Changed battery and altenator but my kia 2000 sportage still won't hold a charge

The battery is charged by the alternator. With the engine running, a voltmeter (not a test light) should be connected to the (+) red lead) and (-) (black lead) post of the battery.
Expect to see the meter indicate 13.5 to 14.5 Volts DC. If you see LESS that this voltage, the alternator is not powering the car's electrical & ignition systems and not charging the battery. The battery is doing all the work, and it will be drained in a fairly short time.
This can be the result of a defective alternator, poor or incorrect electrical connections, open fuse, open "fusible link", etc. Until 13.5 to 14.5 Volts DC is present at the battery post while the engine is running, the battery will NOT be charged.
The battery should have slightly more than12 Volts DC available with nothing connected to it. If it is less than 12 Volts with nothing connected, it is discharged. If it is 10 volts or less, it is completely discharged and may have a shorted cell (even if new!). Either way, the battery should be charged with a home charger, trickle charger, or even connected to another car to be charged. The length of time required is determined by the level of discharge it was in when tested. When the battery is at 12 or more, it is considered charged and can be reconnected to your car. Perform the voltage checks again with the engine running.