How Big of an Alternator do You Need?

  This is the first  step you need to consider when purchasing a new alternator. There are several  steps that are involved. Take your time and try and be as accurate as possible when answering the questions. If you don’t size your alternator properly, you will end up with grief, and the added expense of upgrading the alternator again.

  First we should  deal with an old myth. That being that the larger the alternator the more HP I will lose. It is true, it requires more HP to produce 100amps than it does to  produce 50amps. However, the alternator only produces as much current as the  system requires. In other words you can have a 200amp alternator, but if the system load is only 50amps, a 200amp alternator will only produce 50amps at that  time. So bigger is almost always better. The only downfall to going bigger, is  the higher the output of the alternator in the top end, usually means a sacrifice of output in the lower end. This is why we rate alternators at several  different rpm ranges, to allow you for to compare the outputs at given rpms.

1.) What is your system load?

   System load is the total amount of amperage your system uses at a given time. If you are upgrading because of the addition of a stereo, the  calculation is rather simple. Take the fuse value of the new amps and add it to the original output of your stock alternator. Example a GM with a 105amp alternator, plus a new amp with a 50amp fuse, would require a 155amp alternator.

   If the new additional device(s) has a steady current draw  (unlike a stereo that has a variable draw) continue on here. (This like running lights, neon, plow lights, etc.)You will need to consider at what rpm your new device will operate. If it is at idle, then you need to add the fuse value to your at idle amperage requirements. To figure out what that is, you need to have your system check for draw at idle. This can be down at most competent garages. Simply have your alternators output check at idle, after it has been running  for 5 minutes, with an amp clamp. It would also be wise to increase your rpms a  few hundred to see if this reading increases. If it does, that means your alternator is already under sized for your application. Use the higher of the  two readings for the remainder of the calculations. If you have absolutely no  access to this test, you can use a figure of 75% of your stock alternators  output for a base number. Note though that this is a very inaccurate way of  doing it, and can lead to problems.

If you are starting from scratch like in a racer car or  monster truck, you can simple add up all the fuses in your system. You may want to make several lists. You need to consider what devices will be running and when. Add up everything you will be using when you are at idle. Some examples of  things are: MSD or Crane, fuel pump, fans, electric water pump, headlights/tail lights (don’t worry about brake lights as they are only on for a short period of time), transbrake, air conditioner or heater (pick the bigger of the two, as you  don’t use them both at the same time), stereo system, data logger, etc. Do not consider things like nitrous or fire fighting systems in your at idle list, as you don’t use it at idle. I would also not consider turn signals and such, as they are only on  momentarily. Don’t be surprised if your at idle requirement is between 70-100amps, as that is a typical range.

Your second list should include your at idle requirement plus  the addition of all other electrical devices in your system. This is where things like nitrous and such come in. This figure is the maximum of your system  requirements at off idle engine rpms.

2.) You should now have a figure worked out for what your  system load is. There are a few other things that you need to consider. First is we should build in a safety factory into the alternator. Typically OEM manufacturers use a value of 80%. Meaning the system load is 80% of the  alternators maximum output. To do this in reverse, simply divide your system load by .8 . The only time you may not want to consider this is in a stereo  application, where the bass amp is not engaged constantly, which builds in a safety factor by itself.

There are several reasons for wanting an alternator to have  the 20% cushion. The first being that as wire and devices age, their resistance  increases. As the resistance increases, their amperage requirements go up. So it  is not unusual for us to see vehicles that once had 80amp draws when new, to  have 100+ amp draws 5 years later. It is for this reason that our rebuild  division, always rebuilds stock alternators to be higher output than the  original stock units.

Do not take this cushion lightly. An alternator that is  operated at above 80% of its total capacity for a lengthy period of time, will have its life expectancy measured in minutes, not years. Meaning that if you  ask a 100amp alternator to produce a 100amps, it will for about 20minutes, then it will ask you to replace it with a new one. The same alternator will produce about 90amps for 2 hours before needing to be replaced (maybe sooner though).  However if you only ask it for 80amps, it will do it for a long period of time.  Best however is to only ask it for 50amps, then it should last until the brushes  or bearings wear out.

3.) The last thing to consider is how hot the environment is that the alternator will be operating in. As the temperature of copper rises, its resistance increases. This has an adverse effect on the alternator. Typical  under hood temperatures range from 150F to over 200F. A lot of shops rate their  alternators cold, meaning room temperature. This gives their units a higher reading, which makes them look better, but it is somewhat false, as it will not produce those amperages at operating temperature. At 150F you should expect to see about a 15% decrease in the over all output of the alternator. At 200F this increases to about a 40% drop in output. Very drastic in deed. Our fire  truck/EMS units come with two ratings, their cold ratings and their operating  ratings.

If for some reason you are using one of our competitors  alternators (usually smaller shops), you need to ask them what temperature their  units are rated at. If it is 77F, then you must add 15% (divide by .85) to your  system load requirements. If your alternator is going to operate at over 150F we strongly recommend adding a hood scoop or duct work so that the alternator can  cool better. In one experiment a hood scoop lower the operating temperature of an alternator from 155F to 107F instantly. This dramatically increases the  longevity and stability of the alternator.

You should now have two figures written down. The first being  your at idle requirements and the second being maximum output requirements. Now proceed to the next step:Pulley ratio