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Thread: why not 2 alternators?

  1. #1
    Variable Bitrate
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    why not 2 alternators?

    So i was wandering... ----assuming you have a car a bit old (With extra space, in engine bay), since i dont think people do easily , hardcore carpc's in newer cars--
    you will need a 2nd battery, else your battery will prety often (remember, its a hardcore build , not just a tablet)
    you will need a bigger alternator from the factory one, since maybe you will have big sound system?
    you will need to isolate the batteries... so many opinions. so many problems. so much info, and all becouse both battaries have to be on the same alternator.,.

    so i was wandering, why instead of having them isolated with diode or relay, dont you just add a second alternator in your car (instead of buying one bigger), and have each battery in each alternator?? i dont really think that the 1hp drop will be a problem... just wanted to start a conversation !!

  2. #2
    Low Bitrate ephect's Avatar
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    The cost to fit another alternator out ways the cost of isolator and a high output alt. Getting brackets made up, ensuring the right amount of belt contacts the pulley and meshes in with the current pulley assembly.

    Certainly doable if U have the skills but to get it built by a shop can burn a nice hole in ur wallet.

  3. #3
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    Indeed, why not. But I would not do it because of extra batteries, but merely to obtain the required output power (eg, 2 or more 250A etc alternators).

    Multiple batteries as you describe are not a problem. There is no problem charging off a single alternator.

    A few years back I helped someone on (I think) mp3car with a 2 alternator and 2 or 3 battery setup - ie, the normal main alternator & battery & car electrics and the 2nd alternator feeding 1 or 2 batteries for a big audio setup.
    But even that had a mode where the batteries were be interconnected to the main alternator when the big amps were not in use.
    I can't recall why they wanted 2 independent systems. (For some applications, interconnecting equipment fed of different supplies can be an issue, but audio & PC apps are fine.)

    Other multi-alternator systems that I know of are all commoned to feed the main battery and then feed other batteries using normal battery isolator methods.

  4. #4
    Newbie BennY-'s Avatar
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    i like that idea

    if there are compressor kits for your engine, you may get all parts prebuild (just needs an adapter for the alternator instead of the compressor to get mounted)

  5. #5
    FLAC PhilG's Avatar
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    The Ford Super Duty's have (or had, not sure if still offered) an optional 2nd alternator from the factory on the diesels, I think both worked in tandem to charge the dual batteries installed for the diesel.
    My 2007 Ford F350 Work Log located HERE

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    i dont think the cost is that much, when you compare it with a new heavy duty alternator+ isolator. especially if you consider that since you want the second alternator to also be "normal" , you can easily find a used one. and the comprasor idea seems prety nice indeed.
    but old spark, dont you think that it would make a huge difference in chagring the batteries? you will have all the plus of two completely isolated systems, but non of the negatives!

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    Maximum Bitrate RAWPWR's Avatar
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    Last edited by RAWPWR; 06-27-2013 at 04:54 PM.

  8. #8
    Raw Wave
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    Quote Originally Posted by settra View Post
    but old spark, dont you think that it would make a huge difference in chagring the batteries? you will have all the plus of two completely isolated systems, but non of the negatives!
    What negatives? How many dual battery systems do you know of with problems?
    And if dual alternators are the solution, are you saying that those on mp3car with dual batteries and audio buffs with multiple batteries and all dual-battery emergency vehicle should get a 2nd alternator?

    For charging the batteries it makes no difference.
    Assume the alternator/s is/are set to 14.2V. A single alternator outputing 14.2V (at the battery) is no different to 2 alternators each to their 14.2V battery.
    If the alternator is not putting out 14.2V, then it is undersized.
    If it's not putting out 14.2V because of a/the battery's initial high-current charging, that current will reduce and the voltage will return to its normal 14.2V.
    If it's not putting out 14.2V because of a drastically failed battery, then that battery should not be there. (And giving it whatever current it can absorb means less time to fire or explosion.)

    Furthermore, dual alternators:-
    - worsen battery recharge current limitations,
    - requires 2 voltmeters if battery state and presence of a failed battery is to be monitored,
    - may give rise to other problems if interconnecting signal wires etc between loads (and allegedly used to give problems when the alternators themselves were interconnected).


