A lot of Hikvision ip cameras do have a 12v input, can you connect that directly to a 12v battery, given that a 12v battery is at 14v when fully charged, and 10,5v when empty?
I would check the detailed specs of your camera to see if it lists the acceptable voltage range. My Dahua cameras show 12V ±30%. If that's the same with your cameras, it should work.