Battery/Alternator Draw

Discussion in 'Off Topic [BG]' started by Dave, Sep 25, 2005.

  1. I did some searches and couldnt find a specific answer.

    Im upgrading my power amp thats powering my subs in my car. Im wanting to see if anyone knows how powerful of an amp you have to be running before you need to worry about needing to upgrade your alternator or running a second battery.

    Im looking at getting one of these, do I need to worry about issues with my alternator/ car battery? I dont have anything else aftermarket like lights or anything like that. Just have this and an aftermarket CD deck.

    EDIT: This is going in a 2000 Ford Taurus, if that info is needed.

    Audiobahn power amp

  2. LajoieT

    LajoieT I won't let your shadow be my shade...

    Oct 7, 2003
    Western Massachusetts
    I don't know the answer, but if you've got decent car audio shops around you like we do (NOT Best Buy or Circuit City type chains, around here we have Custom Car Sounds and Long Radio which are ONLY car audio shops) they are generally VERY willing to help out and answer these questions. They've spent a lot of time talking to me about my (very modest self installed) system, knowing I was doing it myself and were always very helpfull and informative.
  3. If your battery continues to be charged adequately while you're driving, and your alternater doesn't blow up, it has the capacity to handle it.

    If you notice the headlights dimming to the bass while you're driving (not when stopped and idling, it's probably a little normal then), its may be underpowered. Try adding some large capacitors to handle transients from the bass drum/bass, see if that clears it up. If not, then you may need more alternator.

    I would be tempted to replace it when it blows, not before. Prove to me its inadequate befor I buy a new one. Alternator blows, you can still get home on battery power most likely, or at least somewhere to get serviced.

  4. Haha yeah I figured that if it didnt blow up, itd be ok. Id just rather get a little less powerful amp and not have to pay for a new alternator. I dont have one of those newfangled money trees, although Im trying to grow one.
  5. Duuuuuuuuuuuude, so anyone else got any info?
  6. Chasarms

    Chasarms Casual Observer

    May 24, 2001
    Bettendorf, IA USA

    Ok. You asked.

    For a sub, you are probably going to run it bridged mono. It's rated at 800 RMS x1.

    It's class A/B. So it is probably in the mid to high 50s, best case, low 60s efficiency. Most makers use non-regulated power supplies and rate their amps at 14.4 volts. (This is the top edge of voltage for a car system with the engine running)

    Assuming you can actually get 14.4 volts to the amp (which is not easy) and giving you the best case efficiency of 60 percent,

    800/14.4/.6 = @ 93 amps. So, that amp will pull 93 amps at max RMS output.

    The Taurus probably has an 80-85 amp alternator and uses 20-30 amps to maintain the systems (more or less when using A/C and/or the headlights or other accessories) That gives you have about 50-55 amps of head room.

    The reality is that you probably are only going to get around 13.5 volts to the amp, depending on the length of the wire you have to run and the size. So, you'll end up with either less RMS output or be forced to try to pull more power to actually make 800W RMS. So, actually, you probably have less than 50 amps of headroom.

    That being said, you will almost never need it. A sub amp for your car is like a bass amp, it's making music. The power consumption is needed in short bursts. It peaks and falls.

    I think that you will definitely experience dimming with the headlights with the bass hitting hard. Otherwise, it may not be that big of a deal. If the power drain is more excessive, the car will fail.

    Your risks include ruining the battery and/or the alternator. A deep cycle battery will handle the high current draws better. It also creates a bit of a buffer in the system. It behaves a bit like a capacitor.

    Of course, a HO alternator would prevent drain altogether.
  7. Awesome, that is exactly what I was looking for. Now if Im not cranking the amp at full blast, will I have to worry about screwing up the alternator? And if I get a deep cycle battery, will that give a little protection against screwing up the alternator?

    Thank you very much,

  8. Chasarms

    Chasarms Casual Observer

    May 24, 2001
    Bettendorf, IA USA

    The battery will not help the alternator. It may in fact make it worse on the alt., as it is likely to pull more current to charge.

