Guys

Hi guys. My car won't start. I think it's the 12V battery--we've had this problem before, but this time when I tried to jump it, instead of the alarm going off, nothing happened for a while and then the lights came on (no lights on the dash though, just the left and right signals and the headlights. And the switches for the lights were definitely off.), But we still couldn't start it. Left it connected for 15 mins, still couldn't start it. Found a newer car to jump it off with (my friend's car is kind of old and crappy) and this time the horn just started blaring like crazy (not the alarm, the horn).

So. What's going on? Is it starting to charge and could it just need a longer jump, or is it completely dead and I need to get a new battery?