Battery simulator

How I can simulate a battery to test my battery charger?
I want to test my battery charger for all the charging phases without spending hours to charge /discharge batteries

To create an easy battery simulator all you need is an operation amplifier with high current sink capabilities
Connect it to the charger output instead a battery
The voltage you set on the inverting output will be the OPA output voltage, the charger will recognize this circuit as a battery
So, by changing the voltage on the inverting output the output voltage will also change and you can test easily all the charger opweration pahses