Charging a trailer battery through a trolley plug

I’m having a question about charging a trailer battery through a trolley plug.
A 13-pin trolley plug is a continuous flow of trailer battery charging straight from the battery (pin 9 and 13)
The question is, can I directly connect these cables to a battery or do I have to put something in between?
Some of the smartest men have said that there should be some kind of regulator that does not let the current flow too high in the case of an empty battery!
There was a danger of burning car wires!
Does the car itself have any regulator or protection sometimes to prevent this from happening?
I hope there is someone wiser who can help.

The long wire will keep the current down except for extreme conditions.
Fuse both ends to protect the wire.
If fuses nuisance-blow use larger wire and fuses.

Or maybe you want protection from running the main battery down.