I've found a very good explanation on schmitt trigger circuit design here -What is a Schmitt Trigger and its application exactly?
But the design is done considering same reference voltage and the comparator biasing voltage v1. It means threshold voltages (high and low) will be varied along with the supply voltage. I want to achieve a constant threshold values to design a battery charge controller. For example Vth(H)=13.8v and Vth(L)=13.0v. How can I achieve this?