2012 Subaru Forester - Horn Blares Solidy and Amber Lights Turn on When Connecting Battery

[KSS]

Recently, my battery went completely flat probably because I forgot to close the cargo door for 1+ days.

The battery was completely dead so I hooked up my charger and managed to get it charged back up (it's a AGM battery, by the way).

I connected the Positive cable first and then connected the Negative.

When I connected the Negative, the horn turned on and the amber turn lights turned on. All were turned on solidly (horn never turned off, lights were not blinking).

I tried pressing the unlock button on my key fob and locked and unlocked the door with the key to see if that would disengage the security system. It didn't.

Any ideas why this might be happening? Hope it's nothing serious.

Thanks

 

[bmart]

Bring a battery back from death is more likely if another battery is connected when charging it going to occur. Your car probably sensed 12V but nearly 0 amps. Did everything work normally after the charge?

 

[KSS]

I went ahead a restarted another re-charge. I'll hook up my battery tester before I attempt to connect the battery to the vehicle again. I "assumed" that the charge was successful before and didn't hook my tester to it first. I've never had a battery run down as low as this, usually I had a volt or two to start off with. It could be that I damage the battery by letting it drain so completely. It's an AGM battery. My first one so I don't know how forgiving they are compared to lead-acid. Will let you guys know.

 

[Dick Foster]

You didn't say how old the battery is but even AGMs can pack it up and even sulfate over time. Especially if not fully charged on a regular basis.
BTW AGMS charge a little differently so it's best to use a charger designed for them. Unlike wet cells, they like a good stiff charging current if they've been fully discharged. For AGM batteries and chargers, I like Odyssey.

 

[KSS]

So, my AGM battery would not take a charge. I used a charger (NoCo Genius) that has its own AGM setting. The battery was taking a charge, but the battery started hissing and was becoming hot to the touch. It was up at the 75% mark, so I decided to stop the charging and give the battery a rest for the day. Next day I attempted to hook it up again to my car. No blaring horn anymore. Instead, I had no power at all. This was my original symptom. This was not a dead battery situation it was something else. Anyway, I took the battery back to my auto-store and asked them to do some testing on it. It was still within warranty so, if something was wrong, they'd give me a replacement.

They did their tests and I came in the next day to pick up the battery and give the good/bad news. The manager hooked up the battery to a hefty diagnostic tool he had. He connected the cables and it was returning 0 volts. Not good after being on a professional charger for several hours. He hooked it up in reverse (+ to -, - to +) and lo and behold, it showed that there were 12+ volts BUT the polarity was reversed. He said he's never seen that happen ever. I mean, how can a battery change it's own polarity? Makes no sense to me But I watched what he was doing closely.

Long story short, they gave me a replacement. I came home, tested it first with my own diag device and everything looked great. Then I installed it, got into the cabin and tried starting the car. Nothing. No lights, no bells, no sounds of the system readying (when you turn the key to the ON position). Suffice it to say, the car would not start.

I checked all the fuses and nothing was blown. I would have checked the relays but I'm no expert in diagnosing issues with relays. So, I need to reach out to a mechanic.

Tomorrow I have a mobile mechanic showing up. I don't have high hopes, but he might be able to find something. If that doesn't work, it means I have to get the car towed to a local mechanic. I'm kind of dreading a big electrical issue bill.

Wish me luck!

 

[Dick Foster]

Maybe your Genius charger reverse charged the battery. Possible, and It's the only thing that springs to mind.