Adding a 'leisure' battery

I went through hell (and considerable expense) on this. Finding a competent auto electrician was the main problem. First, someone wired the additional battery and made the entire bodywork live. Then, with one burnt out radio, another person had a go. The isolator kit blew up. In the end, I have a battery to operate at 12 volts and a switch that allows me to accumulate charge in the leisure battery when driving, and then disconnect via a simple switch when parked up. This final solution cost pennies.