Never attempt to charge a battery in temperatures of less than 2 degrees Celsius, this is dangerous and could cause fire.
Only use the charger supplied with the bike or a replacement from the bike manufacturer, using the incorrect charger is dangerous and could damage the battery or cause fire.
If a battery case has been damaged, do not continue to use it unless it has been checked by the supplier or manufacturer and they have declared it safe to use.
If a battery is paired an with intelligent charger that will automatically switch off when the battery is fully charged it is safe to leave them to charge overnight, providing the air temperature is above 2 degrees Celsius. If you are not certain that the battery/charger set up you have has this facility do not leave a battery on charge overnight.