Ok hi all still learning about what Herman can do

I recently discovered the black box under the seat in my 1991 westie and noticed the orange light wasn't on whilst on hook up so i flicked it and it came on but after a while the box got hot is this A, normal, B,do i leave it on at all times, C what does it actually charge and D if i don't leave it on how does my batteries get charged.Thanks in advance.Marc.
