Tech / Tech Articles

Alt Text: 5 Things You Should Know Before Using a High-Output Alternator

PWM-47796_HH_mlSo you’re considering swapping out your stock alternator.

There are plenty of good reasons to make the leap to a high-output alternator, but you’ll need to do a little bit of homework first. Luckily, we’ve got smart friends to help us with our studies, so you can ace the topic. In conjunction with the alternator experts at Powermaster and MSD, we’ve compiled the five things you need to know before upgrading to a high-amp, or high-output alternator.

This starts with the most basic of questions:

Do You Really Need a High-Output Alternator?

If you’ve got a basic, stock vehicle, chances are you don’t need a high-output alternator. Most factory alternators are rated at 65 to 100 amps and are capable of handling your vehicle’s basic necessities, such as headlights, gauges, fuel pumps, A/C, etc. These alternators also typically come with a 10 to 15 percent reserve to handle additional accessories.

However, many of our readers don’t have a stock vehicle. For example, you may have a custom-built street rod with a unique combination of accessories. Or you may have a high-end stereo system or a race vehicle with an array of on-board electronics. As the electrical load of all these accessories add up, you may find yourself in need of a higher-amperage alternator.

But how do you know?

There are a few ways to figure out whether you need to upgrade your alternator. A few telltale signs are dim headlights, poor stereo system performance, or an alternator that simply wears out quickly. You can also check your electrical load using an ammeter. Simply connect the ammeter in series with the battery’s ground terminal (with the engine turned off), switch each electrical component on and off, and note their amperage draws. Add up the total electrical draw and compare with your alternator’s rated output. The output should be 50 percent greater than the draw.

One final way to estimate your vehicle’s electrical load is to check the accessory fuses. The amp ratings, although slightly higher than the highest draw of each component, will give you a good estimate of your vehicle’s electrical load.

What Amperage Do You Need?

That depends on the current draw, along with any future accessories you plan to add. For that reason, we’ve supplied a list of some common accessories and their amp draw:

Electrical Load of Common Vehicle Accessories

AccessoryAmp Draw
Air Conditioner20-21
Audio Power Amplifiers10-70
Back-Up Lamps3-4
Cigarette Lighter10-12
CD/Tuner with Amp7-14
CD/Player/Tuner without Amp2.5-5
Dome Light1-2
Electric Cooling Fans6-15
Head Lamp Dimmer2
Head Lamp (Low Beam)8-10
Head Lamp (High Beam)13-15
Heater Defroster6-15
Ignition (Racing)8-36
Instrument Panel0.7-1.5
Lamp, Gauges1.5-3.5
Lamps, License Plate1.5-2
Lamps, Parking1.5-2
Lamps, Side Marker1.3-3
Lamps, Tail5-7
Nitrous Oxide Solenoid5-8
Power Windows Defroster1-30
Power Seats25-50
Power Windows20-30
Power Antenna6-10
Pumps, Electric Fuel3-8
Starter Solenoid10-12
Voltage Regulators (1 Wire)0.3-0.5

How Much is Too Much?

You can never have too much amperage when it comes to alternators; therefore, you never have to worry about choosing an alternator with too high of a rated output. Here’s why:

Amperage is basically the amount of electrical current your alternator can supply. And it basically operates off of supply and demand. That is, your alternator will only supply the amount of amperage a particular component demands—and no more. So high-output alternators will not harm your components or charging system, no matter how high you go with the amps.

prf-30709_xlWhat Gauge Wire Do You Need?

A performance alternator really doesn’t require much in the way of modifications. However, Powermaster and other alternator manufacturers do recommend you replace both the ground straps and charge wire. Keep in mind the factory cables weren’t designed to handle the juice of a higher-output alternator, and can restrict the flow of electricity.

In the case of the charge wire, you really can’t go too large. However, here is a chart that matches cable gauge size to total amperage:

Recommended Cable Gauge and Length for Amp Draw

Cable LengthUp to 4 ft.4-7 ft.7-10 ft.10-13 ft.13-16 ft.16-19 ft.19-22 ft.22-28 ft.
0-20 Amps14 AWG12 AWG12 AWG10 AWG10 AWG8 AWG8 AWG8 AWG
20-35 Amps12 AWG10 AWG8 AWG8 AWG8 AWG6 AWG6 AWG4 AWG
35-50 Amps10 AWG8 AWG8 AWG6 AWG6 AWG4 AWG4 AWG4 AWG
50-65 Amps8 AWG8 AWG6 AWG4 AWG4 AWG4 AWG4 AWG2 AWG
65-85 Amps6 AWG6 AWG4 AWG4 AWG4 AWG2 AWG2 AWG0 AWG
85-105 Amps6 AWG6 AWG4 AWG2 AWG2 AWG2 AWG2 AWG0 AWG
105-125 Amps4 AWG4 AWG4 AWG2 AWG2 AWG2 AWG2 AWG0 AWG
125-150 Amps2 AWG2 AWG2 AWG2 AWG2 AWG0 AWG0 AWG0 AWG

What is Pulley Ratio (and Why Should You Care)?

