With the key switch turned off, the battery is only physically disconnected on one if its two poles, but this should be sufficient to keep the controller circuit open and allow the battery to charge normally.
There should be no need to disconnect the battery lead unless you need to remove the battery, but some people prefer to unplug the battery just to ensure that it cannot inadvertently be left switched on.
If your parking area is dry, secure and the ambient temperature remains above freezing overnight, you should be fine charging the battery "in situ" while it is still connected and switched off.
If your bike is kept in a locked shed it might be better to keep the battery and charger indoors. The typical garden shed is not very secure, and if someone breaks into the shed and steals the bike, at least you will still have the battery and charger.
I had one of my sheds broken into eight years ago and my kids' motorbikes were taken:
When I contacted my home insurance company they told me they were not liable as these bikes were "motor vehicles" and were therefore not covered under the home insurance policy, so I should have taken out separate insurance cover.
I argued that these were just kid's toys solely for use in the garden, they were not registered and were not suitable
(or intended) for road use, but they would not change their minds.
However, when I asked if the ride on mower
(which was in the same shed but was not taken) would have been covered, they confirmed that the ride on mower would definitely have been covered had it been taken too, because they classified that as garden equipment.
So, a petrol driven ride on mower is apparently not a motor vehicle, yet a child's toy motorbike
(which is only used on the same grassed area) definitely is!
Alan 
