I need to design a buck-boost converter operating in CCM for charging lead-acid batteries. But I don't know what minimum inductance should be taken... Here are the parameters you need: Vin from 30V - 160V, Vout 50V, Iout less than 4A. Switching frequency is 20kHz. In load side, I am not sure whether take the battery internal resistance to calculate minimum inductance for inductor. If I choose around 0.5 ohm for load, simulation shows there is a very high current across inductor. Please help me to solve it. Also, I am wonder how to determine which part of duty cycle is CCM region. Many thanks!

 

It's not the battery impedance that determines the inductance needed, it's the I/O voltage, current, and duty-cycle, among others, that determines that.

Here's a calculator for the minimum inductance needed for a switching regulator.
For your range of input voltages, calculate for both the high and low voltage, and use the maximum inductance calculated of the two as the minimum you need.

The current must be controlled in some manner to limit it to 4A maximum you want.