I have a little plug connector just under the saddle. I've modified my charger lead so I can just unplug the bike and connect the charger. Also makes removing the battery very simple.
Here's one of the same type of connectors.
When rewiring BSA's ( and any other old Brit ) we connect the battery to a T shaped blade plug,
Thus like Greybeard we just unplug the bike and plug in the charger.
Apart from eliminating the possibility of cross wiring, it allows the battery terminals to be painted over with
Liquid Electrical tape" so no more corrosion at the battery terminals.
Contry to popular belief there is no necessity to fit the waterproof type that Greybeard is using, just put a dab of di-elctric grease on the plug before connecting them.