I'm far from an expert, there are many here on the forum that have better engineering skills than I do.
DC Power has several pages that will explain this better than I can.
Here is a good one to start -->
https://www.dcpowerinc.com/articles/supply-and-demand-electrical
Just keep in mind they are trying to sell you something that they can provide.
Charging System Harmony
Let’s just say that your vehicle was originally outfitted with a 120 amp alternator. This means that the vehicle probably requires a little over 100 amps of current to power the accessories in worse case scenario – like say driving on a hot, rainy night. Given such, you may elect to have the A/C on, be using your high beams for extra visibility, and of course the windshield wipers . . . and who drives at night without the stereo cranked up? Your stock alternator is designed to do this without failing . . . at least until the warranty period on your vehicle expires. Your charging system is in harmony and all is good.
Adding Aftermarket Electronics
So, what happens when you begin to add aftermarket electronics? Well, to a point, your stock alternator can still do the job as the designers have allowed for say 15% extra above the maximum requirements of your vehicle so that the stock alternator doesn’t fail when presented with worse case conditions. In the above example, 120 amps – 102 amps = 18 amps. Add one set of 7” high-powered Halogen lights to your four wheel drive import truck and you’ve exceeded that!
Adding aftermarket electronics can exceed the capabilities of your stock charging system. Specifically you can:
Exceed the maximum output the stock alternator is capable of.
Exceed the current capability of the wiring of the accessory return path.
Your charging system is no longer in harmony as the demand you’ve placed on it exceeds its ability – this is not good. Let’s look at the above more in-depth . . .
Exceeding the Maximum Output the Stock Alternator is Capable of
When the alternator is working at maximum capability and unable to keep up with the demand placed on it, the battery begins to act as a buffer. This has several side effects:
The battery is required to “fill in” what the alternator can’t – albeit at a much lower voltage. A fresh battery will do this for a while, but it requires charge from the alternator to keep it fresh . . . charge that isn’t available. After a period of time, the battery will be drained to the point where it will be unable to start the vehicle.
The on-board electronics have less voltage available to them then they were designed to operate on. This can cause intermittent or erratic operation – given the amount of safety equipment installed in today’s vehicles, this is certainly not desirable!
The filtering capability of the battery is lessened as it becomes weaker. This can cause erratic operation of aftermarket accessories.
Over time, this can cause alternator and / or battery failure.
Exceeding the Current Capability of the Wiring of the Return Path
Any time you exceed the current capability of a length of wire, it will heat up. The heat increases the resistance the cable has to current flowing through it. As resistance in a length of wire increases, it requires more voltage to pass the same current through it. Over time, this can cause the cables to become very high in resistance and greatly diminish the overall performance of the stock vehicle accessories.
Restoring Charging System Harmony
Now that you agree that your stock charging system is in need of upgrading, DC Power has the answers. Contrary to what your buddy’s friend that sweeps the floors at the local service center tells you, a bigger battery won’t solve your problem! Of course, this should be obvious to you by now. On the other hand, a bigger alternator and upgrading the accessory return path will solve your problem.
A high-output DC Power alternator will provide all the power you need to supply those current hungry aftermarket accessories – fog lights, driving lights, upgraded headlights, winches, audio amplifiers, in-car video systems, etc. We make them up to 400 amps, so we’ve got you covered.
In addition, since your aftermarket accessories will likely exceed the capabilities of the stock accessory return path, we recommend upgrading it accordingly. In most cases, this is as simple as replacing the stock 10 AWG or 8 AWG wire from the battery (-) to the chassis with a 4 AWG wire. If the aftermarket accessories that you are adding have a combined current requirement of over 100 amps, then we ask that you call us so that we can help you upgrade the accessory return path accordingly.
How Big of an Alternator Will I Need?
An easy way to determine just how big an alternator required is to simply add up the sizes of the fuse values in each of the aftermarket electronic add-ons you plan to add to your vehicle, except for audio amplifiers! For audio amplifiers, if you’re using them to reproduce music, you’re safe to assume that they will consume no more than 50% of the fuse value in doing so – even at maximum output. On the other hand, if you’re attempting to outdo Steve Meade, then you may just want to call us directly for our guidance!
For example, let’s say that you had a vehicle with a 120 amp alternator and you intended to add the following aftermarket electronics to it:
Capacitive Discharge (CD) Ignition Box – 5 amps
Three Gauge Cluster – oil pressure, temp, and voltmeter – 5 amps
Dual 13” electric fans – 20 amps x 2 = 40 amps
4 channel Audio Power Amplifier – 20 amps (has two 20 amp fuses)
Mono Audio Power Amplifier – 50 amps (has one 100 amp fuse)
Note: The owner’s manual or installation manual of your accessories should provide this information. If not, we are happy to help you determine this.
This is a total of 120 amps of current required by the aftermarket accessories. Add this to the estimated 102 amps required by the stock electronics and accessories (120 amps less 15%) and this means that worse case scenario the stock electronics and accessories AND aftermarket accessories will require 222 amps of current.
A DC Power 275 amp alternator would fit this application perfect, and provide a 20% buffer should you choose to add additional aftermarket accessories down the road. Another solution is to add a DC Power 190 amp alternator AND one of our dual alternator brackets (we offer them for both Ford and GM applications). This would allow you to keep the stock alternator and add the DC Power alternator to power all of the above aftermarket accessories with ease!