In short, pulley ratio is a comparison between the crankshaft pulley diameter and alternator pulley diameter. This ratio is derived by dividing the crank pulley diameter by the alternator pulley. For example, a 6-inch crank pulley with 2-inch alternator pulley will yield a 3:1 pulley ratio.

The ratio has a direct effect on how fast the alternator spins.

In order to understand the importance of pulley ratio, you first need to understand the “power curve” involved with alternator output. Although the alternator’s output is dependent upon engine speed, it follows a unique curve. At idle, small changes in the alternator’s speed can make a big difference, so the pulley ratio becomes very important.

Powermaster supplies its alternators with pulleys matched to the alternator’s power curve. The company follows this common rule of thumb:

  • Street use = 3:1 ratio or slightly higher
  • Drag racing = 1.75:1 ratio
  • Circle track = 1:1 ratio

So why should you care?

Because differing ratios can affect performance, you should take care to maintain the same pulley ratio if you decide to use dress-up pulley sets. A mismatched pulley ratio and alternator can lead to big problems, especially at idle where alternator performance is critical. That’s because these high-amp units typically lose output under 2,400 rotor rpm. Rotor rpm are a factor of pulley ratio multiplied by engine speed. So, if you have a pulley ratio of 2:1 multiplied by an engine speed of 870, you’ll get a rotor rpm of 1,827.

At 1,827 rpm, you’ll see a significant drop in alternator output.

Again, the ideal ratio depends on your application (street, drag racing, circle track racing), but you need to understand the effects of altering pulley ratio.

With all this in mind, you’re ready to choose the right alternator for your application.

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  1. bryan danks says:

    ALong these same lines what if your vehicle uses 16 volt battery or batteries

  2. I have a 1984 elcamino.what modifications are needed,just wire diameter?

  3. hi david, thank you for this information!

  4. How can u tell what ratio pulley u have?

    • Get a string and a tape measure, and a calculator.
      Wrap the string around the alternator pulley one time, so the end touches the strings free end, hold or mark that point on the string, then measure from the mark to the end. That’s your alternator pulley (AP) diameter.
      Now do the same measurement for the crank pulley (CP).
      Pulley ratio = CP divided by AP. (PR=CP/AP)

      • the measurement you provided was for circumference(around the pulley) not diameter. Diameter = radius x2. radius is the distance from the center to the edge so diameter is distance from edge to center back to edge again. You can also calculate diameter using your circumference measurement above divided by pi.

  5. I have an amp with a 150 inline fuse for my subwoofer and 2 little amps for my mids and highs one with a 20 amp fuse and one with a 30 amp fuse. all together my stereo is 2000-2600 watts. not cappy overrated brands. will a 240amp 6 phase alternator be enough, I just ordered one and I’m waiting for it now. I also have an optima yellowtop under the hood and one in the trunk. both rated at 48ah. is this enough or what else will I need

    • 2000-2600 watts RMS or max?

      • Rms. This is my situation

        • If it’s a Class-B amp (typical car amp) then it’s about 65% efficient, meaning a bunch of power is turned into heat instead of sound. So your 2600 watt rms amp actually needs 4000 watts to do its max. (2600/0.65=4000)

          Nominal alternator charge voltage is ~13.5vDC.

          4000 watts / 13.5v = 296 amps of current draw.

          So, if you’re driving down the highway (the alternator’s best output), then you’re still discharging your battery. If you’re idling the engine in a parking lot, it’s even worse.

  6. Pingback: Quick Tech: Tips for Choosing and Installing an Alternator - OnAllCylinders

  7. Pingback: Quick Tech: Tips for Choosing and Installing an Alternator | MEP Precision Racing

  8. Maybe in the future some questions could be addressed on Tach installations. Bought a new Auto Meter Sport Comp 3905 and was wondering if all companies now recommend not using solid core ignition wires and they question which ignition system is used. Do these points just come up with this mfg.? Great info on the Alt. most things I read and like this one keep for ref.

