Additional Lighting Current Draw Question

[bdbecker]

I've got 4 - 150W lights, along with 2 - 30 amp/350W relay kits on their way to me in the mail. I'm also looking at installing some auxillary backup lights in the near future. My question is this - how can I figure out if my factory alternator can keep up with the additional load on the system? I'd hate to be out and about having fun at night, only to turn off my truck and not be able to get it started again because my extra lights, and other accessories like a fridge, pulled too much out of my battery when I was driving.

 

[Tony LEE]

Regardless of the rating of your alternator or battery capacity, there will come a time when due to lots of slow running at night or unusual demands by the fridge and other loads, your normal starting battery ends up too flat to crank the engine. The only way around it is to have a reserve battery that is charged via a smart isolator and that can never be discharged except when you need it connected.

There are lots of ways to achieve this, and the cost will range from $100 upwards, but once you use it once, you will consider it money well spent

 

[corax]

say you have a 100 amp alternator, conservatively 100A x 12V = 1200 watts. This is the max current draw before you start draining the battery

So the next step is to find out what your alternator outputs and do the math. Some parts stores or Sears can test your alternator in the vehicle and tell you its actual output.

 

[bdbecker]

Thanks guys!

PS - I really like that quote corax.

 

[evldave]

To find out what your stock alternator puts out, go here

http://search.waiglobal.com/vehiclelookup/

Just enter your vehicle and the results will include your alternator output on the right.

 

[4x4mike]

To cut down on my draw I updated my Hella 500ff's to 55w HID and most of my other lighting is LED. With 2, 55W hids pointed forward I've got more light than I need. If I were bombing down roads 100 mph I'd probably need more but I don't. All of it custom homemade by me and I have no worrys about over drawing the system. Even running close to the max output of your system is going to stress it. Your alternator won't last as long nor will your battery, they are just getting used more.

 

[bdbecker]

So here's where I sit now...

My alternator pumps out 130 amps, assuming that the fact it is 7 years old has no effect on it's efficency.

The lights I'm going to be adding will be pulling 50 amps when on. Headlights should be 4.6 amps (low beam) or 5.4 amps (high beam). If I crank my stereo all the way up, add another 8 amps. So now I'm sitting at over 60 amps of current draw, and that doesn't even take into account all the other electrical nonsense under the hood like the fuel pump and ignition coils.

Does anyone know how to measure the current draw while the truck is running? Is it as simple as borrowing a clamp meter and hooking to the positive battery cable?

Thanks...

 

[BBsound]

you can install an ammeter which will tell you how much you are charging, or discharging.

 

[corax]

Does anyone know how to measure the current draw while the truck is running? Is it as simple as borrowing a clamp meter and hooking to the positive battery cable?

Thanks...

Yep, positive or negative cable with an inductive clamp on it - even better, turn on every accessory you have (including A/C which makes the elec fans run) to see what your current max draw is and then factor in that you'll very seldom run with everything on at once

 

[dzzz]

So here's where I sit now...

My alternator pumps out 130 amps, assuming that the fact it is 7 years old has no effect on it's efficency.

On gas you need to include spark plugs.
Alternators made for emergency vehicles put out a lot of power at low RPM. Stock alternators do not.
Regular starting batteries are not designed to go much below 90% charge. When you current battery is replaced consider something better.

 

[78Bronco]

So here's where I sit now...

My alternator pumps out 130 amps, assuming that the fact it is 7 years old has no effect on it's efficency.

That would be the max output but your output at idle to 2000 rpms will be less.

 

[bdbecker]

Just talked to my Dad, turns out he's got a clamp on ammeter (I figured he would after 27 years of turning wrenches for a living) so I'll finally know for sure tomorrow night when I go home. I'm just chomping at the bit to install those lights, but I don't want to go drilling holes until I'm sure my system can handle it.

Also, taking a little vacation next week, probalby be off the grid for a few days...

 

[fordem]

Yep, positive or negative cable with an inductive clamp on it - even better, turn on every accessory you have (including A/C which makes the elec fans run) to see what your current max draw is and then factor in that you'll very seldom run with everything on at once

No it won't - if the engine is running, putting a meter on a battery lead will show the charge or discharge current - the electrical load is going to be fed from the alternator.

Putting the meter on the alternator lead will show the alternator output at that instant in time which may be more or less than the max current draw.

If the engine is not running - an ampere meter on the battery lead will show the discharge current, and since the entire load is being fed from the battery, that will be the max current draw.

It is possible to measure max current draw with the engine running, but, you'd have to find the correct place to put the ammeter, and that may vary from one vehicle to another.