Figuring out how hard im pushing my electrical system

**Craig W** · 02-08-2005, 09:43 PM

hey guys, i have a carputer, dual touchscreens and dual amplifiers in my Pathfinder. 2 questions:

1) how do i tell how many amps or whatever my whole system is pulling in order to see how hard im pushing my current alternator.

2) how do i tell how much my system is using when the car is off? i have a few things powered all the time and want to see how much they are actually pulling with the engine off.

the only tool i have is a multimeter, will that do the trick?

thanks guys,
Craig

**bobs987** · 02-08-2005, 09:53 PM

I know there is a way to do it with a mulitimeter by unhooking the ground and putting one lead from the meter to the frame and the other to the neg. lead on the batt. im not sure what setting to set it on ill find out tomorrow, or anyone here can answer now???

Any One?????

**Craig W** · 02-08-2005, 09:54 PM

yes, find out for me if you can, it would be much appreciated.

Craig

**Grayscale** · 02-08-2005, 10:17 PM

Originally Posted by bobs987

I know there is a way to do it with a mulitimeter by unhooking the ground and putting one lead from the meter to the frame and the other to the neg. lead on the batt. im not sure what setting to set it on ill find out tomorrow, or anyone here can answer now???

Any One?????

you'll blow the meter, this will be funny.

**meddler** · 02-08-2005, 10:20 PM

I TAKE NO RESPONSIBILITY IF YOU BLOW UP YOUR METER, PC OR ELECTRICALS IN YOUR CAR.

Set the multimeter to read amps. Some meters have a different socket to plug in the lead. You can then disconnect the supply wire that goes to the +VE terminal of the battery (the one that supplies everything you want to measure) and measure between the wire you have disconnected and the +VE terminal of the battery. You can then measure current draw with the system off. CAUTION. Most meters are only rated to measure 10A. So if you think that your system uses anywhere near this amount of current when switched on DO NOT TRY TO MEASURE THE CURRENT DRAW!!!

The other option is to get hold of a clamp style ammeter. Maybe go to you local mechanic or car audio place and see if you can use theirs if they have one.

**Craig W** · 02-08-2005, 10:30 PM

I TAKE NO RESPONSIBILITY IF YOU BLOW UP YOUR METER, PC OR ELECTRICALS IN YOUR CAR.

hmm, the SUV is 2004 and $40G, not sure i am comfortable with that

**meddler** · 02-09-2005, 12:42 AM

It's just the standard disclaimer

Then the best option is the clamp ammeter. You do not need to disconnect anything. You just put the red clamp section around the wire and it tells you the current draw. See pic.

**davem** · 02-09-2005, 01:19 AM

I did a search on shop.yahoo.com and all I the clamp ammeter's I found were made for measuring AC circuits. They could do DC but still had the 150 mA restriction (which makes em useless)!

1. Anyone know where I can get a good one of these from that measures DC Amperage?

2. What about when I am measuring my system when it's off? Something is draining my battery and it CAN'T be the Lilliput Monitor (low video standby current draw) but that's the only thing I leave on continously (also the alarm but that's always been there). The power to the inverter (which then powers the normal ATX PSU) gets hard-cut by the ITPX startup sequencer which tells a relay to cut the power to the inverter. Anyone know if the relay could still be sapping power or what could be happening (i know it's a vaaaague question)?

3. Will these sort of meters be sensative enough to measure milliamps -- which should be the current draw when everything's off?

**KyferEz** · 02-09-2005, 09:51 AM

Clamp meters don't (Edit: Most don't) measure DC current.

To measure high current draw safely, get a high power resistor, at a very low resistance, 0.1 ohm or preferably less. Measure the resistance to make sure of what it is. Now, connect the resistor in series with the car's power line (such as you would have done with the ammeter).

Turn on everything you want to test. Read the voltage accross the resistor. Now you can calculate current. I=v/r. Use the measured resistance of the resistor.
Example: Lets say I measure the resistance is 0.1 ohms, and there is 4 volts across the resistor. Then I=4/0.1 => I=40amps

Be careful, the resistor will get HOT, and if you don't get a big enough resistor, you need 160 watts for a 40amp load using a 0.1ohm resistor, you will damage it (you will see it start smoking!). If you are quick about doing this, it will be ok even if you use a somewhat-too small of a resistor.

Or, you could get a much smaller resistor and it won't need to be as powerful. A 0.001 ohm resistor will only need to handle 1.6 watts.

Most any multimeter can measure low current. Just start with the highest setting current setting and work your way down, so that the current doesn't damage it or so it doesn't indicate an out-of-range condition.

KyferEz

**bobs987** · 02-09-2005, 10:14 AM

If your jst going to measure the current draw when the carputer is of i doubt you will be drawing anything more than 2 Amps let alone 10 Amps, you should be fine with a regular 10 Amp mulitimeter as long as the system is not on.(You still just want to measure the draw that happens over night right?)