Question: I have a new battery and alternator but still not keeping my battery charged.
Answer: First check to be sure the alternator belt is not too loose. If so, it will slip and reduce the voltage coming from the alternator.
About 1/4 inch to 1/2 inch of play should be about right when you push down on the alternator belt.
And you should not hear any squealing from the belt while the car is running.
Next, check the battery cables for corrosion.
If they are old, they could have corrosion that has worked it's way up under the outer covering.
If so, you may not be getting enough voltage to the battery.
Use a battery charger to get a full charge on the battery.
Start the car and use a volt meter on the battery terminals to check the voltage coming from the alternator.
It should read close to or just above 13.5 volts. If not, you may need new battery cables.
Finally, if the voltage is correct, then there is a power drain from something that is running or there is a dead short somewhere.
You will just have to listen and look carefully for the source.