just a bad battery or something else?

My 2004 C4S stalled in a weird way last night. While idling at a stoplight, the engine shut down, all (many?) dash indicator lights came on briefly, then all went dark. Turned the key to the off position, back to the "on" positio and a few lights (on the left side) lit up and then went dark. After that, turning the key on/off made no difference. Nothing electrical worked including hazard lights (!) and frunk switch. it was like there was no battery installed in the car. AAA came, we used an aux battery to open the frunk open, and then we hooked up the same aux battery in parallel with the main battery. Hazards started working and voila! the engine started up immediately with a turn of the key. Dash volt meter showed ~13V-ish. Drove home, shut it down and restarted it a few times. Engine turned over quickly which would indicate a healthy battery. The only thing I can think of is that the battery somehow flaky and goes from "good" to "zero" but I have no idea what mechanism could make it behave that way. The fact that the hazard lights didn't work is a clue that this isn't an ignition switch issue since the hazards should be powered with unswitched 12V. Has anyone seen this kind of failure mode? It's an interstate battery from costco purchase in 2022, so I'll be replacing it under warranty tomorrow, but I wonder y'all have any ideas.

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Let's brain storm. Think about it... the battery has one big responsibility; that is, to START the car. That's it. After that, it's responsibility of the alternator to keep things electrical happy. The alternator dies, then what's next? Yep, you guess it, the car is now relying on the remaining charge of the battery to keep the electrical things happy. The battery dies. The car dies. That simple. Never convenient. Now, the Y cable on the alternator is a common problem on these cars, but replacing the battery is a good start. Then test the alternator. Then you may want to look into replacing the Y cable. My 2 cents worth. And yes, it's happened to me.

Thanks @ZuffenZeus, I tend to agree the battery is the problem. But the fail was just really strange, esp since the car started perfectly fine once I got home. I don�t understand how a battery could be so flat the hazards don�t work yet after 10min of driving be perfectly fine to start the car. More background info: Y-cable and starter have been replaced within the last 5-6 years so charging system is good (solved the well known hot-slow-crank issue) . Battery was also dead earlier this week after to no use within 2wks, so I had just charged it up Thursday night. All indicators show �bad battery.� I am just stumped by the fact it seemed like the battery wasn�t there, vs being really weak. Live and learn I guess!

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No harm replacing the battery, but I'm still leaning to the alternator (or connectivity issue - e.g. bad ground) as being the issue.. Just me.

My 2 cents-rarely have I seen a battery just go dead, like turning a light switch off. There will be signs, i.e. dim lights, slow cranking, just weird electrical anomalies. I would test the battery, test the charging system, maybe even scan for codes. Ignition switch? Me thinks you have a connection problem. Either at the battery, maybe you knocked something loose when charging last week? And there is always the ground on the passenger side of engine to chassis. Do some diagnostics before throwing parts at it.

I'm with ZZ here suspecting alternator. If the battery was dead Thursday after sitting for two weeks that also makes me suspect the alternator. But here is a quick check. What does the voltmeter read when you start it from cold? If the alternator, battery and Y cable are new, it will read just over 14V. If you start it up and it reads 13V, you have an alternator or Y cable issue.

A AGM battery will go "dead" for no apparent reason. And no battery survives a full discharge (completely dead) and a recharge. Replace the battery. Get a AGM battery and keep it on a battery maintainer when not in use.

Thanks for the thoughts everyone. Here's some more data: Parked the car Friday night after the tow truck followed me home and it sat over the weekend while I was out of town Last night, turned the key on, all lights come on and volt meter shows 12.3V. Turn the key more, engine turns over strongly and quickly, engine starts, and volt meter increases to ~14V. Battery is ordinary automotive battery, not AGM The above tells me starter, alternator, Y-cable are all OK. Battery is acting fine too since even after sitting for 48hrs (unlocked, so not sleep mode), it still turns the engine strongly. I'm not replacing the battery just yet and will continue to diagnose. Next I'll see if there are any codes. It still feels like a big clue that the hazard lights were dead when the car died. I've started inspecting the wiring diagrams in the bentley manual but haven't made sense of them yet. If anyone understands the wiring of the hazard circuit and how it could be totally disabled please chime in.

The hazard lights work off battery power. No battery power no hazard lights. It is possible that you have a bad/dead cell in the battery. That would break the circuit and then you have no 12V. I wouldn't take any chances, just get a new battery.

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Not saying your battery is bad, if it tests out ok, you're good to go. But here is some auto trivia for ya.... Actually, its not uncommon for this exact scenario you describe. Contrary to popular belief a multiple cell lead acid automotive battery is not like a aaa/aa/C/D/etc, flashlight battery that mostly goes lower and lower till its dead. A 6 cell lead acid auto battery produces 2v per cell for a total of 12v. All cells must work in series to produce 12v. Its very common for just 1 cell to get shorted out and go completely dead making the battery a 10 volt. A shorted cell 10v bat usually won't run/start a car at all with its corresponding lower amperage. Not uncommon for a seemingly perfect battery to get a shorted cell right after it started the car, never to start it again...Now rarely the short can be intermittent at the start of the battery/cell failure, especially if its a lose/dislodged plate banging/shorting around in there. The short happens for a number of reasons. It can be mechanical damage from vibration or physical damage that dislodges a plate. Or more common its from multiple total discharges/overcharges, that sulfate the plates. Some snake oil remedies you can buy and pour in the cells to 'remove the sulfate build up from the plates, good luck with that... An AGM battery normally never has the lead acid 'wet' auto battery above problems because of its AGM architecture.... jmo

Thanks @DBJoe996 and @allcool . @DBJoe996 I definitely plan a new battery. I just want to confirm it is the root cause before I get rid of it. @allcool it�s interesting to know that the lead acid battery could fail this way. For sure the battery is capable of starting the car now (I did it repeatedly last night). I don�t know anything about battery testing but will research it and see if the local auto parts store can test and confirm the battery is bad. I�ll report back with more data as I have it. Thsnks everyone for the help!

