How do i know if my car battery is broken or the alternator?

Can i take the positive or negative off my battery while the car is running to find out?How do i know if my car battery is broken or the alternator?
start the car and take the red cable off (positive) if it dies it is the alternator if it doesnt it is the battery
Yes you can. Just take either off and if the car stops running, then your alt is bad.How do i know if my car battery is broken or the alternator?
if the car is starting the battery is not the problem. If you are having to jump it then yes take off the positive cable after you have it running if it stays running the alternator is good. with the cable still off shut the car down then take the cable and put it back on. If the battery is good and there is a short that is draining the battery there will be a spark when you reconnect the cable,. helps to be in a dark area when looking for the spark. The bigger the spark the bigger the short.
the first two answers are wrong

do not unhook your battery

simply go to autozone (or buy a voltage checker that you plug into your cigarette lighter)

or any place like that

and get your battery and alternator tested

if the voltmeter shows 14.4 volts when the car is running then the alternator is working

and if its shows 12.5 volts when the car is turned off then the battery is fully chargedHow do i know if my car battery is broken or the alternator?
As I read the first two answers one thing is clear. The engine may run but you've learned nothing about the voltage and amp output of the alternator. Too bad the el-cheapo car companies don't put volt meters on the dash. If the voltage isn't between 13.5 - 14.1 volts the alternator is sick. Amperage has to be checked with every electrical accessory on full blast.



The alternator voltage accessment by Alex is all wet too. Although an alternator may charge up to 14.5 volts for a few minutes but the voltage regulator had better trim that voltage back to no more than 14.1 volts or the acid and water will boil out of the battery if the voltage is left unchecked over a 6 month time period. That my friend is a fact.
If you have a Batteries Plus ( http://www.batteriesplus.com ) near where you live, you could also have it diagnose there for free.
not on a fuel injection vehicle. you can buy a battery at interstate batteries for 30 bucks, and have your alternator rebuilt for under 50 bucks.