Saturday, Jul 12, 2014 at 09:27
Sorry but there is ALWAYS voltage drop..its a matter of how much.
Lets start with the ciggerette lighter socket.
Is this an actual cigearette light or a factory fitted accessory socket that looks like one or an accessory socket that has been installed specifically for the purpose.
The cigarette lighter sockets themselves are not a clever item, they are notorious for poor contact quality and the best of them are rated at 15 amps maximum...and they are not happy things at that.
problems with cigarette light sockets and fridges are the norm rather then the exception....they where not originally designed as an accessory socket.
moving on to where this "cigarette lighter" socket is and how it is wired.
If it is an actual "cigarette lighter" it will probably be wired to a 15 amp fuse ( maybe a 10 in some modern cars) and in the dash with a fairly short length of wire......think about this for a minute..an actual ciggy lighter will draw its 15 ish amps for about 30 seconds before it pops out...that is the limit of the design.
If it is an accessory socket that looks like a cigarette lighter.....it may not be designed to support much load to start with...a couple of the ones I have seen are protected my a 10 amp fuse or less.
they realy are intended for charging mobile phones running sta nav and other fairly light loads..and they are not wired in particularly heavy wire to start with.
so if we have a factory fitted accessory socket..it might be marginal to start with.
Then we look at the 10 meter extension cable.
If we are working with a 12 volt nominal supply, voltage drop is ever present...there is always voltage drop.....we only have 12 volts and if only one or two of those volts go missing stuff stops working or works badly.
If wiring a ciggy socket ( or something to fill the same function) in the rear of a vehicle.....we need to be looking at 6mm automotive wire as a minimum over the approximately 5 to 7 meters involved...that is 4.5mm2 or 50 amp cable.
This extension cord is 10 meters...that is a long way in 12 volts.
Using 6mm ( automotive) or 50 amp cable would be the minimum I would entertain.
If this cable has been made up on 4mm automotive or 15 amp cable, it does not surprise me you are having problems.
If this cable is 4mm automotive twin..it will be about 4mm x 7mm dimension in the jacket....6mm automotive twin will be about 6mm x 10mm dimension in the jacket.
In electrical design in 12 volt systems the actual amp rating of the cable is only significant in very short lengths of cable....everything revolves arround voltage drop and using heavy cable to combat it.
so think about this for a moment..if you are connected to a socket in the rear of the vehicle, and a 10 meter extension cord..you will be running about 15 meters of cable back to the battery, pluss the poor contacts in two sets of ciggy sockets.
I don't need to do any calculation or measurements to expect to see problems.
If the fridge motor wont start...I don't know how the autoelectrician measured the voltage drop on load.
Serioulsy....unless you want to fit a purpose made accessory socket ( and probably not a ciggy socket) and have a pretty heavy extension cable made...forget the idea of an extension cable and leave the fridge in the car.
cheers
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