So everyone says you need a properly rated fuse, and you do.

If you want to power an 800 mA device from a car battery power source, you

must first realise that a car battery is NOT 12 volts. It will be 13.8

volts.

So you will need to regulate the voltage to a proper 12 volts.

You can buy a regulator chip. A 3 pronged device that you attach to your

13.8 DC source. Make a little circuit board using that strip pre drilled

stuff.

The live / positive lead will go from the car positive, to the front left

prong. Heat sink at back, writing on front.

The regulated 12 volts will now be restricted to 1 Amp, and it comes out at

the right prong, you will attach the positive out to the right prong, but

then you need to restrict the current to 800 mA. You must attach the

negative wire to the centre /=/ negative, If you are happy with 12 volts at

1 Amp you just connect the negative wire to the centre prong and there you

go. For lower current you will need ( a 'PROPERLY CALCULATED' current

limiting resistor.)

The centre prong is common/ to us/ this means the negative. A resistor

attached to the 12 volt 1 Amp out 'right prong' is to be fixed /shorted the

centre prong. You still take your power out from the left prong, its just

shorted to negative / centre prong via a resistor.

The current limiting resistor value is calculated using 'Ohms law'. (Don't

panic)

So now you are using the regulator you will soon have a nice steady 12

volts. you will have bought a 1 Amp positive chip, with a number something

like LS7812C? and be getting 12 volts / 1 Amp.

So now using Ohms law you need to work out the resistor value.

The on line

Ohms law calculator is here, so its not going to be a

nightmare,,,

http://www.cvs1.uklinux.net/cgi-bin/calculators/ohms_law.cgi
I did your suggested math.

Your

Answers: voltage 12 V current 800 mA resistance 15 Ohms

wattage 9.6 W

So you need a resistor of 15 Ohms to reduce you 1 Amp current to 800mA.

also give some scope on the Wattage of the resistor. Use at least a 12 Watt

or bigger resistor.

I will repeat myself here. This resistor is taken from the positive out /

left lead and fixed to the centre 'negative lead. This shorting out is what

causes current limiting. Not the most power saving way of doing it bit it

will work and probably cost you less than £5 to knock up. Similar circuitry

is used for building posh battery chargers.

Best of luck hope this helped, with the ohms law calculator you will be able

to work out the correct limiting resistor value for any appliance.