First find the output power and voltage from the brick that powers the electrical device - in most country's it is mandatory to have this on the power brick label (although you often need a small magnifying glass to read it).
For my 50cm TV it reads 12V, 3A and has a DC symbol (solid horizontal line above dashed horizontal line). If you multiply these together you'll get 12V * 3A = 36VA (VA = Watts for non inductive loads, add 20% for anything with electric motors). If the low voltage output A, VA or W is not reported you can calculate it from the input V and A (or VA or W) as the input will be equivalent to the output + 10% for conversion losses.
The label should also indicate the polarity of the connection to the device, most will use a barrel type with the hollow centre being the +V. This is most often shown graphically as a + sign inside a capital C with a - sign attached to the middle left of the C. If the picture is a capital S on its side the output is AC and the solution isn't so simple.
If the label is missing use a multimeter to check the output from the brick (if you don't already have one go to a cheap (pound) store NOT an electronics or auto store, you'll get a small, workable one for under £5). Most good stores can be persuaded to allow a test of the brick while not connected to anything . . .
Measure between the inside and outside of the barrel connector on the 20V DC scale. With no device connected the no-load voltage should be somewhere around 18V. Record the voltage and whether the centre (inside) of the barrel is +V or -V. If the reading constantly changes try the 20V AC setting
If the device is for 12V DC at or below 10A then you are in luck. [12V DC is a 'generic' term for 11 to 14V DC, which is the usable range in a car; 13.5 to 14.2V with the engine running down to 11V minimum required to operate the starter motor] There are several choices for providing power - 1) a dedicated cable with the correct plug on the end, 2) a single 'cigar lighter' receptacle on a wire, plus a cable with sockets for the cigar lighter and the device at either end 3) a single 'cigar lighter' receptacle in a wall plate outlet, plus a cable with sockets for the cigar lighter and the device at either end, etc.
A typical DC outlet will handle 10A easily, so long as heavy enough cable connects both the +V and -V all the way to/from the battery (use a fuse !). However screw terminals or Anderson plugs are recommended for higher power devices. These are simple and cheap to fit, they can be found at automotive stores, caravan suppliers and ship's chandlers (outdoor, waterproof types).
If the device is AC or any DC voltage other than 12 to 13.5V then a 200W DC to AC inverter may be used with acceptable losses of ~20%. You will have 12V Battery DC --> Inverter 220V AC --> Power Brick --> Device.
bye.
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