question about current

Ok im new to this so bear with me. If I hook up my multimeter to my cars battery and measure amperage, will it show the max amperage the battery puts out? I don't really understand how the current draw concept works. Since a battery can supply about 600 amps, whats to keep that from burning out my 45W headlights? Theres no resistor hooked to my headlights is there? Does an electrical accessory only draw as much current as is needed? How does that work? Please help me out in understanding this. Thanks.

bmw5002 wrote: If I hook up my multimeter to my cars battery and measure amperage, will it show the max amperage the battery puts out?

If you are measuring static current draw it will be the parasitic current draw while the vehicle is at rest. If you are measuring the dynamic current draw while the vehicle is operating, then you may not use your DMM. You will need to use a inductive / halll effect clamp on meter to measure the current safely. As the current draw will exceed the standard 10 ampere range of a DMM. You are measuring the current that is being consumed by the vehicles electrical system, not what the battery is capable of producing in a instantanous fashion. ie. CCA (cold cranking amps)

bmw5002 wrote: I don't really understand how the current draw concept works.

Thats why you are here, to ask questions and learn. I would also suggest that you take a few moments to research the information on line. This will explain alot more than I have time for.

bmw5002 wrote: Since a battery can supply about 600 amps,

The current rating you are referring to is the CCA. This is what the battery can supply for XXX amount of seconds, based on a relative room temperature of 70-80'F.

bmw5002 wrote: whats to keep that from burning out my 45W headlights?

The primary battery is called a SLI. Starting, lights, ignition source. The primary goal of any battery is to start the vehicle. From there on the battery is simply a load, which the alternator must replenish what was taken during the ignition start up. If the alternator is not operating properly. ie. Burnt out diode packs, bad regulator. Then the secondary role of the battery is to supply the needed current to offset the demand(s). This is called the RC value. Which is the reserve capacity, which is rated in minutes.

bmw5002 wrote: Theres no resistor hooked to my headlights is there?

If you have DRL (daytime running lights) then yes there is.

bmw5002 wrote: Does an electrical accessory only draw as much current as is needed?

Yes and no. That is dependant upon the circuit load. ie. Your radio will draw anywhere from 1 amp, to 40 amperes etc for example based on your listening volume. Whereas, your headlights will draw consistently the same amount of current based on the value of resistance within the filiment of the bulb, ballast, etc. Knowledge is power. But only if you apply that knowledge in a positive way, which promotes positive results in others.

EVIL Teken . . .

Wow, awesomely informative reply! It cleared up alot. Thanks. What I'm trying to do is rewire my lo-beams of my headlights. My bulbs have two filaments, with the center prong acting as the ground or supply for both filaments, and the two out prongs grounding or supplying separately the two filaments. Its a weird problem, and its an old car, so I don't mind rigging something up to fix it. I've tried almost everything and this is my last resort. When I turn on my lo-beams, nothing works. When I turn on my hi-beams, only the hi-beam side of the bulb lights. So the problem is that the lo-beam side is faulty somewhere between the relay and the lights. Heres a wiring diagram for my car (ignore that thing "Where are the high and low fuses...", its from a different post about my headlight problem): So I think I'm going to patch into the wiring harness for my multifunction headlight switch and wire up my own relay. I wanted to make sure that by connecting the relays coil between the battery and the lo-beam switch on my multifunction cluster, the small wires and the relay coil wouldnt just go up in smoke from all that current from the battery. The battery would also connect to the switched side of the relay and directly to the lo-beam connector on my headlights. So I'm really bypassing the car's electrics for the lo-beams with my own. So if I'm understanding correctly, a relay coil rated at 12v and 600mA will be fine just hooked right up to the battery? What about ohm's law? It would seem that you would need a resistance of 20 ohms with that, given r=v/i. Do you understand where I'm confused here? Say I had a device that required 12v and no more than 60mA. Would I need a resistor for that? Thanks alot for the help Teken!

First question I would be asking and wanting a accurate answer is this: Why have you not replaced the headlight(s) first, to see if it isn't in fact a burnt out filiment within the bulb?? Knowledge is power. But only if you apply that knowledge in a positive way, which promotes positive results in others.

EVIL Teken . . .

Lol, teken believe me, I'm not a total dunce. I tried EVERYTHING, minus tearing apart the wiring harnesses. I replaced the whole multifunction cluster. I replaced both headlight bulbs. I checked all the fuses and relays. So its safe to say it's either a broken or severed wire, or a bad ground. And I'm not ready to tear the car apart looking for that exact spot. So I believe this is the last option. Plz check the other thread someone posted about LEDs and resistors, I think I finally passed my mental roadblock.

bmw5002 wrote: I wanted to make sure that by connecting the relays coil between the battery and the lo-beam switch on my multifunction cluster, the small wires and the relay coil wouldnt just go up in smoke from all that current from the battery.

I believe you are getting too involved with the whole battery situation. The battery has only one role in your problem. That being it is a constant source of +12 volt power. It does not provide 600 ampere's or higher, all the time. It only provides the required current the *Load* that being the lights, or anything you require at that moment while the vehicle is off. The 12 volt source, or power is being supplied by the alternator at all times, while the vehicle is operational / running.

bmw5002 wrote: So if I'm understanding correctly, a relay coil rated at 12v and 600mA will be fine just hooked right up to the battery?

The relay is rated to operate within 12-14 volts. The coils internal resistance is typically 80 - 90 ohms, which translates to a holding current of 150 - 130 mA of *coil* current draw while it is turned on (energised). A standard Bosch relay has a internal coil resistance of 86 ohms, this translates to a coil holding current of 140 mA, while it is on. Knowledge is power. But only if you apply that knowledge in a positive way, which promotes positive results in others.

EVIL Teken . . .

So to answer your question specificly regarding the head lights. You must know the power output of the bulb(s), then you will be able to install the correct size relay rated in amperes. You cannot use a standard 30 ampere relay to do this. Many people have, and do use them. But I can assure you that the life span of the relay is short lived. Typical for a head light assembly is 40-50 ampere's. You have to remember, that the inrush current while it is first turned on, is extremely large, and that the inrush / start up current at extreme temperatures (cold) is almost 2X greater. Hence the use of a larger relay based on instantanous use, opposed to continous use. Tell me how much power they consume, and I can tell you the exact amperage the relay(s) must be. Knowledge is power. But only if you apply that knowledge in a positive way, which promotes positive results in others.

EVIL Teken . . .

They are 45W. Thats about 3.8 Amps right?

3.75 ampere's each lamp. Remember, that number must be relative to the source voltage. That is why the figure may vary, as the battery is typicaly 12.6 to 12.8 volts at rest. 13.6 to 13.8 while operating. So the load current will be aprox 7.50 amperes to 6.52 amperes for the pair continous, not instantanous though. Hence the use of much larger relays during start-up Knowledge is power. But only if you apply that knowledge in a positive way, which promotes positive results in others.

EVIL Teken . . .

Have you tried to apply direct voltage to the low beams yet to see if they do come on? From my stand point I would have suggested that you measure the voltage drop of each wire leading to and from the lights. Only during operation will you see the true voltage drop of the circuit. Knowledge is power. But only if you apply that knowledge in a positive way, which promotes positive results in others.

EVIL Teken . . .