When you put 14V at the battery terminals current will flow into the battery. It doesn't matter how the 14V got there. You can measure it with a voltmeter or an appropriate Arduino circuit.
If the battery is in a low state of charge then it will take a lot of current at 14V. To hold it up at 14V requires a lot of power to keep that high current going. If you don't have that power available then your source must either blow its fuse or reduce the voltage. Fortunately wires have positive resistance so just the wiring keeps the system stable with no dangerous runaway.
Thanks very helpful so do car batteries need protection from overcharging? How does the van avoid overcharging by default if there's nothing between the altenator and the battery?
@moderategamer, I am still not clear what you had in mind when writing Reply #10 - there seems to be something you think needs to be avoided but you have not said what it is - something that you could do if there was more distance between the connections? I have the sense that you are trying to say something like "if the placing of the parts was different I could do X"
Posting a diagram of the circuit might help. See this Simple Image Guide
As @MorganS has said, just measure the voltage at the battery terminals.
In an electrical cable without a resistor the voltage will be the same everywhere.
I just want to charge the leisure battery while the altenator it running and to not charge when it isn't the fuses were just there to protect the battery in the chance of over current. I was wondering if the altenator is charging the battery how could I measure the battery level if it's being pulled up by the altenator to avoid overcharging.