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I wonder how much current is drawn from car battery when the headlights and the radio are fully open. Is there a way to measure the resistance of the radio or the headlights without unscrewing or unscrewing few screws? Or any other methods? Something like plugging to a varying resistive load into cigarette lighter to determine the resistance? Or connecting a capacitor to determine the charging time (rc network, the all resistive loads connected to car battery are r; and capacitor plugged into cigarette lighter is c) to find resistance connected to battery? Any suggestions are welcome.

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    \$\begingroup\$ DC Current Clamp ... \$\endgroup\$ – brhans Dec 19 '14 at 20:48
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    \$\begingroup\$ Headlights are easy. They tell you how many watts they consume. P = V * I. So divide the ways by the cars battery voltage. My 24W headlights take 2 Amps at 12 Volts \$\endgroup\$ – Passerby Dec 19 '14 at 22:28
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    \$\begingroup\$ Put the DC current clamp meter around the battery earth strap, and turn on the headlights. All that current must get back to the battery somehow! (via the erath strap) But if you do try that, remember to not start the engine; the current must flow from the battery, not the alternator. \$\endgroup\$ – John Honniball Dec 19 '14 at 22:37
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    \$\begingroup\$ Keep in mind that the voltage will be perhaps 15% higher with the engine running. \$\endgroup\$ – Spehro Pefhany Dec 20 '14 at 3:54
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    \$\begingroup\$ Voltage drop across a portion of wiring can be used as a current sensing shunt. \$\endgroup\$ – Russell McMahon Dec 20 '14 at 16:52
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@brhans has the most straightforward solution as a comment. Locating and getting a clamp around the wire may be a bit of a tricky endeavour in a car. An easier solution may be to just pull the respective fuses and put your ammeter in the terminals instead. Naturally before you go blindly plugging your meter where fuses were before you should check to make sure that the meter's fuse is rated higher than the current you intend to measure. The radio and headlights are likely to be on their own dedicated circuits so this should be pretty easy.

My meter's high current scale will only go to 10A; this is likely the rating for the radio's supply but the headlights may be on a 20 or even 30A circuit. You'll have to check with your car's manual and/or fuse panel insert. If your meter can't safely measure the current that the original fuse is rated for you might have to resort to other means to measure the current.

The process is simple: pull the fuse, connect your meter (set for current measurement) in the socket positions that the fuse was in, take measurement. You will want to make sure the lights/radio are off before you pull the fuse; opening a high current circuit under load can be unpleasant for you and the vehicle (arcing).

Clearly this is only intended as a temporary, vehicle-not-moving solution. If you want to permanently monitor the current then you can get a little more creative and use either inline shunts or a hall-effect current measurement device. Your question seems to indicate that this is a temporary requirement, so my first answer (pull the fuse and measure current across its sockets) is probably the easiest.

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OK this is a late night 'crazy' solution. But I wonder if you could tack (solder) voltage tabs (wires) onto an automotive fuse and measure the current with the 200 mV scale of a good DMM... I guess you'd have to calibrate it your self.

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I added this for the event that the headlights are on a fuse that means that they may be drawing too much current to be measured by your meter in series.

For each measurement, hack an extra fuse to make it accessible to measure the voltage drop across it using your voltmeter. After measuring the voltage across it, you just have to remove the fuse and then with your bench power supply, put enough current through the fuse to duplicate the voltage reading, and now the amps reading on your power supply should tell you how much current was flowing in the original measurement.

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