Battery or alternator?

[DUFF TT]

Hello! I've owned my '17 GT for not quite two weeks, and I'm running into some odd battery issues.

Some quick car details: 2017 Mustang GT, 6spd, 1,800 miles, twin turbo with ngauge and boost leash electronics (among other gauges). Car is in pristine shape, never driven in inclement weather, never stored outside, never rained on, etc..

The previous owner said that he kept the car on a battery tender essentially 24/7, as he didn't drive it much and built it for shows. To make a long story short, the car wouldn't turn over yesterday (I had driven it three days ago, but didn't have it on a battery tender), so I took the battery to an AutoZone with the intent of swapping it for a new one, as the battery appears to be original unit. They offered to charge the battery overnight for me, then test it today, which they did. When they tested it, it had charged to close to 80%, and the volt readings were permissible. I can't remember the exact figure, but the tech at AutoZone seemed to feel it was in the acceptable range. By the time I got home, all of 10 minutes later, I re-installed the battery and put it on a battery tender. This tender will let you know when it's charging, when the battery has hit 80% charge, and when the battery is charged enough for the tender to go on standby mode. It took several hours for it to register as hitting 80% charged. I left it on for several more hours, but it never went to standby mode. I decided to drive the car, and it started right up just fine. I drove for approximately 30 minutes, nothing crazy. After I parked and turned off the ignition, I tried to restart the engine again about 1 minute later. It started, but barely. It behaved as though the battery was low on charge again.

I'm no mechanic, by a longshot, but I visually inspected the alternator the best I could, and it looks brand new from the outside. The belt looks new, and spins freely and without any weird noises. No fluids of any kind drip from the car, and I don't see where anything would've dripped down into the alternator. In my mind, the battery is the most likely culprit, right? Yes, the car is tuned to 900+whp and all that, but it's a case of "horsepower because horsepower is cool" situation, and not a "horsepower because I street race and beat the hell out of the car" situation. I really, really don't want to turn the car over to a shop unless I absolutely have to. I'm willing to take the risk on a new battery versus taking the car in somewhere first. What say you guys? Could this behavior result from a wonky battery? When the guy at AutoZone first inspected the battery, he said it sounded like it was "flooded" to him. There is no corrosion on either lead, no goo or stains in the battery tray, etc.. Appreciate your opinions!

Edit: After changing up my search terms, I found some older threads on here that discuss other weird battery behavior, and it seems like these batteries just kinda suck? Makes me more confident the battery is indeed the culprit, as I read a thread where someone else had charged a battery, had it immediately tested and was deemed ok, then it was totally dead the next day, That smells a lot like my situation, but I'm still very interested in your more informed opinions!

Edit with outcome: I swapped in a new battery, and the car started up like normal. I drove it for about ten minutes, then parked it. Four hours later, started it up again and no issues. The old battery was 100% the issue, and everything is now good to go. Appreciate the help, everyone!

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[2022 Mach 1]

It's a five year old battery. While tenders are good for occasional use, your battery isn't designed to be on one for 5 years straight. The battery has been cycled continuously now and needs to be replaced.

 

[cerbomark]

put a new battery (it s old enough) and then see what the alternator is putting out for volts.
Now you ll have the 2 most likely issues covered.

 

[Mike Pfeifer]

Sounds like the battery is self discharging. Replace the battery, then reassess the situation.

 

[AvalancheSVT]

5 years is plenty of goodie out of a battery, man. replace it even if it ain't bad but it probably is.

 

[Cory S]

Battery cell going bad. Can’t hold a charge long at all…..

 

[K4fxd]

Replace the battery and don't get too focused on the volt meter. Unless the tuner disabled "battery managment" the voltage will vary between 12.6 and 14.x. while the car is running.

 

[DUFF TT]

Appreciate the comments, everyone. I'm going to replace the battery today, and will update with results for posterity's sake. Be on the lookout for my next post, coming soon, where I ask how I can manage to move what I believe to be a catch can out of the way so I can access the low pressure AC nozzle to recharge the AC (with pictures!).

 

[baevid]

I had to replace my battery a few days ago after it suddenly wouldn’t start even though it was cranking fine all week. Mine was a rapid clicking so the battery was for sure done after about 3-4 years.

 

[DUFF TT]

Issue resolved. See first post for details. Thanks!

 

[galaxy]

It's a five year old battery. While tenders are good for occasional use, your battery isn't designed to be on one for 5 years straight. The battery has been cycled continuously now and needs to be replaced.

Ehhh, that statement's not very true anymore. New chargers are pretty much all smart chargers these days and they don't sit there and cook/cycle the battery non stop. Nothing beats a properly maintained battery, and there should be no time limit on how long it can sit plugged in. I have two vehicles that are not DD's, and live on a charger. If it's gonna sit for a week or six months, it's plugged up. My truck's battery is at least 15 years old. And I can take it off the charge for weeks at a time with nare a sign of degradation. I know the horror stories on the factory Mustang battery, but mine is five years old next month and in the same condition. I attribute that 100% to constant and consistent battery charger maintenance.

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