The plastic bungs should be red and blue, the red being + (the positive) and blue - (the negative). Just remove these that's all before you connect. Clip the red first and then the blue and visa-versa when unclipping. That's the advise in the book anyway but it isn't a hard & fast rule. Regarding the 12v change over switch make sure that the switch is initially in the neutral position (centre) as there will be no electrical feed and you won't blow any fuses on the car. When you have the grey plug (12S) connected to the car, then switch it to car. This will connect the current electrical feed to charge the caravan battery and also run the fridge on 12v power (flick the red switch on fridge and it should light up). When you are on site and ready to unhitch, make sure that you flick the car/caravan 12v switch back to neutral first then unplug from car. This will cut the feed which won't blow any car fuses. When that task is done then flick the switch to caravan and the power will be coming from the caravan battery. I failed to put the switch to central once and then wondered why it wasn't charging the caravan battery afterwards. It wasn't till I took it to the dealer that he said the auxiliary fuse had blown. That was my fault but it was a lesson learned so it is important to cut the feed before connecting or disconnecting the car plug whether you have twin electrics or the single 13 pin connection.
If you are on EHU (electric hook up) then the caravan battery will be charged via the onboard charger automatically. Just leave the 12v switch on the caravan setting.
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...Clip the red first and then the blue and visa-versa when unclipping. That's the advise in the book anyway but it isn't a hard & fast rule...
Always put the red (+) on first followed by the blue(-) and the reverse to disconnect, this is to prevent arcing/sparking if you inadvertently have anything 12v switched on at the time of connecting the battery.
I didn't actually know the reason why you had to connect and disconnect the battery terminals in that order but I have been doing this recently just following the guide in the book.
Some batteries have a plastic tube on the side to allow the gas to escape when charging,there is usually a bung in it to stop the acid from spilling during transit,this may be the bung they are referring to.I would have thought the others would have been called covers,to my way of thinking a bung is inserted into something.
Saxo1
I have just bought a battery for another purpose, and there is a label on it to remove the small red bung at the side to the battery to act as a breather. I think Saxo is right here.
The reason for fitting the positive first and removing it last originates from the motor trade,if you are using a spanner to loosen the positive terminal with the negative connected, as most modern cars are negative earth if you touch any part of the vehicle with the spanner while in contact with the positive terminal and the negative still connected you short circuit the battery with quite spectacular results,I have seen spanners welded solid to the bodywork where this has occurred.
Saxo1
Maybe the plastic bung is actually being referred to the short breather pipe at the top of the battery. When I bought my 110ah battery new two years ago there was no bung on the breather tube, only the red & blue plastic pole caps so I'm not familiar with this protection cap. If there is a bung on the breather pipe then it needs to be removed because when the battery is being charged the acid activates and releases the gases which needs the breather to stop a build up of pressure.