I agree with Griff.
I rarely actually charge my batteries up separately (except when I give them a boost to desulphate them). I have fitted a battery cut-off on the negative on the first battery and "touch wood" haven't had an issue. I run the ferret every 3 to 4 weeks during the winter, probably more frequently in summer. I have recently fitted a NOS 2 speed generator and find that 10 mins on fast idle is more than enough, the voltage is up to 16v with ease.
The only thing I have found is that after a few weeks it can take some starting, it's a fuel issue. After a few weeks I find that the fuel needs priming. As you will find out it's nigh on impossible to get to the priming handle on the fuel pump, so I "gently" pressurise the fuel tank to prime it. After that - it starts on the button, and much reduces the wear and tear and drain on the batteries.
Fitting a battery cut of is an essential to prevent drain.