Why does my battery light keep coming on?

The battery was probably okay in the 1st place. From your description, it sounds like your alternator is on the way out - probably a shorted (or nearly so) diode allowing the battery to slowly discharge through the alternator. A diode is like a one-way switch that allows alternator current to go out of the alternator to charge the battery and run the electrics, but does not allow current to flow back into the alternator. There are three of those "one-way" switches. If you lose one, you've lost 1/3 of your alternator's capacity. So when you slow down, the alternator doesn't have the capacity to provide the needed current.

You have a bad diode in the alternator and the battery will go dead again. You are only getting a slight surface charge of the battery. Hope you tested the old battery after charging it with a battery charger or you may have replaced and not really needed it.

Your battery wasn't really bad, it just lost it's charge because your alternator isn't working and doesn't charge the battery.

You need a new alternator.

This new battery will probably go bad in a matter of days.

it's not a bad diode... it's more likekly worn brushes inside the alternator...

simple voltmeter check at the battery terminals would verify it:

battery voltage before you start the car

Voltage goes up to 14.xx V once you start the vehicle and let it idle, if not, you confirmed that it's a bad alternator, brushes, regulator, diodes...doesn't matter = you need to replace it with a rebuilt unit (most carry lifetime warranties)

Practically undoubtedly the alternator is not charging or providing enough vigor. Determine the drivebelt tension and appear for no less than 14.5v on the alternator. (simple, but perhaps garage job?). Alternator will recharge the battery, the sunshine will go off, the blowers and the a/c will come on and keep on once ample vigour is restored/available.

Could be something as simple as a slipping fan belt. Who told you to buy a new battery?

did they test alternator and check belt..it may be bad alternator killed old battery..and may kill new one after a while..any auto parts will test alternator for free,,u need 14.6+ volts....and be sure battery cables are clean and tight

i assUme ur car is running good...tuned up,,not on verge of stalling every traffic light,,

First off the battery will not help the power of the car.

Second off you probably have a weak alternator.When the engine RPMs drop it's not making enough power.

Could be the alternator or the belt that drives it. Have them checked.

alternator