviously, the whole car goes dead when I remove the negative battery cable… except the battery backup system for the alarm! (car alarms are REAL LOUD when they go off inside your garage!!! Just in case it hasn’t happened to you yet, LOL!). I timed the profile of the initial current and the gradual drop in current draw (numbers here from the Fluke meter that I trust more) as car systems wake up and go back to sleep when they aren’t used…
Initial power-up when meter leads are connected between negative cable and negative battery post – 10.5 Amps on the 10Amp Fluke Ammeter connection (I don’t know how it read more than 10A either).
After 1 minute, amps are down into the 7 Amp range, At 5 minutes Amps go down to 4. And at 10 minutes, there is a brief drop to 2.5 amps then current goes down to 00 on the Fluke’s 10A scale. I repeated the test using the 100 mA scale of the Fluke meter and after 10 minutes, I get 10.7 milliAmps.
That’s IMPOSSIBLE. 10 mA can’t mostly discharge a battery in 5 or 6 days of not driving.

Car’s Voltmeter (on the dash) reads about 12.8-12.9 volts while driving. An independent Porsche service shop says that’s good/normal/typical for this year/model and that factory Ammeter reads a little low normally.

I pulled fuses one at a time anyway to see if I could find the source of the 10.7 mA draw and came up with nothing. No one fuse drops that to zero. Some of the fuses cause a current draw temporarily after they are re-inserted, but I always wait till it drops down to 10,7 mA before pulling the next fuse.

One thing that was suggested as a possible source of the problem is the amplifier for the audio system not shutting off. So after waiting for the car to reach 10.7 mA again, I inserted the ignition key and turned it to the accessory position and turned the radio on (I had habitually been turing it off before turning the ignition off when parking the car thinking that perhaps the audio amp stayed on only if the radio was turned on when the ignition key was turned off but that made no difference compared to leaving the radio on when turning the car off. Turning on the ignition key did produce a big current draw initially, but it dropped fairly low fairly quickly even with the radio running at low volume. Turned the key off while the radio was still working (radio goes off when you remove the ignition key) to shut-down the car. After several minutes, it settled back to 10.7 mA. So I don’t think the audio amplifier is killing the battery.

One thing I’ve noticed that I have no information about is… after the car has been driven an hour or so and parked in the garage, when I connect it to the Battery Tender, the RED light on the Battery Tender illuminates indicating the battery is charging. After 1 hour to 2 hours, the green light on the Battery Tender is blinking with the RED light still lit indicating the battery is approaching fully charged. After another 30-60 minutes, the RED light goes off and the green light comes on steady indicating a fully charged battery. Is that normal for ANY car (our other car is an electric car so I can’t check that easily)?

If there is no parasitic load on the battery, what else could POSSIBLY run down the battery over 5 or 6 days of not being driven?

I should mention that we live in an area where a LOT of places we go to run errands are within 2 miles of home. So I might drive a mile, shop, open the liftgate, drive another mile (Air Con running full blast more than likely), park, shop, open the liftgate, drive another mile, park drop something off, drive another 2 miles to an appointment, etc. So I could drive 5 miles and stop/start the engine 5 times and open/close the liftgate 4 or 5 times… I could see that running down the battery over time because 5 miles of driving time isn’t enough to recharge the battery after 5 engine starts and 4 or 5 liftgate actuations (the motor closes the liftgate also, but that’s probably not a lot of current compared to opening the gate.

Anyway, my mind was blown to see the “resting” current go down to 10.7 mA after 10 minutes (that was unlocked, if you lock the car, it goes into rest/sleep mode in less than 1 minute, so we have even been locking the car while it was in the garage but that didn’t seem to make any difference. I thought SURE I would find the parasitic draw with the remove-the-fuses-one-at-a-time trick, but when the resting current hit 10.7 mA even before pulling any fuses I realized that this wasn’t going to be a normal parasitic drain problem.

Any ideas or help with the “is it normal for the battery to appear to not be fully charged to the Battery Tender after being driven for an hour?” question would be appreciated. I’m guessing this is normal behavior for a Battery Tender since it charges a battery differently than an alternator, but I want to be sure I’m not full of crap on that assumption.

