What usually happens in these cases is that the parts which were tested were not tested properly.
A battery can only be tested with a load tester. It will test good with a hydrometer or a voltmeter even when it doesn't have enough reserve power to reliably start an engine.
An alternator test must include a measurement of both the charging voltage (between 13.5 & 14.5 volts when idling) and also how much current it draws from the battery when the motor is not running. But the real test is of the alternator when under load; it sounds to me like the alternator is perhaps capable of keeping the battery charged during daytime driving, but cannot supply enough power to run the lights, accessories, and ignition; and keep the battery charged at the same time. This is often caused when one or more diodes in the alternator fail (which will reduce the maximum power output of the alternator by 33% to 66%; depending on how many diodes have failed). When that happens; the battery will gradually go dead during night driving; but may do much better during the day. The way to test the alternator is to start the engine, and first measure the voltage across the battery terminals with the engine idling and no lights or accessories running; it should be between 13.5 and 14.5 volts. Then turn on the headlights to high beam and set the heater fan to the highest speed. With the engine still idling; if the voltage drops below 13 volts during this test, and the alternator drive belt is not slipping; the alternator should be replaced. If an alternator has one or more shorted diodes in it; it will slowly drain the battery while the car is parked. A good electrical system will draw less than 50 milliamps of current from the battery while the motor is stopped and no electrical accessories are running. If it draws more than that, there is either a bad alternator diode or an electrical short somewhere.
Battery cables are properly tested by measuring the voltage difference between the two ends of the cable while the starter is running. A good cable should have less than 0.5 volts drop through it when the starter runs. Be sure to test both the positive cable and the ground cable. It often turns out that there is substantial voltage drop at the connection between the battery cable clamps and the battery posts. Battery cable clamps will develop invisible corrosion on their inside surfaces. The best way to clean this is with a tapered reamer type battery cable service tool.
A starter should be tested by both measuring the cranking current draw (typically between 150 and 200 amps) and also measuring the voltage drop across the solenoid during cranking. There should be less than 1 volt lost across the solenoid during cranking.
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