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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?
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.
[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.
That would work… except if the draw starts hours or days later and isn’t always there, I’d have to be there to see it. Which makes connecting a current measuring device to a computer that could log and plot current data over time would be the best solution if I can figure out how to do it without spending a ton of money. I figured when I searched for USB Ammeter I’d find all KINDS of results but all I got was little meters you plug into a USB port to measure current flowing through the port… not from some outboard device you want to measure… MUST be out there somewhere. There are current and voltage monitors that run constantly for solar panels for example… just need something that works in the operating range of a car.
[quote=”Johnny Stang” post=173187]Could you use just a simple ammeter like the gauge in the dashboard type, the kind you’d buy at Autozone, between the alternator output lead and the battery? They’re not super accurate but good enough to see a big drain. See if the drain is happening on that side of the circuit? With most vehicles the positive side of the battery only runs to the alternator (thin cable) and the starter (heavy cable) so any leech in the electrical system should be seen on that lead. Watch it for something significant whenever you get bored and go look?[/quote]
I think I need something with more accuracy in case the drain on the battery is in the 50 mA to 2 A range… so that really requires a digital display that just moves the decimal point depending on the Amps being drawn so you never have to adjust the range, you just have to pay attention to the location of the decimal point. A conventional analog ammeter with a needle moving left or right to indicate charge or discharge won’t necessarily tall enough about what’s going on. It MIGHT, but don’t want to gamble on being mislead or not getting a useful result. So probably need that sort of functionality but with a self-ranging digital dieplay and maybe something that stays on all the time so even at night I could go out to the garage with a flashlight and read the meter without opening a door.
The Flule meter is probably 20 years old, but measuring amps hasn’t changed in that time. 😛
The problem with clamp-on devices for measuring current is that to work well, they have to have the cable being measured held in the center of the clamp and the circuit being measured should be stationary since the earth’s magnetic field changes the readings. If the clamp moves while taking a reading, that also changes the reading… and the clamp can only go around 1 wire (positive or negative, but not both at the same time). And the meter will time-out and turn itself off after 5 minutes too… already happened to me often while making the measurements I’ve already done. I hadn’t seen a clamp on the end of test leads before though… that could turn out to be handy later if the clamp becomes the only way to see what is going on right at the battery with the battery being located, inconveniently, under the driver’s seat. I’ll give up some accuracy, but may get enough info… I’d have to see about running the leads outside the door though, when the car is in “sleep” mode, opening a door immediately wakes it up.
Yes, I did replace the original battery for 2 reasons…. the original was 2-inches too short and did not fit the retaining brackets in the battery box AND I wanted to see if another battery behaved the same way. The battery in the car had a manufacture date of 12-2015 so it was a new wet (lead-acid) battery. The replacement was made in May 2016 and is an AGM (absorbed glass mat) filled with gel instead of liquid acid. The original battery was 900 CCA, the replacement battery is 950 CCA (battery capacity spec is “740 CCA or higher”).. Both batteries act the same so no need to replace the battery again. I fully charged the new battery on a Battery Tender before installing it… the store said it was fully charged, but I don’t think it had a full deep charge at that point, it took the Battery Tender about 22 hours to get the new battery to a fully charged state..
Will take a look at the battery info link shortly… thanks for that!
Now I’m beginning to question the alternator… have to see what I can find out about the design… whether it is 1 phase, 2 phase or 3 phase, If it is 3-phase and it “lost” 1 phase it could be undercharging the battery, even on longer drives and that could build-up quickly with a couple of weeks of short trips. If it is a 1-phase design and you lose 1 phase… you get NADA out of the alternator so that’s definitely NOT my problem. With a 2-phase alternator, losing 1 phase is pretty severe… the issue might be more severe than what I’m having, but not sure. But should be some way to test this. I do have an oscilloscope so I could look at the AC coming out of the alternator if there are connectors accessible before the rectifier(s)–not always available though.
I doubt condensation is an issue here. It doesn’t rain from April to late October and humidity is typically 20%-40%. Daytime highs in summer run 78-104. But there could be some odd build-up of old dust from the brushes in the alternator I suppose. If I can organize a sufficiently long… like 5-day period where I don’t drive it at all I’m definitely going to start with a fully charged battery, then connect the Battery Tender to see how long before it goes to full-green after sitting for 5 days. I have a 2000 BMW station wagon that sits in the driveway for MONTHS and the battery doesn’t die even if I just start the car half a dozen times in 4 months to move the car… only running the engine for 10 minutes at the most. And it held up to all the short trips with ZERO problems.
I’d also thought about leaving a meter connected for an extended period of time to see if the parasitic drain starts happing hours or days after the car is in “sleep” mode. Problem is, the battery is under the driver’s seat…. VERY problematic when trying to get a meter connected. And you have to measure current in series with the battery so you have to remove a post (I use negative for the obvious reasons) and all the current operating the car and charging the battery has to go through the meter. This car over-ranges the Fluke meter that only goes up to 10 Ams for a brief period when you first reconnect the battery after connecting or removing the ammeter. I would have to buy some sort of specialized ammeter that can pass however much current is required to start the engine AND still read in milliAmps for seeing what happens when the car sits for hours or days (to see if the 10.5 mA “sleep” draw changes). It would be interesting to see what the car is doing in the current realm while driving… it just can’t happen with the 2 meters I have… the Fluke only goes to 10 Amps. The clamp-on meter has 100, 20, and 2 Amp settings, but it’s a clamp-on meter so it’s impossible to read with the driver’s seat down… because the battery is under the driver’s seat. So I need an auto-ranging Ammeter I can leave connected permanently… something that can take max current that will ever come out of or go into the battery… and that meter should have negative, 0, and positive Amps so I can see if the battery is being drained, charged, or neither at any given point in time… driving or parked. I have NO idea what the maximum current is… probably starting the engine puts the most draw on the battery. So not sure where to look for an ammeter like that… beefy enough to handle all the starting current, but sensitive enough to measure milliAmps when the car is “sleeping”. It would also help if I could connect a computer and capture readings every minute or two so I could plot the results on a chart. Maybe I should be looking for a USB Ammeter??? (half joking, there may very well BE such a thing). Just did some quick reconnaissance and came up with NOTHING that could hadle 12volts and up to 100 Amps… most of the shunts for ammeters that are rated for 100A can work at no more than 75 mV!!!! A shunt that can handle 12-volts and 100 Amps (headroom is always good) would be huge and that would have to be installed between the negative battery terminal and the negative battery cable and you would connect the Ammeter across that shunt to measure current (via voltage drop across the shunt presumably). If anyone has any ideas where to look for a device that will do this, please let me know… willing to try almost anything at this point. Has to be a solvable problem here somewhere.
Also – anybody know of a reference that shows battery life with various loads on the battery (like 10 mA, 1 A, 2 A, 4 A, 7 A, 10A, etc.)? Not having any luck finding anything like that either.