Just as a sanity check (I have to ask, you probably have already done this) but are the leads on the battery tight? I've had cars fail to start due to the leads not being seated correctly. I could see some vibration or something else causing some sort of inconsistency like this. I highly doubt your battery is bad, this sounds like some sort intermittent short or wiring failure.

It also only takes a few minutes to pull the battery and take it to your local parts store. They can do a full check up and detect a bad cell and load test it. In your case it really sounds lile a bad wire connection.

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+1 for loose leads. Note: you can�t always tell by looking at them or even seeing how tight the bolts are. Even tight connections can corrode or have high resistance to a connection. You can remove the leads, clean them, re-attached. Then check the other cables, such as ground / Y cable.

@Mike Murphy , @brontosaurus , @De Jeeper , it's interesting you raise questions about the battery connection. In fact, the ground connector for my battery has a cross-threaded for some years now nut which makes it very difficult to tighten and un-tighten. I cannot say that it is 100% tight right now or when the car died. However, it is very stiffly connected to the battery such that i cannot pull it off with my hands even when trying hard (ok, no jokes about me being weak...). The terminals are very clean so it's not corrosion. I suppose I could just crank down harder on the cross-threaded nut and call it a day but I remain concerned this is not the root cause and I'm ripe to get stranded again. If indeed it's the ground conection on the battery, could this explain why the car died while idling? The alternator and charging system is strong (14V right after starting the car). I will not try this for fear of an inductive blast that would kill all of the electronics in the car, but what would happen if I yanked the ground cable off the battery while the car was running? If the car would keep running, then this proves that a loose battery connection could not kill the car while it was idling. BTW, I checked with a simple ODB-II reader and no codes. I also checked with my durametric and only the alarm module indicates any fault codes, something about "limit not being reeached," along with messages saying "this does not indicate a problem." This weekend I will get the battery tested at O'Reillys and see what they say. I'll also see if it's straight forward to saw out the old ground conection bolt and replace it with something that works well. Thanks all for the inputs! We are on a bug hunt!

Do NOT PULL OFF A BATTERY CABLE WHILE RUNNING !!!! .. Without a complete circuit to control the voltage of the alternator output the 120 amp alternator can put out 17v-19v or even more and A/C voltage.........Bulbs will start blowing and sensitive electronics can be damaged... I have seen people do this and get by with it, but only a fool would try it on a modern highly electronic car.....If you want to see if the Alternator is working properly there are safe specific tests for this...Voltage/load/amperage tests.... Yes a crudy battery post can cause all this problem you are describing... A crudy connection will not allow the battery to charge ......can be intermittent as all hell ,....can cause stalling at idle...can cause intermittent no starts.... and then can work perfectly .... I could go into a highly technical novel about what is exactly goes on when there is no complete circuit ...but it should be enough to say it MUST have a complete circuit... Usually you will see at least one "pit" on a post when you have had intermittent continuity of a large amperage circuit like that...Multiple "pits' means it has had multiple occasions of intermittent high amperage "arcs".... Funny story....I remember George Barber who owns Barber Museum and Barber Motorsports Park...had the battery replaced in his Cayenne by the guys that restore cars and motorcycles for the Museum ( Museum quality work) after a no start,,,,then it still wouldn't start with the new battery ...They towed the Cayenne to me at Porsche to diagnose it...I found the plastic cap still on the negative battery post with the clamp tightened over it......I got a good laugh out of it and had some fun with the Museum guys...Moral of the story, " never overlook the most simple things "...

Quotes source posts: 19872870, 19872870

Completely agree! I only mentioned it in the context that I'm confused how a loose connection at the battery terminal could lead to my experience of the engine shutting down at idle. Good suggestion, I'll check this weekend

Hi All, I tested the battery toady and it checked out fine. The car also starts easily every time I have tried it. Voltage with key on is 12V and with engine running is ~14V right after starting, so all indications good. However, based on the condition of the battery terminal and connector it does seem like the connection was not perfect. See pictures below. So I've cleaned up the terminal and connector (last picture) and replaced the terminal clamp bolt so I can get nice solid connection. I still can't explain how the connection could fail so suddenly and completely and then recover so magically, but maybe some of the Lucas Electric mojo leaked out of the Sprite and into the 911. I've made sure my manual trunk pop cable is in place behind the towhook plug and am getting her back on the road and will take my chances. If it happens again I'll try to analyze more methodically before jumping. Thanks for all the help! Negative terminal and connector Negative connector Negative terminal Positive terminal looks way better Terminal and connector clean and and beautiful now

Yep, here is your "mystery electrical problem"....Allways check the simple things first .. As I mentioned above, this will cause all your symptoms....The red area circled is the only place that is transferring current and the small contact is why there is arcs/pits from high amperage causing a spark from the small contact area...( I have actually seen those small sparks cause an explosion of the battery if the battery is gassing/bubbling from overcharging ect) .. Glad you were diligent enough to follow my advice to look carefully and find the culprit...