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Thread: Calculations to determine if my alternator/batteries can withstand my electronics?

  1. #1
    Constant Bitrate doncarbone's Avatar
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    Calculations to determine if my alternator/batteries can withstand my electronics?

    Here's my setup:

    2003 Mitsubishi Lancer OZ Rally
    90amp OEM alternator
    Stock Battery

    Kenwood deck - max current consumption 10A
    400W monoblock amplifier
    500W 4-channel amplifier
    1200W inverter

    None of the above are installed yet as I am having to determine what kind of batteries I will get and whether or not an alternator upgrade is needed.

    I would like to have one deep cycle/fast charging battery in the trunk in parallel to my starter battery separated by an isolator.

    I am worried that my alternator will not be able to charge the batteries faster than they are being drained with all of the electronics mentioned.

    How can I calculate/estimate whats needed so I can be a better prepared for when I install everything?

    Thanks

  2. #2
    Raw Wave
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    Figure out your average audio power consumption.
    Add your car's usage (night & day).

    Compare that to your alternator's average output.
    Factor in battery recharge current (assume a 30% penalty on whatever it discharged).


    Simple!

    EG - 90A alternator is ~900W output; or max 1250W (13.8V@90A; else [email protected]).
    1200W inverter at full load ~1500W.
    900W amplifiers at full power ~1100W; or 550W if "Music Power" rated, or less if "VVanker Watts" rated.
    Kenwood - probably 5A or ~70-100W.
    The car - anything up to 70A (950W) though probably only a few hundred Watts plus lighting.
    Plus battery recharge Wattage x 1.3.


    Or to sample and confirm the above - get a dash voltmeter. (ie, 13.8-14.4V; 12.7-11.4V OCV etc.)



    Battery size is only important if reserve time is insufficient or discharge rates are regularly excessive.
    But I do recommend a 2nd battery mounted next to the amp(s) to help alleviate cable limitations (voltage drops), and for cranking reserve.
    Connect the 2nd battery (to the main battery/alternator) only when the alternator is charging. (Normally requires appropriate fuses (2) and cable, and an SPST relay if a charge light is available.)
    If internally housed and good transient response is required - use an AGM battery.
    Keep the standard main battery unless you have reasons for changing. I suggest you stick to wet cells (ie, not AGM - especially if there the 2nd battery is AGM).

  3. #3
    Constant Bitrate doncarbone's Avatar
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    Quote Originally Posted by OldSpark View Post
    Figure out your average audio power consumption.
    Add your car's usage (night & day).

    Compare that to your alternator's average output.
    Factor in battery recharge current (assume a 30% penalty on whatever it discharged).


    Simple!

    EG - 90A alternator is ~900W output; or max 1250W (13.8V@90A; else [email protected]).
    1200W inverter at full load ~1500W.
    900W amplifiers at full power ~1100W; or 550W if "Music Power" rated, or less if "VVanker Watts" rated.
    Kenwood - probably 5A or ~70-100W.
    The car - anything up to 70A (950W) though probably only a few hundred Watts plus lighting.
    Plus battery recharge Wattage x 1.3.


    Or to sample and confirm the above - get a dash voltmeter. (ie, 13.8-14.4V; 12.7-11.4V OCV etc.)



    Battery size is only important if reserve time is insufficient or discharge rates are regularly excessive.
    But I do recommend a 2nd battery mounted next to the amp(s) to help alleviate cable limitations (voltage drops), and for cranking reserve.
    Connect the 2nd battery (to the main battery/alternator) only when the alternator is charging. (Normally requires appropriate fuses (2) and cable, and an SPST relay if a charge light is available.)
    If internally housed and good transient response is required - use an AGM battery.
    Keep the standard main battery unless you have reasons for changing. I suggest you stick to wet cells (ie, not AGM - especially if there the 2nd battery is AGM).
    If the alternator is only outputting 900W to 1250W and my total setup is roughly 3650W, isnt that a serious problem? Obviously my system won't be blasting at 100% the whole time, nor do I drive 24/7, but it seems my alternator won't be able to keep up with the demand - can you confirm this?

    I can confirm that almost any time I will be driving my vehicle, my computer will be turned on (connected to inverter), my touch screen will be on, my deck is on, my HVAC is on (its bloody cold here), and I'm sure when I get my system installed I'll be playing it virtually the whole time I drive to and from work.

    What do you recommend I do, or do you believe I am not running any serious risks?

  4. #4
    Raw Wave
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    Answer my first questions...

    What will you average load be?
    What is your average alternator output?

    We can take it from there.

  5. #5
    Constant Bitrate doncarbone's Avatar
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    Quote Originally Posted by OldSpark View Post
    Answer my first questions...

    What will you average load be?
    What is your average alternator output?

    We can take it from there.
    I can only guess as to what each of these averages may be.

    Average alternator output I'll go on a whim and suggest it would be 80A. It's factory and my car is 7 years old.

    As for average load...assume I drive my car at least 2 hours daily and during this time, my audio equipment is being played at an average of 30% volume. That's roughly 350W including my deck, right?

    At all times my inverter will be powering my netbook (at least 100W) and my touch screen (8W) for a total of 108W. [Later on I will be connecting a larger PC and an Xbox to this inverter, that is why its much bigger than needed.]

    Then factor in say 300W of additional power being needed for my headlights, interior lighting and HVAC, again accounting for the fact I often drive at night and with my heater on (its about -10 here at night and the windshield fogs up quick).

    So that's 2 hours of driving with an average total consumption of 758W.