    Obviously less stress on the alt is better, but it's not that big of a deal. An alternator is a lot cheaper than your stereo system.
  9. Its like a bass amp, the louder you play, the more juice it takes. If you play it at "normal" volumes, comparable to "normal" car stereos, you're not pulling near the full power out of the amp, ergo the alternator either.

    In the old days when you had an ammeter, you could tell when the battery was supplimenting power to the car cause the alternator couldn't keep up (like idling with the lights on) vs when the alternator was supplying all power to the car, with some left over to charge the battery.

    So your stock alternator "could" handle a million watt bass sub depending on how loud you played it.

    It helps to get the figures on how much normal alternators can put out current wise, but you need to know also what the current draw of the amp is at your normal listening volume.

  10. I think you should forgo the sub, and get more tweeters. High end rules!!!

    Who likes that boomy bottom end around here anyways?
  11. Well, I do already have 8 tweeters, guess a few more couldnt hurt. :D

    Ill mount like 25 tweeters in my sub box and have mind blowing highs.


  12. 59jazz

    59jazz Infinite Rider on the Big Dogma Supporting Member

    As long as your avg. draw is not 50% or greater than the altenators output spec, you'll be fine...............Great detailed facts Chasarms!
  13. Thank you and a BIG thank you to Chasarms once again.
  14. Mike N

    Mike N Missing the old TB

    Jan 28, 2001
    New York
    If you have trouble with the alternator, you can probably find a kit, or a shop near you to upgrade yours to a higher amperage. Places like JC Whitney used to sell alternator upgrade kits that would turn 66 amp alternators into 105 amp alternators.

    I have one of their kits in my truck (along with dual 900 CCA batteries) and have no problems.

    You may also check with a local parts store and see if Ford offered an optional high amp alternator for your Taurus. On some cars, the base model got a weaker alt than a similar car with power this and that, heated seats etc.......

    When I'm at the shop tomorrow (if I remember), I'll look up and see what alternators are available for your application.

    Lastly, I would only run a deep-cycle battery in a boat, or a racecar with no alternator. I wouldn't use one in a daily driver.
  15. Alrighty then, any more info is much appreciated.

    The information on the amp says it uses a 60 amp fuse. Does that mean the amp is going to, for the most part, stay under that?

    My car, other than the radio and your various accessories with the engine, uses very little electronics. It has power windows and door locks, but they are broken. Headlights are about it. I'm not the kind of guy thats going to crank my system for everyone on the block to hear, but I do listen pretty loud.

    Now I do understand that the amps not going to put out 800 watts constantly, but will it draw 60 amps all the time, or just in short bursts?

    Thank you, that would be greatly appreciated.

    Thank you guys for all the great information.

  16. Yes, normal operation will be under 60 amps. The louder you play it, the more amps it will draw.

    Windows and door locks only pull power when you use them, so they effectively don't exist anyway. The short period they're in use wouldn't harm anything anyway, even if they exceeded the alternater's capacity. You'd just be discharging the battery for the 10 sec it takes to open the windows, or pop the locks.

    THe power the amp pulls is proportional to what's being sent to the speakers. The power going into the speakers comes from the cars electrical system. The amp isn't 100% efficient, so 800W into the speakers actually requires > 800W from the alternator.


    Thank you, that would be greatly appreciated.

    Thank you guys for all the great information.

  17. Mike N

    Mike N Missing the old TB

    Jan 28, 2001
    New York
    Standard alternator for your Taurus is 105 amps. A 130 amp alternator is shown as an option.

    If you look at your alt you may find a stamp or tag of some sort indicating which one you have.

    I would tend to believe either would be sufficient for what you're doing.
  18. Its great to have such knowledgable people on here, I think Im going to go cry now...

    Seriously though, thanks Mike for going out of your way to find this out for me, that will let me sleep a little better knowing Im probably not going to blow my alternator.

    Thanks again,

  19. Irragi 300amp alt/optima yellow top.

    And everything will be good.

    But I have run 2200 watts through my stock alt and an optima red top so that may even work for you.
  20. Mike N

    Mike N Missing the old TB

    Jan 28, 2001
    New York
    300 amps? What are you running, an onboard welder?

    Please tell me you upgraded your battery cables to accept this.