Topic

[cj1]

Car’s Voltmeter (on the dash) reads about 12.8-12.9 volts while driving.

What does your voltmeter read engine running?

[Rob]

[quote=”cj1″ post=173221]Car’s Voltmeter (on the dash) reads about 12.8-12.9 volts while driving.

What does your voltmeter read engine running?[/quote]

Im confused on what your asking…. are you saying your car is reading 12.8v while driving or are you asking Scotty what his voltage is?? because 12.8 is a little bit low… for when your alternator is engaged… but could be ok… with that number I would have it checked out….should be more like 13.2-14v when running… but it could be disagaged or could be slipping

[cj1]

Scooty mentioned that his dash voltmeter read 12.8-12.9 wondering what he measure with his own voltmeter.

[Rob]

[quote=”Scooty” post=173213][quote=”Rob781″ post=173210]Im willing to bet you that the backup battory is drowing a lot more power then you think it is…. if the battory is like over 3-5 years its t its end of life and will draw more power…. and how your actually measure it will not show your true draw that it is drawing… I have to go for a little bit I will come back and explain why your orginal testing is bad if the backup battory is the problem… and why you will not see that in your first test trying to find the draw that is drawing the main power and why it is giving you a fauls reading’s[/quote]

OK, that’s promising. Never thought the alarm backup battery could be the culprit here… I have NO DOUBT that it is now 8 years old and is original to the car. I have NO idea where it is located either…. probably NOT in an easily accessible location since you wouldn’t want the crooks to have easy access to disconnecting it… so not looking forward to possibly having to dig into the guts to replace the alarm system backup battery. While waiting for your follow-up, I’ll see if I can find out where the alarm backup battery is located. May require purchasing service manual and electrical diagrams… not too costly as downloads of non-Porsche documentation, but same info used by outfit that provides shop manuals to independent shops. Seems pretty accurate in the bits I’ve seen. There is a second covered “box” next to the big battery under the drivers seat though… big enough to hold a battery as large as maybe 2/3 of a brick.[/quote]

I would go over to the dealer and ask them if that is a stock alarm…. because if its not they will not beable to find the battory and means someone had to install it for them…. now is this your car or a custmores car your working on??? if your working on a custmores car… I would replace the engines battory it could be going bad that is one of the first signs of it failing… even if you test it on a load tester 100 times it could say good but the battory could be bad…. so personaly I would replace it… just to satify them.. examples is the cemistery in the battory and the plates could be failing… I would recomand having multiable auto part stores test the alernator and battory… each place runes diffrent testors… some are bettor at things then others…
if its yours and dont mind waiting for a week and want to test if its the cemistry in the battory is failing…. if you disconnect the battory for a week… and read the volts on it like every so often… NON-LOad test… it should be fine still a week lattor… then I would connect the battory to the car and test it… upen the trunk is what I thought you said… and try starting it… if it works fine most likiy its not the engine battory at all…you shount really be seeing a voltage drop over those few days should stay about the same…

[Rob]

[quote=”cj1″ post=173226]Scooty mentioned that his dash voltmeter read 12.8-12.9 wondering what he measure with his own voltmeter.[/quote]

thats very intresting….. that could be pointing to a bad or failing battory or alternator…or even posaboe bad cable….
ya he sould have to check that out to have that out ruled

[Rob]

So when your testing it you probly tested it when the backup battory was fully charged only reading 10mA(0.01A) for the whole system… but when the backup battory is charging… you could have adraw of maybe 500mA(0.5A) to maybe 1000mA(1A) by trying to charge it…. now when a battory starts failing it will not hold a charge as long… but will still hold a charge for sometime…. at 10mA I calcuated that from what data I could fine… thinking a car battory is like 100Ah at 10mA of draw it should die in like 41 days but people where saying something like a avg battory is 100Ah-50Ah… so maby like 41-20 days before the battoy is fully daed… even at half life it should even have egough power to open the trunk. I would think…
so if lets say we are charging the backup battory at like 1A of draw you would have like 100 hours -50 hours before the main battory is dead
with 10mA going to the system its telling me that the charging circut is not engaged it should take more power then just 10mA to charge the backup battory
now 10mA is like nuthing for power… basiccly your just powering some electronic circuts.. not even like a bulb or even a computer harddrive I dont think would run on as little bit of current as 10mA
if that doesnt make sence let me know what your confused on and I could explain my self more…
if you want to get an idea where the alrm is follow the ingition wires under that dash and look for any jumpper wires in the fuse bock… might help… if not pull out all the fuses one by one untill the alarm freaks out…LOL and see where the fuse wire goes… might give you an idea