    Lets work with that.

  6. #6
    Raw Wave
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    Oh well, 80A at even 10V is 800W which is more than your 758W.

    But assume [email protected] = 1100W hence leaving ~350W headroom, or about 20-25A.

    Starter say 10 secs @ 250A is equivalent to 100 secs @ 25A, so a few minutes to recharge at 25A (averaged over at least 10 minutes in practice).

    Based on what your figures, I'd suggest keep things as is - except for the Big-3 upgrade - and get a dash voltmeter ($10-$40 eBay; I recommend 3-digit digital, eg, 12.3 (not 12.34) etc - one of the few times I recommend digital readouts as the prime display).
    I recommend voltmeters anyhow as they pre-warn of problems and strandings, but certainly for those with variable demands and limited charging.


    Remember too that half volume is not half power - it's an exponential/logarithmic function - eg, 2x the volume needs 4x the power (or whatever the relationship is).
    But there are so many variables.

    The vehicle's age shouldn't matter. It's new enough to have a high output from cold (ie, 14.4V else down to 13.8V with headlights after cranking) and it shouldn't deteriorate except when the brushes are are their end.
    Only the battery will be effected by age. It may look good sitting at its "full" 12.7V, but whether that is its new "full AH capacity" or only 10% of that is only determined by loading it....


    PS - I forgot to mention the vehicle's power - like EMS, air-con clutches etc, but that too can be highly variable. A few hundred Watts is reasonable.

  7. #7
    Variable Bitrate Wayne613's Avatar
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    Quote Originally Posted by OldSpark View Post
    Remember too that half volume is not half power - it's an exponential/logarithmic function - eg, 2x the volume needs 4x the power (or whatever the relationship is).
    But there are so many variables.
    Sorry, donīt wanna steal the thread, but I was wondering since Oldspark is here..

    I always thought most of these would operate at their rating much like say a power supply, so if it was rated as providing 100watts RMS, it would be running at or near that constantly regardless of how much you were actually using from the pool so to speak.

    I assume this doesnīt actually hold true for amplifiers and some other common devices and is actually dependent on draw?
    2008 Ford Mustang GT/CS CARPC(99%)
    Software: Ride/Road-Runner, Digital FX skin 5.x, iGuidance 2011, GPSgate on Win7 64bit

  8. #8
    Raw Wave
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    Rated output is what (watt) they are capable of.

    Your car may be 100kW (135HP), but probably not when reversing out of your drive. (However driving in at full power is totally acceptable!) (IMHO.)

    Same with most loads.
    A 100W amp puts out 0W at nil volume (and may consume 10W doing so - ie, overhead, or standby consumption) or 100W at full volume (hence 100W in plus 10W overhead plus maybe another 10W in power-proportional losses like I^2R (i x i x R) conductor losses or ~120W in for 100W out).

    LOL - you remind me of the "obvious once you know", aka "common sense" or logical - again, provided you know.

    Amplifier loads are very proportional to volume (as illustrated above).
    Other loads maybe not - eg, a TV because most of its power is to process the picture (even with a dark hence "nil" display), or a PC even if not running anything.
    I think George the Bush Jr now understands that NOT sending emails will not have much effect on (California's) electricity demand. On the other hand, reducing acoustic noise will. (Hence why loud music will be banned in the near future.)


    Does that clarify? Is it now obvious? I can give better examples or descriptions if desired.

    That was a good question - it's very confusing until clarified.
    (There are only bad answers.)


    PS - In general, I think it reasonable to say all loads have their:
    - basic overhead (minimum required to standby; power LEDs, fans etc)
    - their output power (loudness, or winching power, or lighting, ec)
    - their output-dependent losses (i2R losses; extra cooling; extra circuits).
    All the above can vary from near 0% to near 100% depending on what it is (eg, amp, PC, GPS)

  9. #9
    Variable Bitrate Wayne613's Avatar
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    That makes sense now. I just wanted additional clarification, as I wasnīt really sure where amplifiers were concerned. It is still fairly obvious to most others probably once thought about, with most using a cap or extra battery near the amp load point, but it was still fuzzy for me. Thanks!
    2008 Ford Mustang GT/CS CARPC(99%)
    Software: Ride/Road-Runner, Digital FX skin 5.x, iGuidance 2011, GPSgate on Win7 64bit

  10. #10
    Raw Wave
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    Oh no - the C word!!!

    We did go thru the cap stuff didn't we? Or rather, you understand (or accept) that a battery next to the amp instead of a cap is most likely a far superior and far cheaper solution? [There are exceptions, but that is a few % of cases if that. And whether it be here or on the12volt.com, I have yet to see a case where a cap is warranted.)
    But the cap argument is more to PREVENT purchasing a cap (again: use a superior & cheaper battery). If already bought, may as well use it unless you can swap etc.]

    And it is understood that - apart from extending reserve time (the time it takes the battery to go flat) and to stay within battery discharge (rate) limits or extend battery life - there is no point adding batteries. They need to be recharged, and that incurs an efficiency penalty of ~30% or more. (IE - amp uses 100A, the alternator supplies 80A, therefore the battery supplies the 20A shortfall. The charger will have to supply an "equivalent time" of (say) 26A to replace the drained 20A.)

    Not meaning to hijack of course! (Open a new thread if it's a problem. Or maybe I could look up references and links...)


    PS - And batteries & caps are NOT extra loads on an alternator. Or rather, to state it correctly, caps and extra or bigger batteries lessen the load and SAVE wear and tear on the alternator etc. (Generally speaking.)

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