  9. Roy Arild Tveit says:

    140 amp alternator and agm battery (Exide EM1000) fried my alternator cable to the starter.
    Recommend to follow the recommended sized charge wire

  10. We’ve been lately installing one of the largest subs setup we’ve ever done and been experiencing a hard time setting multiple amps in our truck and your gauge wire chart just saved me from a lot of headache.

    Thanks for sharing this article.

  11. I have a 2014 Chevy Cruze LT running 2 12 inch sundown subs and 1500 watt amp wired off .5 ohm. As you can imagine my lights flicker when its turned up, and also when I’m driving its like the car doesn’t want to go. Kind of jerks. Do I have to upgrade the alternator or add a second battery?

    • Charlie Cox says:

      You want to do the Big 3 upgrade 0 gauge and you’re going to need to upgrade your alternator also you’re going to need a second battery if you don’t do this you’re going to burn up your amps

  12. Hector730 says:

    Get another alternator. Upgrade the chargimg wire and ground wire and you should be good

  13. Very informative.

  14. Hello,

    I have a 2001 toyota highlander. I have installed a sound system consisting of the following…
    1 crescendo BC2000 powering 2 sundown SA 12’s
    1 sundown 50.4 powering 4 memphis prx 6.5’s

    It dims my headlights pretty bad and I was wondering if i’d need to upgrade my alternator or just my battery? Please let me know


  15. Dustin Hoertsch says:

    I am installing a 240 amp alternator in my 09 avalanche i have a 1800 watt rms amp(class d) i am also installing a second battery what size fuse do i use from the alternator to the second battery and do I need to upgrade the stock 175 amp agm fuse that is on the fire wall

    • Hey Dustin, here’s a basic bit of advice regarding fuses: You should fuse to the wire size you’re using, measured in AWG (American Wire Gauge). So let say you’re running a 4 AWG line in this application, then a 125 fuse would be acceptable to that size (gauge) wire. I found this website: that’s chock-full of info on the subject–click the “fuses” tab to learn more.

  16. For all you guys talking about amplifiers for stereos – you are not measuring correctly. A 240AMP Alternator should be plenty – and probably way overkill. You are measuring watts at consistent pulling. For example your sub’s working on one continuous note. I had a high powered stereo that ran 4 x 10’s. The draw was 50amps at every punch the woofers made with dance music. Other music was considerably less. I used zero gauge wire for the amplifiers ( 3 Alpine V12’s 1000watt x 2 and 400 watt x 1) and a 120amp fuse straight from the battery terminal. My 120 amp alternator, upgraded from 80amps was sufficient to power the stereo as well as upgraded headlights A/C etc. I also changed my battery to a higher CCA – over 500, even a second deep cycle battery used for the stereo alone was recommended for times when the car was stationary without the engine running – I didn’t do this however and never had a problem. Upgraded wiring helps for sure. Even the higher power headlights needed wiring upgrades because the wire resistance was not strong enough. Drawing too much power through a wire that can’t handle the power draw, can cause fires.

    Anyway, if you remove your sub’s but still have all those watts of power, you will not use anywhere near the power you once were. The air that sub woofers have to move, is a massive drain on the system. You are able to use all your other speakers directly from the head unit because their amp draw is little. Your subs can not run off a head unit. In other words if you add amplifiers to all your other speakers, you are effectively supplying them more power – could be 500 watts to each speaker – but the amp draw is insignificant. I think that the remainder of your stereo system (without sub woofers) would be using less then 30 amps. Unless your stereo is running full boar, without a break between beats or songs or turning its volume down and what have you, then it isn’t going to draw 240amps out of your alternator.

    As one person said, a capacitor can store the power produced in between those beats, drawing 50 or 60 amps – and then utilize that power for those draws. I never used capacitors and didn’t need to with heavy duty wiring and larger capacity battery.

    • You can’t use your head unit to power any decent speakers, c’mon. Even the better end head units 50 watts rms (claim) per channel or less. To each his own but if you’re running 2400 watts rms lows I don’t think a supposed “200 watt rms” head unit is adequate for mids and highs.

  17. Mohamed Mahmoud says:

    what is recommended to setup a 16 volts system in the racing car N/A more than 500 HP with an GTR injectors and AEM PN: 30-2853 coils and Electronic Water pump with electric fan and head lights …

    actually in this car after the 6000 the voltage drop down up to 11 and 11.5 volts …
    cus of that i decide to upgrade the charging system to save the volts at least to 13.5 volts …

    so which kind of 16 volts batteries and alternator and what is the best amps for it …

    note the car now supplied with an 12 volts system with 12 volts alternator and 150 Amps

  18. Jeremy Ansell says:

    Dave Fuller – This is a very helpful article – Thanks for putting it out there. I’m and IT manager, and a backyard shade tree mechanic, but can’t always find the core info I’m looking for on the web – Appreciated man!

