# Bad battery vs. bad alternator



## jk2000bmw740iL (Jan 6, 2009)

how would you check to ensure it's bad battery vs. bad alternator? any smart tricks?
my 2000 BMW 740iL starts today but it doesn't the next day. it starts again if I jump it.
what are the possible source of battery draining? or not holding the charge? or not getting charged properly?
thank you!


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## fear740il (Dec 19, 2008)

jump the car to get it started and go to schucks, pep boys ect and the will do a load test for you.


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## CodyItaliano (Feb 19, 2009)

if you have a volt meter, or multimeter, you can check up on the battery like that. check the voltage after the car has been run and is ready to sit. after leaving it sitting, like over night, check the voltage again. do this a few different times in a week and see if there is much change between when the battery was charged by the car running, and the time in between while its sat. thats about the only thing you personally can do unless you have a specific battery and or alternator tester. but like it was stated above, go to a local auto parts store. they will be able to test your battery and alternator on the car and should be able to tell you if there is anything wrong. 

on the battery, it may not have the required cold cranking amps, may not be able to hold a proper charge or have a bad cell. as for the alternator, it may have a bad regulator not allowing it to charge much above 12.5 volts, might be lacking amperage output, or could have a bad rectifier. these would be things that a simple test at a parts store would be able to tell you if either part is defective in some way.


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