What car what year?

Anything aftermarket on it?

>>14230082
1990 Honda Accord

It's all stock.

3 year old battery I would go ahead an replace it. They dont last worth a fuck in the desert. Other then that I cant really help, electrical is my final frontier. thats where I would start though.

>>14230093
How are your ground cables? They're the only other main thing I can think of

>>14230075
Buy a polimeter if you dont have one and check if any of the fuses have any consumption , if they dont, replace the battery

>>14230075
Is your alternator belt at the correct tension?

>>14230561
Oh shit, you said while driving, then check if the ground cables or the cables that go to the alternator have any resistance or are in bad shape

>>14230075
Here is something to try,

Charge your battery with a battery charger (the kind that runs on 120volts) then start the car. If your battery light comes on immediately, it's a problem with the alternator or battery or both. If it takes a while for it to do it then you have a very large load on the electrical system somewhere (i.e. A short)

But I'd put my money on the battery. Try replacing that first, it's not a bad idea anyway.

>>14230075
I had a similar problem and cleaning the battery terminals fixed it for me, as stupid as that sounds.

1. Acquire a decent multimeter
2. Check battery with multimetwr car off, should be like 11.9-12.2v
3. Start car, do same test. If charging, you should be something like 14-14.5v across the battery.
4. If not, use continuity on your multimeter and check from the alternator power out to battery positive. A beep confirms the connection is good. Sometimes there is a inline fuse between the alt and battery that can burn out.

If your alternator is working, I would follow other anons suggestions. Do a battery health test and look for drains/shorts.