My hyunda getz
 

my getz 2004 will not hold its charge so I changed the alternator and the 2nd hand one I put on is still not charging. I had the one I took off checked and they tell me there's nothing wrong with it. the battery is newish any ideas about why it won't charge up

Not holding a charge sounds like an issue with the battery.
So what caused you to change the alternator?

Did you put a multi-meter on the battery with the engine off?
Was the voltage near 12.25 to 12.75 volts?

Did you put a multi-meter on the battery with the engine running?
Was the voltage near 13.75 to 14.75 volts?

If battery voltage was low then there may be a parasitic drain on the battery
Would then take multi-meter and put it in line to measure the current draw of the vehicle with engine off and car closed for 30 minutes
(it sometimes takes 30 minutes for all units to shut down)
If current draw with vehicle off is less than 50 mA then it is good
If more than 50 mA then got to find the thing drawing too much current

If the alternator output voltage is not within specs then check wiring to field coils on alternator
In some vehicles they are from the ECU and determine the charging voltage
Or how much current to produce to handle the load of engine running with the options on

In any case, the first thing to do is to clean terminals and grounding points
This will knock out the issues of corrosion or wiring contact

Good luck

many thanks the fan belt was slipping but was not loose, it was down to 11 after putting a meter on so we guessed it was the alternator but the one I took off has been checked and come back good but it's not charging the battery is newish so i am at a lose, I will have the battery checked i have to keep taking the battery off and put it on charge but will take you advice