    IMO a single HO alternator is better than 2 smaller alternators - mainly for mechanical loss reasons etc.
    Dual alternators for load sharing or redundancy may be ok, but for redundancy they must both be fully rated for the total load anyhow (ie, just one handles all the loads; the 2nd of same or bigger rating is later added). But for redundancy I merely carry a spare alternator.

  9. #9
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    I have seen Car Audio systems that had large amounts of batteries that had as many as 4 alternators on them. Each alternator powered a separate bank of batteries. They had a wall of woofers and were purely SPL machines. In the multi alternator systems I have seen I don't believe any of them were combined to charge the same batteries. They were totally separate charging systems for totally separate battery systems. And each battery system powered a different bank of amplifiers.

    Having said that it makes sense to have a second alternator if you have a very large audio system. Instead of over loading your normal system alternator if you can afford the space then a second alternator makes sense. As oldspark mentioned you should really be monitoring the voltage of each to make sure they are working fine.

    I have in my possession a dual output alternator. It is a hot rated 130 amp alternator but apparently they are not made any more. Basically it is two alternators in the same case. They were used mostly in ambulances and high demand commercial vehicles. Mine was purchased for a car audio system but was never used.

    If you can install 2 separate alternators in your vehicle and you leave one running your normal systems and the main battery and the other is totally separate and powers the auxiliary battery you should be fine. However, unless your system is very large a second alternator is likely over kill. Would be easier to get a bigger alternator and a battery isolator regardless of what type you decide to get.

    I don't agree with the opinion that if it worked great you would see it more often in commercial vehicles. I think you see the high output alternators because they are simpler, not because they are better. You are only likely to see dual alternators in commercial vehicles if they can't get a high enough output alternator or because they need a backup for safety reasons.

    If you DO add a second alternator you need to make sure they only share the ground. If you cross over their positive portion you can have all sorts of issues. They need to otherwise be powering totally separate systems. Especially with the smart sensing alternators.

  10. #10
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    Quote Originally Posted by redheadedrod View Post
    I have seen Car Audio systems that had large amounts of batteries that had as many as 4 alternators on them. Each alternator powered a separate bank of batteries. They had a wall of woofers and were purely SPL machines. In the multi alternator systems I have seen I don't believe any of them were combined to charge the same batteries. They were totally separate charging systems for totally separate battery systems. And each battery system powered a different bank of amplifiers.

    Having said that it makes sense to have a second alternator if you have a very large audio system. Instead of over loading your normal system alternator if you can afford the space then a second alternator makes sense. As oldspark mentioned you should really be monitoring the voltage of each to make sure they are working fine.

    I have in my possession a dual output alternator. It is a hot rated 130 amp alternator but apparently they are not made any more. Basically it is two alternators in the same case. They were used mostly in ambulances and high demand commercial vehicles. Mine was purchased for a car audio system but was never used.

    If you can install 2 separate alternators in your vehicle and you leave one running your normal systems and the main battery and the other is totally separate and powers the auxiliary battery you should be fine. However, unless your system is very large a second alternator is likely over kill. Would be easier to get a bigger alternator and a battery isolator regardless of what type you decide to get.

    I don't agree with the opinion that if it worked great you would see it more often in commercial vehicles. I think you see the high output alternators because they are simpler, not because they are better. You are only likely to see dual alternators in commercial vehicles if they can't get a high enough output alternator or because they need a backup for safety reasons.

    If you DO add a second alternator you need to make sure they only share the ground. If you cross over their positive portion you can have all sorts of issues. They need to otherwise be powering totally separate systems. Especially with the smart sensing alternators.
    most commercial hd vehicles that use two alt simply remove the ac and have a new bracket put in place. I have worked out of many bucket trucks, and this is the norm. they generally run two batteries and run.the batteries and charging systems in parallel. for me, this seems excessive and doubles the chance of failure.

    if you need more then 200 amps, you need to rethink your system. amps x volts =watts. how many of use really peak over 2400 watts?
    Progress,.... that is what I keep forgetting ;)
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