using step-down charge module

I have two 12V 2,3A batteries (paralery connected to get 4,6A) and I was recommended to charge them using step down charge module (I bought this one) I have 18V 5A power source.
How to set it to charge the batteries properly? I suggest to set the voltage to 12,5V, but what should I set to current?

See this article on charging lead acid batteries.