PS I cant tell you if these numbers and cacultions are correct but I beleave they are resible on numbers(aka probly close)

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[Rob]

some places where I think the backup battory for the alrm might be
areas where I would think to check… under the dash, behind the radio, in or around the glove compartment, in the center collom, in the alram speaker(aka part of it), uner the seats, the trunk, near the car battory, near the starter relay near the fuse bock’s

[Daniel]

One way to have a sensitive ammeter that can also handle large current surges would be to have 2 huge diodes rated for the max current expected in inverse parallel giving you a max voltage drop of 1 volt or less for standard silicon diodes. Also in parallel with the diodes would be a resistor and a voltmeter, say you use a 1 ohm resistor then amps would directly equal volts, IE your 10mA would become 10mV but be soft capped by the diodes starting at about .6V so any reading above .6 amps wouldn’t be accurate but at least you would know that it was at or above that current when it shouldn’t be.

Sent from my SM-G900R4 using Tapatalk

[Rob]

[quote=”danielthechskid” post=173404]One way to have a sensitive ammeter that can also handle large current surges would be to have 2 huge diodes rated for the max current expected in inverse parallel giving you a max voltage drop of 1 volt or less for standard silicon diodes. Also in parallel with the diodes would be a resistor and a voltmeter, say you use a 1 ohm resistor then amps would directly equal volts, IE your 10mA would become 10mV but be soft capped by the diodes starting at about .6V so any reading above .6 amps wouldn’t be accurate but at least you would know that it was at or above that current when it shouldn’t be.

Sent from my SM-G900R4 using Tapatalk[/quote]
that would be nice of a test to use a data logging O-Scope and read the draw over time and see if amp draw changes over time

[Doug B.]

Sorry I’ve missed some posts, apparently this forum doesn’t default to sending you Email alerts when there are posts in a thread you are active in… have to look for that setting in user settings.

This is my own car. An independent Porsche shop I trust said the dashboard Voltmeter reading just under “13” while driving is correct for the ’08 Cayenne S I have. I haven’t connected my own voltmeter while driving because of the awkward main battery location under the driver’s seat. But I’m fairly sure this isn’t a charging problem since driving the car daily for some reasonable period of time keeps the battery charged enough that you would never know there’s a battery-drain problem. It only discharges to the point things stop working when it sits for 4 to 7 days without being driven. The first thing you notice when the battery drain has gotten bad is that the motorized liftgate stops working. First time this happened I was reading through parts of the Owner’s Manual for the first time (haven’t had this car all that long… 3 months or so and the 2 Owner’s Manuals have around 300 pages each! typical page size for owner’s manuals… something like half the size of 8.5″x11 paper). It said in the Owner’s Manual that the first sign the battery charge is weak is that the motorized liftgate will stop working… it unlatches OK (solenoid) but just sits there instead of raising or lowering when it should be in motion. Car still starts when the liftgate stops working… a VERY interesting situation. I would never have predicted that.

Re, the battery saga…. every battery size database in the world apparently has the WRONG size battery listed for this car… every fitment guide says the battery is an H8 size battery but the REAL/CORRECT battery size is H9. The battery in the car when we got is was a wet (liquid acid) H8 size battery that didn’t fit under one of the retaining brackets in the battery box under the driver’s seat. The manufacture-date sticker on that H8 battery (not the original battery from Porsche) said 12-2015 so it had undoubtedly been replaced early in 2016 before we bought the car. When I found out that the REAL correct size battery for this car is H9, I decided to replace the H8 wet battery with an H9 AGM battery. The H9 is the same width and height with same battery post positions as the H8 battery, but it’s about 2″ longer than the H8. CCA on the H8 battery was 900. CCA on the H9 battery is 950. Porsche wants a battery in the car with 740 CCA or more. So the battery in the car now is an H9 AGM with a manufacture date tag of May 2016. I charged this new battery with a Battery Tender (not “Jr”, the “regular size” Battery Tender) before putting it in the car… that took most of a day on the brand new battery, so the charge the new battery had been given was clearly not “complete” though it probably would have started the car.
There was no difference in how the drain on the battery happened with the new H9 battery vs. the previous H8 battery. These are BIG batteries, among the largest car batteries you can get.

