Thoughts or comments.
First, read below about chargers and Fires.
Are you trying to design an 18v Ni-cd charger? If so, you will have to find a way not to over charge and cause a fire. You will need to limit the charging current. There are two types of Ni-cd chargers. Many years ago I bought a cheap 18 v ni-cd black & decker drill. It had a 500 ma transformer and took almost 24 hours to charge at a very low ma. I have 18v dewalt chargers that are fast chargers. I've never measured the amp draw of a fast charger, but I can say I fried the wiring in my truck's cigarette lighter by using a 12v in dewalt charger which is designed to charge all sizes of dewalt batteries.
I'm not an expert, but the new chargers, that can charge batteries ranging from 7.2v-18v, simply monitor the charging current and shut off when it drops to a certain lever. My 12v truck charger has to have an inverter circuit to change 12 volts in to around 20 volts to fast charge 18v batteries.
So, thoughts and comments, go buy the charger you need unless you are desinging something special. In that case, go to your local college and sign up for an electronics class. One of the first things should be making power supplies, which is basically the same as making a charger.