Can low battery cause misfires / rough running?

Hi, Can a drained battery caused misfires? For some reason my battery gets drained when car sits for a while, I suspect my stupid aftermarket stereo (courtesy PO). When I started the car after it had been sitting for 2-3 weeks it barely started, could tell voltage was really low. It started but ran like crap, CEL with misfire codes and would barely idle. I had to jump start it a few times. Today I started it again with a somewhat charged backup battery, it ran much better. Or could it just be coincidence that it has been sitting for a few weeks and had some moisture in gas or something? Car is a 1999 996 3.4 cab 100k miles

A low battery in and on itself should not cause issues, since power delivery is done by the alternator once the car is running. A really bad alternator that puts out a very noisy current might, though. If you tried to start it a few times with a bad battery where it refused to fully turn over but still inject fuel, that might explain the rough running after jump-starting. Moisture in gas / gas going bad shouldn't occur within 2-3 weeks, that's more a 6-months / several years type of deal.

Yes, a bad battery can cause all sorts of lights to come on the dash with engine problems to boot.

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Actually a low battery will in fact dramatically reduce the the performance of ignition system causeing mis-firing and other flags. . The alternator does not supply all the power to the car once started, it actually depends on the RPMS, current draw on the car and such.... But the battery is still a stable support that is absolutely needed to keep the ignition hot enough to deliver a good spark and power for the ECU to operate effectively.

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When the engine is running, the battery is nothing more than a big capacitor. The state of its charge does not matter, since a good alternator will keep the voltage above 13.5v right from the start. All the low but good battery needs to do is reduce ripple. An actual bad battery (one that doesn't work at all and will no longer charge) fails to fulfill its purpose as capacitor, allowing all the transient spikes of the alternator to wreak havoc on the system which may cause a variety of problems. I have never seen a merely low battery cause misfires or other ignition troubles. All my cars with fully electronic ignition, injectors and ECU started right up after push-start with completely flat battery (not even a clicking starter) with no issues whatsoever as soon as it started. Moreover, inside the ECU specific variable voltage tables are programmed that compensate for low voltage to deal specifically with low initial battery during startup. This is the case even in my old 1994 Volvo (Motronic 4.4), and the 996 uses a way more modern revision (Motronic 5.2.2 or later). My own 996 has had a low battery for the longest time due to a bad Y-cable, and not once did it run bad after start-up. Every now and then, it failed to turn over though, so that battery was definitely low. The alternator provides more than enough current to feed every single system with ample power while simultaneously charging the battery, even at idle. That's literally what it's been designed for - otherwise sitting at idle would drain the battery and we all know that's not how it works. If you have definite misfires, rough idle and CEL's, your problem is not with a low battery but with something else.

YES... My E46 330XI (RIP) would go completely bat-**** crazy when the battery got old.. I finally put a full on AGM sealed on in,, cost a bunch but it outlasted every other option I tried. FWIW the E46 used a big wonking battery like my C4..

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While I will not argue your experience, the fact remains that a battery that is producing low voltage can cause misfiring. I have not said that is the only cause this poster is experiencing, but he should not assume at any point that the battery is definitely NOT the cause of the misfiring. I am stating the facts, if a battery is at a low voltage point and staying there and not being charged it will in fact affect the electrical system of the car in several ways... and one of the symptoms can be misfiring along with, flags, dash flashing and more. This is not a guess or assumption this is based on personal experience, working with battery products in street cars, races cars and working with several shops. BMW are notorious for this as well as many other cars. If the battery voltage is low the Alternator is NOT going to be a cure all because often the cars IBS, Intelligent Battery Sensor, will notice the low voltage and start the flags and other issues. The Car may throw flags, ECU will act funky and cause mis-fires. This is often a problem posted in many forums... once they replaced the battery all the symptoms when away. I am not at all trying to diagnose, nor claim this is the solution or sole problem, but it is absolutely a possibility and does happen. So the original poster should note and check that absolutely. Last the Alternator does NOT supply all the power needed to run the car correctly once its rotating... for example if you have heated seats, or other high current draws being used at the same time, they will be covered by the battery as an assist.... Or else most all the cars would have smaller batteries. The battery actually needs to be there and functioning at a proper voltage for the system to work correctly.