This car has the Porsche factory alarm system… factory “alarm activated” LEDs in door panels, correct audio tone (2 chirps) when locking car, correct switch that disables interior motion detection inside the car used for leaving pets or people in the car with doors locked. I have been not been able to find a resource that reveals where the security system battery is located. There are no Chilton or Haynes manuals for this car. The factory service manual is 9900 pages (the head nearly explodes at that page count!) and sells for over $2000. I found a “free online manual” that is possibly the worst excuse for a manual I’ve ever seen, it had no info about the alarm system except for replacing the “horn” for the alarm system. I’m a bit hesitant about considering the “subscription” manuals offered by AllData as I can’t find out in advance whether they have the level of detail that it covers the location and removal/replacement of the security system battery. Another site offers manual downloads but the source is not made obvious nor can I tell if those downloads cover the security system battery or not. A Russian site offers a moderately expensive download with “thousands of pages” but how do you trust THAT to deliver what they say they will deliver? I may take a chance on the inexpensive download since I’m unlikely to find the security system battery on my own.

[Doug B.]

Let’s assume for the sake of argument that the alarm system backup battery is somewhere behind the dashboard. The main battery is under the driver’s seat. I was measuring the drain on the battery by disconnecting the negative battery terminal and putting the meter’s mA input in series with the ground cable so all current through the negative battery lead was going “through” the meter. When I turn off the car, it takes 10 minutes for the current to drop to 10.7 mA (shop says that’s “normal” for this model series). If I lock the car, it goes to sleep faster, like 30 seconds or so before the draw drops to 10.7 mA. (shop also says that’s normal).

If the security system battery is drawing power from the main battery and the security system battery has it’s own ground (say to the firewall), I would not measure the draw on the main battery from the security system battery via the negative battery cable from the main battery? I would have to re-measure the draw, and probably over days, through the POSITIVE main battery cable to see if the security system battery was really drawing on the power of the main battery?

If I need to measure the draw through the positive battery cable, I’m going to have to be REALLY careful – wrap the loose cable in cardboard or something so it can’t touch anything metal.

I can’t find the Ah spec for car batteries anywhere. But I suspect 100 Ah is probably in the ball-park. 10 mA is .01 Amps so 100 Ah should last 10,000 hours at 10 mA draw. Divide that by 24 hours/day and you get 416 days to kill the battery. WAY beyond what happens in real life… our little-used BMW wagon has sat for 4 months and just barely had enough juice to start the car, so there are more considerations for how long a battery will hold a charge than you get from a 10 mA draw alone… the battery discharges slowly over time naturally even if there is no draw on it (sitting on a shelf long enough will discharge a battery) so that “natural” drop in stored energy won’t be measured. 50 mA would only drop theoretical life to 83 days… might be more like 40 days in real life. So I’m pretty sure if I have a draw on the battery, it has to be in the .75 to 1 A range to pull the battery down to a low enough level for the motorized liftgate to stop working (car still starts when the liftgate motor fails to operate, but you can detect the starter cranking isn’t quite as robust with the battery partially drained). If the alarm battery is causing the draw, the draw could be “delayed” somewhat (I think…) since the alarm battery wouldn’t draw anything from the main battery as long as its voltage is high enough… but say it takes 3 or 4 hours for the alarm battery to drop low enough to begin drawing power from the main battery… I could go that long without seeing an excessive current draw from the main battery and I haven’t waited that long yet to see if it happens… but I would likely only see a draw on the main battery from the alarm battery if I was looking at the positive battery cable on the main battery.

Does all of that sound right?

[Richard Kirshy]

Why don’t you just ask the Porsche shop where this backup alarm battery is located.

As to the one battery feeding of the other.
I would tend to lean towards some type of sensing/charging circuit rather than any type of direct tie to both batteries.

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