Effect of duty ratio on buck converter with battery load
You don't give much information : what are the voltages of your voltage source ? Of the battery ?
Let's say the input voltage is E=14V, and the battery voltage Vb=12V.

If your duty cycle is D=0.1, you will get a small charging current, because your buck converter will operate in discontinuous mode, which means that the current will be non-zero only during a small part of the period.
If you want to operate in continuous mode, you must have at least D*E>Vb.

The two operating modes are explained in the  wikipedia article. See figure 6 : you can operate at an output voltage close to maximum, but in discontinuous mode, and at a (very) lowcurrent.