Avoid current spike in buck converter
The issue stems from the relatively large parallel capacitance to the battery. The low impedance of the battery exacerbates the situation.

Here a 12V battery with the internal impedance set as per your statement (0.004R) is placed in parallel with a 4.7mF capacitor with an initial voltage of zero.



Essentially you have a 12V low-impedance source in parallel with a 0V low-impedance capacitor.

How to improve it this


Set the initial voltage on the capacitor to 12V. This will mask the issue as you REALLY need to deal with this inrush in practice
Add some realistic impedance to the capacitor 
contemplate some form of soft-charge circuit for this bulk capacitor that can be shorted out via a MOSFET