  19. I have 3000 watt inverter in my 2007 Ford E-250 van. I instaled ot to run a charger for my power wheelchair. Works well. Recently, I was in a political campaign. I pulled a 5’X 8′ utility trailer I had outfitted with a sign frame and lighting. The lighting was two, 100 watt equivalmnet, LED floodlights and a strobe, all AC. When I drove around at night for any thing much longer than an hour, with headlights, trailer lights and wheelchair charger all energized, I would have to start van with a jump box if I shut it down. Stock alternator was not keeping up. In order to be able to run max load on that 3000 watt inverter continuosly without pulling down battery charge, what output do I need on a high output alternator? Would you recommend a second, 12 volt battery, as well?

    • Jeremy Baker says:

      This is a well known problem with RVs that have a inverter. Stock alternators and regulators do not charge well for deep cycle use with a big inverter. The voltage is too low to push much amperage. There’s a fairly easy and inexpensive diy fix. You solder a couple diodes on the D+ wire that powers the field in the alternator. This makes the alternator put out a higher voltage of around 14.4 and pushes a lot more amps into the battery.

      • Can you explain what you did with the diodes in a little more detail please? Not everyone here is as savvy as the next person. I know I’m not…

      • I am using it to run the power to my house using a windmill and alternator. Like Mike, some of this Greek to me. I have wired car and houses but just the basics. TY

  20. Can you do an accompanying article on regulators and whether you need smart regulators with dual battery or agm setups.

    • I’ll add that I’m looking at the Balmar fully programable, 8 stage smart regulator with temperature sensors on both the alternator and up to 2 batteries.

      • Jereny Baker says:

        If you read the comment above by Sando you will see that your proposed upgrade to smart alternator and dual batteries is probably unnecessary. A slightly bigger battery, bigger cables, might do the trick. If not a minor alternator upgrade might. Smart alternator regulators are used in yachts with massive battery banks to charge and huge loads that are critical when out on the water. They can be used in RVs with large house battery banks also.
        Then there is a the diy upgrade I mentioned earlier. Solder a couple of diodes in the D+ Circuit of the alternator regulator to raise the the voltage 0.7 volts and push way more amps to the battery.

  21. Hello! My name is anthony and I have a 2009 Nissan Versa hatchback…. I’m running a 2400 watt amplifier for my two 12’s and every time I go to crank my music up, my lights are dimming…so far, I have a capacitor and it’s not showing any type of progress… I also know I have to swap out my old positive and negative battery cables… will I have to swap out my voltage regulator along with the electrical system….

  22. TREVOR SHERMAN says:

    I have a 2004 oldsmobile alero with 2 12in skar audio evls and I have the big 3 upgrade and a yellow top optima but I’m still having dimming issues so I want to add a high output alternator but I cant find one is there any place I should look in particular.

  23. Pingback: Best 12 Inch Subwoofer 2018 [Reviews and Comparison]

  24. skip the capacitors they dont store enough energy to handle back to back bass notes and eventually become another parasitic draw that never finishes

    alternator companies that have good ratings
    power bastards

    as noted above amps are not constant draws so alot less total power draw will allow the batteries to charge
    also this is only at full power and at the power level you have its not practical to do that

  25. skip the capacitors they dont store enough energy to handle back to back bass notes and eventually become another parasitic draw that never finishes

    if you really need the xtra then a 2nd battery and an alt are the answers and of course as big as you can 2nd ground to the frame and motor and a big 2nd alt to battery wire

    alternator companies that have good ratings
    power bastards

    as noted above amps are not constant draws so alot less total power draw will allow the batteries to charge
    also this is only at full power and at the power level you have its not practical to do that so you will charge when you are not bangin at full tilt

  26. How do I purchase a high output200amp my 1992 740 volvo Wagon.

  27. Jeremy Baker says:

    I read an electrical engineer commenting that capacitors dont help much or at all with high gain amplification. Has that been the experience of anyone here? The last few posts say as much. Often a problem is some resistance somewhere.
    Too small cable, poor ground, poor connector or connections,……or a faulty diode in the regulator. It will still work but put out 1/3 less power. The other thing to check is if your alternator is getting hot. Power output drops quickly when it gets up to higher temps, 200 degrees and over. Vans can be a problem especially at idle with limited fresh air.
    And as the article says make sure the alternator is turning fast enough. Maybe someone put in the wrong alternator or pulley.

