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.