Car would not start, changed 5yr old battery. 1wk later same thing. alternator checked out ok. belt and tensioner ok. Seems to happen when car is warm. any ideas?

alternator (generator) problem. Could easily be the drive belt tension or a faulty alternator.
Having said that, the first thing to check would be the battery itself.
It can cause exactly this problem. Change in outdoor temp, moisture etc can just flatten a battery. Charge up the old one and see if it holds a charge. If it looks like it's leaking or corroded, put a new one in. If the same thing happens, check generator.
Batteries are strange things. work for years and years then one day...nothing. Can happen with new ones too, rare now, but still happens. Extreme changes in weather, low fluids (although most batteries are sealed now).
So......battery first (check fuses too) then if persists...generator.
Hope this helps.....good luck..

Several things to check. Check alternator drive belt tension, check alternator voltage output, with the engine running at fast idle connect a volt meter across the battery terminals, around 13 to14 volts is ok. check if there is a light on in the glove box or boot. how old is the battery is holding a charge .

You need to check the belt that drives the alternator for proper operation. The belt may be old and worn, or a pulley may be not freewheeling, but seized or with a bad bearing. Possibly the tension on the belt is too loose from a worn out tensioner.

First, spray a little water on the belt with engine running and see if noise goes away, possibly a worn belt. Pull the belt off and check all pulleys. Spin them by hand and see if spins easy, no rough feeling or a catch when spun. Check alternator pulley, idler pulley, tensioner pulley, PS pump pulley and A/C compressor pulley-check them all for free wheeling. If good check the tension on the belt when installed. If unsure, have a mechanic check tension with a belt tension tool.
If belt is good and pulleys good, check alternator output with a voltmeter on the battery with engine running. If voltmeter is below 13.5 volts, the charging system is still not working properly. Have it checked out.

Check for a loose positive battery cable wire. If that is ok then your alternator is starting to go South

sound like faulty alternator.check battery voltage should be 12.6 volts.replace battery cables if they are very old.if battery and cable okay.start engine check alternator voltage out put at the battery.with engine idling.alternator out put should be 13.5 - 14.5 volts.if not.check alternator, fuse,check alternator battery feed wire and voltage regulator wires for damage or loose connections.check alternator drive belt tension.make sure drive belt not slipping,worn, glazed.replace drive belt if glazed.

This sounds like a bad alternator, its sounds like it sometimes charges and somtimes not, check the wiring to alternator and if all is good then repolace the alternator.this should be under warranty. i would ask them to just replace the alt. hope this helps.

sounds to me like you better find another dealer or possibly an auto electric dealer they should have done it right the first time

It sounds like the alternator is not charging. First check the belt tension, sometime the tensioners go bad and the alternator isn't spinning. If that's OK then it's time to replace the alternator

, do a simple test on the charging system...check battery voltage. should be 12v or better. start engine & check volts at idle, no accessories on. volts should read 13.5 -14v. Turn on all accessories(including headlamps on high) volts should drop slightly but should not creep down. Definitely not below 12.5. If voltage drops lower than 12.5, the regulator (in the alternator) is not working. either change regulator or change entire alternator. Alternator drive belt should have sufficient tension so that alternator cannot be turned easily by hand. If it can, check belt tensioner.

Couple things First: Check your belt running to your alternator and the tensioning arm( needs to be really tight) If you have a muti meter check the output of your alternator and the voltage of the battery. Does it matter if the engine is hot or cold?