  28. Aadith Menath says:

    There is a load of 350watt,36volt on a 36volt lithium ion battery.I need to charge this battery using an alternator coupled to an engine.What alternator should i use to charge my battery. Do you have any suggestions related to the battery that i should use.

  29. Aadith Menath says:

    I should charge my battery with very less time.

  30. Ernesto Urbay says:

    I have a 1995 f150 5.0 v8 and just install 2 l7 kickers 12″ with a 2000 watt amp and my battery signal be dropping lower the what’s suppost too I just install a 6 fared capacitor and the battery signal keeps dropping every time the volume it’s up and punching the truck comes with a 95 amp alternator what size alternater should I upgrade with out any modiffication and should I get a yellow top Optima battery if I do yall think that will get fix

  31. Ernesto Urbay says:

    I went to autozone and try asking for a bigger amp alternator and they told me I will mest up my truck I I install a bigger amp alternater they suggest to buy the yellow top Optima battery for 300$ should I upgrade alternater or get the battery for the problem to be fix. Any advice out here thanks.

  32. “The ideal ratio depends on your application (street, drag racing, circle track racing), but you need to understand the effects of altering the pulley ratio ”
    This part helped me a lot!

  33. Blaine Durousseau says:

    I have a 1996 suburban that’s been retrofitted into a mini camper. It does have an upgraded sound system (amp, sub, stereo, speakers) some extra interior LED strip lights, and a 3000W inverter waiting to be hooked up to run various appliances. I already have 2 optima batteries(one to red top to run the starter and one yellow top just to hook to the alternator alone to run the accessories. I’m looking at a 200 amp alternator to put in but had no idea pulley size would matter. What would be ideal for my application?

  34. i have a 1979 buick electra limited 350 v8 5.7 liter, should i stay away from re-manufactured car parts?

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  36. I’ve got a 76 stingray two fosgate amps one a360.6 and a r 750-1d 60 amp alternator that keeps dieing what size alternator do I need

  37. Paul McAndrew says:

    I have a stock 2006 Mustang GT. My alternator failed. Upon recommendation of many, I purchased a Pa. Performance alternator through CJ Pony Parts. It turned out to be a 200 amp. I was asked by a friend if I purchased the 200 amp wiring kit for it. I did not know I needed one, so I did. I have not installed the kit yet. I am trying to wrap me head around why I needed the kit especially with a 200 amp fuse box in the wiring. An additional fact is I did change the stock under drives to Steeda under drives years ago.

  38. Do you have any define suggestions of Alternator for poor stereo system performance? I am having an issue with my stereo system and because of lockdown can’t take any help from anyone 🙁

  39. I have a Chrysler Sebring Touring sedan (not a convertable), since I’ve had the venicle I never put anything extra in it (Like a sound system), my alternator is a 120A even though everything was factory, yet all OEM alternators keep killing my battery even though the alternator does keep my battery charged, I replaced the battery and alternator twice, after the second time (and without a second thought) I replaced all of the lights on the outside with LED’s around the whole vehicle outside hoping that would actually help, sadly it has not, and now I will be replacing the battery and alternator for a 3rd time, and even though a higher output alternator may solve my current issue, a high output alternator isn’t “technically” made for my vehicle but I did manage to find one that will work in my vehicle (with spacing and it has identical plugs) and it puts out 160A as opposed to the 120A OEM part… does anyone have any ideas as to what the underlying issue could be?

  40. This was EXTREMELY helpful but I am not using it is a car. I want to understand their output in making a windmill to power the house. THANK YOU

  41. Nick Cerone says:


    Can you recommend a Half size battery that is bullet proof long
    lasting? The battery needs to be able to be positioned on its side.

    I ran a D51 Optima Deep Cycle for almost 2-1/2 years absolutely no
    problems, I have a track car so the battery comes in and out of the
    car quite often, and I over torqued one of the terminal posts and it
    started leaking, so I bought another one identical, Couldn’t hold a
    charge for its life even with a Ctek 1 week top off.


  42. What types of fuel pumps ? because in market have different types of fuel pumps with different features. can you please explain ?

  43. Keith Daruwalla says:

    My new car audio system to be installed in the near future, will draw 119 amps from an alternator at 13.7 volts. The car is a 2018 Mazda 3 GS, and has a stock alternator, which probably has a max capacity of 100 amps. Therefore, I should upgrade the alternator, correct???

  44. Erica Eidson says:

    I have a 2000 chevy silverado band after replacing my alternator with a new higher amp alternator, my voltage is significantly less than before. I run 2 stereo amps about 4000-5000. Which is better the high amp lower volt or the lower amp with higher voltage?

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