my car is hard to start when the battery is low. i charged it last night and i just started it the voltage was good i had the door open for about 5 mins. i turned the motor off and restarted and the voltage was considerably lower. how can i check if the alternator is bad and how can i check if the battery is bad?

 

A lot of auto parts stores around here have bench testers for alternators that are free and battery testers that are free. That's the best way of doing it if you dont have the stuff at home. Optimum volts for a good battery 13V with about 2-3 ohms.

 

Those alternator testers in parts stores don't always tell the truth. I had mine test fine but it was very screwed up, I took it back later and exchanged it. Problems gone.

 

Did you happen to look at the readings? the auto parts guys dont usually know what they are looking for most of the time.

 

Test the alternator by checking batt voltage with eng off. Start engine and re-check batt voltage. It should now be higher. If it's not, the alternator or wiring is bad.
(This tests may indicate a good alt when you may still have a bad diode which limits the output, but dosen't stop it completely.
If it fails the test, you know the alt (or wiring) is bad)

To check the batt; disable ignition so it will not start.
Connect voltmeter to batt. Crank eng for 20 seconds while watching meter. If batt voltage drops below 9 volts with starter engaged, charge batt and re-test. if it fails again, it's bad (or possibly the starter/wiring is bad and drawing too much current).

These tests are rather crude but will give you an idea what your up aginst.

If the batt has caps a batt hydrometer is a good way to check the batteries state of charge. Most all the auto stores sell them for abt $10 or so.