Unless the battery is damaged and has an internal short, I don't see it causing the problems you mentioned. However, I'll let others argue about that. As a note on your power drain: When I bought my 987S back in 2006, the first thing the dealer said to me is "You can't let this car sit around for three weeks without running or the battery will die. It's just how Porsches are" I guess I thought that was a fairly common problem on that vintage Porsche. So for years, I would disconnect my battery over the Winter or even when I was not going to use the car for a month. It was a bit of a pain and I killed more batteries than I really should have. Big waste of money. Finally I smartened up and spent $60 for a battery maintainer. Pays for itself in battery life, not to mention, batteries get heavier each year...ouch...OK, OK..not the lithium-ion ones

A high percentage of plate style batteries short internally as thier mode of failure, A battery with a dead cell will cause the alternator to over run the voltage, pushing too much at the ECU, this will mess up a lot of electronics, same with a shorted cell except it will absorb all the current available and under feed the ignition. I've fixed a lot of Porsches at the track with weak batteries.. It really shows up over 5500 RPM Like your hitting a really crappy rev limiter

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Yes, it takes a surprising amount of power to run high energy spark at high RPM.

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Correct. Some cars will actually increase idle RPM to compensate for this.

Charged the battery and it runs much better. HOWEVER it is still a bit rough during idle, I don't think a >$100k would idle like this. Sounds like it's overcammed almost, my cam deviation numbers are within spec. Car has 100k miles and is 20 years old, so it's probably combination of things: Coils, Spark plugs, vacum leak, injector etc. (Also drove the car for the first time in a while, realized how awesome these cars are.Even if it has 20 year old shocks that are tired) thanks

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A low battery that doesn't want to charge isn't a low battery, but a bad battery, to be fair. A merely low but otherwise okay battery will not cause the issues you described. I will contest this one, though. If you simply look up some alternator curves, they all start producing appreciable amounts of currents well before alternator RPM's of 2000. There's a reason for the pulley size, since it has been designed to deliver power to the car under all circumstances. Quick-and-dirty measurement of the crank-vs-alternator-pulley-size shows a 2.63 ratio between their RPM's, meaning that while idling at 800RPM, the alternator will be doing 2100RPM - more than enough to supply the entire car. And apart from that, I have literally never in my whole life seen a battery drain with the engine idling, even with all electrical loads engaged. Maybe if you have a lot of aftermarket stuff installed and are actively trying, but then again, when you really cross that barrier, as wildbilly says, the ECU may simly compensate. I agree, I have never contested that.

Love the expert consensus on this question.

Hi Boxman...this will be my last statement on this.... I just want to make sure that if any readers come across this post that they are at least considering the battery in any diagnosis for mis-firing and such. There is no reason to argue this point, because your claim is stating a Car owner should not be considering the battery at low voltage if the car mis-fires, that is not accurate, the battery should be one of the first things in any diagnoses for misfiring. Always check the simple and obvious potential causes FIRST. So the Battery is the one of the first things you want to check for when all of a sudden you are mis-firing. It's probably the first thing any mechanic will check when diagnosing electrical issues... Is the battery good, and it is charged? Do not pass GO. Now I understand you would not trust what I say... although batteries are my business. I can understand that, a lot of people talk guff. So anyway I just wanted to give you another source that claims the same thing as I stated above. Its a horribly written article, but just wanted to post up something that is basically stating the actual function of the battery/alternator relationship. and I don't know who these Company is or care, but the fact remains, as they state, as is absolutely correct, the battery does assist the alternator. Maybe you will trust them more than my statements. In the end it not about who's right or wrong and us going back and forth, its about the Poster understanding there is a relationship to potential mis-firing and the state of charge of the battery. So they should look at the battery. As he said it is also running much better now with a new battery. I never said it was all his problem but I think that conclusively shows a battery was playing a role.

One shoddily written article, 'proof'.. Mate... It's not even written in coherent English, let alone that there's any scientific argumentation to support the random statements that are being done. What's the author basing himself on? It even says the air-conditioner runs off the alternator. I mean, it's such a glaringly bad article, I'm shocked that someone who has a business in batteries would link that as argumentative evidence at all. Again, a low battery is not a bad battery. A bad battery can cause misfires. A low battery that is in otherwise good condition will not. End of. The Porsche 996 alternator runs at over 2000RPM at idle and delivers plenty of power for the entire car. You have batteries to sell, I understand that, so you are invested that people consider their battery for any and all issues and eventually replace them "just in case it's the battery". Don't get me wrong, I get it. But it's funny how the ones that have something to sell are always the ones that are most adamant about the stuff they're selling being the solution for all kinds of problems... whether they're really there or not does not matter

It should be mentioned that Durametric reported 13.0volts when my car was running bad. Could it be that with a drained battery the alternator can not charge and keep the car alive at the same time? Car idled at 600-700 rpm and was struggling

There r known issues with the "y" cable that connects the alt and starter to the battery. Mainly corosion and its questionably undersized. If u have an issue with this cable it can effect the charging rate and starter performance. U may want to start there???

What De Jeeper says. If your voltage is constantly low, you'll want to look at the Y-cable first, alternator/voltage reg thereafter. In the meantime, you can try to attach a standalone charger to your battery to see if it still charges fine. If it does charge properly, you can confirm that it's something in the charging circuit of your car (cable, voltage regulator, alternator).

If your battery is failing,, IE becoming a load not a good power source many many bad things will happen, Both my 996 and my 330XI behave badly if the battery is run down too low and then started,, takes about 10 minutes at a high-ish idle to get a charge back in the battery if its been killed by lights being left on and them jumped. Until the battery gets some charge,, instruments, and engine ECU and ignition are not happy.. if a cell dies the BMW gets very angry havnt had the 911 do that yet since it got a brand new battery just before I purchased..

How does a thread about something like this turn into an argument. It either does or it doesn't and I am suspect of anyone that claims to say it doesn't without being aware of every failure mode of a battery and every car's electrical system. Its like those true or false questions on exams that use the word "always / never" - you can't say for sure there isn't some strange case out there that exists so why would you argue to the contrary? OP stated that charging the battery seemed to help. So GO BUY A NEW FREAKING BATTERY ! It's a sub $200 part that MUST be replaced periodically, takes 10 minutes to swap, and you already have evidence to point to it as the cause. Replace the battery, and let us know if it resolves your issue. You can also drive to an Autozone or similar store and have it tested in the parking lot for free. EDIT: after re-reading I see the debate is about low batteries and not failing batteries. How old is the battery in questions? It may (probably) just be failing.

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The purpose if this was not if I needed to save $200, it was to start a technical discussion about it. Battery is almost brand new, like 1 months old. But for some reason it got drained in my car, could be anything like the aftermarket stereo or a light that was one. With a fresh battery the car starts like a champ but it idles poorly for a 10s and I almost have to rev it. But after a minute or so it seems to run great. Haven't been able to hook up my durametric to it, will get some more data. I think this is what happened: 1. Something is not right with the engine. Vaccum leak, bad injector, bad coil etc 2. Since the car barely could Idle it could not produce enough power to charge the battery, having a low voltage situation obviously doesn't help so it become like a loop of bad things. Car seems to prodcue enough power to charge the battery now.

Weak alternator can cause a delay in field excitement, which can cause the alternator to actually draw power, its usually the first sign of a weak diode, if a battery gets totally drained its not that uncommon for it to take a old alternator with it.. Bad connections are the 80% cause..

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It's not unusual for the battery to be a bit drained after not being driven for 2-3 weeks because of the normal electronics in the car. This is typical of many modern cars. Is this the first time you've left it without driving for that long? If it had enough power to turn over, it should run fine, though maybe not briefly with the abnormally low idle you describe. If the new battery is worse than the old, then it's possible it's defective (if it has a flaky cell, the battery voltage would drop by more than 2 volts when the cell is out and that could easily cause it to run poorly). The idle is probably a separate issue. You might want to try to using Techron fuel system cleaner to see if it helps. Also, you might want to try this (a "trick" key sequence for fuel/throttle system to clean itself -> https://rennlist.com/forums/996-foru...ml#post7886266

My 2 cents worth,as a automobile repair professional and 4 decade Porsche enthusiast. Bad batteries are the root of all evil in a modern vehicle.I see everything from falsely triggered warning lights,clocks that are always resetting themselves,electronic braking brakes that will not release to running and performance issues.A fully charged battery that can pass a full load test,not a electronic tester,but the old VAT 40 type that pulls 400 to 500 amps out of the battery while testing,is required as a minimum.And no,.....jump staring it and using the alternator to charge a dead battery is not the way to do it.A full charge on a wet battery can take 24 or MORE hours,with a AGM style battery taking up to a week to be brought back from a zero volt state,and it has to be done in stages. It could be that all the adaptive values have been lost in the ECU as the battery went flat and it can not compensate for minor intake leaks and sensor degradation that has happened. ............"" The alternator provides more than enough current to feed every single system with ample power while simultaneously charging the battery, even at idle. That's literally what it's been designed for - otherwise sitting at idle would drain the battery and we all know that's not how it works.""..................You would think,but not in a early 928.The alternator pulley ratio is way to low and alternator way to small.You can watch the headlights get dim at night time at idle.

@928Fixer, good thing we�re on the 996 forum and not the 928 forum then, and good thing I mentioned on multiple occasions how a low battery won�t cause these issues but a bad one might, a distinction I made several times. I appreciate your two cents, just most of it doesn�t apply to OP�s car or the things I said.