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    Results 1 to 16 of 16
    1. #1
      Join Date
      Jan 2014
      Location
      Austin, TX
      Posts
      636
      Country Flag: United States

      How to determine battery requirements?

      I'm looking to relocate the battery in my '64 Corvette to the real estate previously occupied by the spare tire carrier. What I'm trying to figure out is what is the smallest battery I can use reliably.

      The running gear on the car is very simple....stock 327 solid lifter motor with 4 spd trans. Electrical load will include at maximum:

      -Holley Sniper EFI/fuel pump
      -Electric radiator fan
      -remainder of factory system
      -LED lights throughout

      I'd specifically like to use the Odyssey PC680....advertised for small imports (Civic, 240SX, etc.) or, the optima yellow top D51. They're both pretty small and would be easier to mount using the holes in the frame crossmember previously used to mount the spare tire carrier.

      Any input on either battery for my application would be greatly appreciated.

      Thanks!
      1972 C20 Suburban
      1964 Corvette Coupe

    2. #2
      Join Date
      Sep 2008
      Location
      Jacksonville, Florida
      Posts
      629
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      It really depends on your alternator. Looks like you might have about 40-50 amps (with headlights on) of draw, as long as the alternator can supply that at idle, you won't be dipping in to the reserve power of the battery. If the alternator is only supplying enough amps at idle to cover the loads, then you will not be charging the battery from the drain of starting the vehicle. A quick trip to the store, it may not restart with too small Ah battery. If you can cover the amp draw and a little extra for charging, then you just need enough cranking amps to start the engine...how many times without charging the battery with the alternator is going to help determine size (amp hour rating).
      Craig Scholl
      CJD Automotive, LLC
      Jacksonville, Florida
      904-400-1802
      www.cjdautomotive.com

      "I own a Mopar, I already know it won't be in stock, won't ship tomorrow, and won't fit without modification."

    3. #3
      Join Date
      Oct 2014
      Location
      DFW, Texas
      Posts
      422
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      I'd suggest spec'ing the battery size off of the cranking amp requirements of the starter, and account for less output under cold conditions.

      Everything else should be dependent upon your alternator, you shouldn't really "need" much of a battery after the engine is actually running.
      1972 Plymouth 'Cuda - Not LS-swapped, 5.7L Hemi [MS3 Gold Box], T56 Magnum 6-speed - 'Cuda Build Page
      1976 Dodge D100 - Warlock
      2016 Subaru WRX - E30 Tune

    4. #4
      Join Date
      Jan 2014
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      Austin, TX
      Posts
      636
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      Thanks for the input guys. The alternator is the factory alternator 50 AMP alternator. As far as calculating amp hour rating requirement....how does that math work?
      1972 C20 Suburban
      1964 Corvette Coupe

    5. #5
      Join Date
      Oct 2014
      Location
      DFW, Texas
      Posts
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      Quote Originally Posted by FLYNAVY53 View Post
      Thanks for the input guys. The alternator is the factory alternator 50 AMP alternator. As far as calculating amp hour rating requirement....how does that math work?
      You could procure an inductive ammeter and crank the engine over. Take that number [amperage] and add 40-50% to it as a safety factor?

      Taking a look at the factory battery CCA is probably a very good place to start with minimal reason to deviate from. If you have a newer gear-reduction starter, it will consume less current and you can probably drop the CCA requirement by a factor you feel comfortable with.

      I'd say your 50a alternator is going to limit you. Not only is it not enough to support all of the loads, but I expect that it is not going to kick out 50a at idle/low rpm. If you're idling with the headlights on, radiator fan running and the EFI going, you're quickly getting to 50amps. Source yourself a higher output alternator, along with solid low rpm output. 100a+, in my opinion.
      1972 Plymouth 'Cuda - Not LS-swapped, 5.7L Hemi [MS3 Gold Box], T56 Magnum 6-speed - 'Cuda Build Page
      1976 Dodge D100 - Warlock
      2016 Subaru WRX - E30 Tune

    6. #6
      Join Date
      Dec 2010
      Location
      Fredericksburg, VA.
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      3,155
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      A 50amp alternator is not going to be enough since you are running EFI & pump plus electric fan. 50 amp was barely sufficient for the stock wiring back in the day, no way it will keep up with the added load. I would be looking at 140amp or better in order for the alt to have some reserve capacity. Here is some good reading about charging systems on these older cars.

      http://madelectrical.com/electricalt...evymain1.shtml
      Steve Hayes
      "Dust Off"
      68 Camaro

      Given sufficient initial acceleration, even pigs can fly!

    7. #7
      Join Date
      Jan 2014
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      Austin, TX
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      Good read.....much appreciated!
      1972 C20 Suburban
      1964 Corvette Coupe

    8. #8
      Join Date
      Apr 2006
      Location
      Atlanta, GA
      Posts
      128
      As other have said- that alternator is a weak point- on a good day it might put out 50A- but that's NOT going to happen at idle when the electric fans are running. Most Alternator set-ups can only muster full output at 1500-1800RPM.

      Here's a thought- I've owned a couple C2's and currently a couple C3's- have you looked at running 2 (TWO) 6 volt batteries in a series- that way you'll have the same AH rating as a one big 12 v battery.

      Oh- when you upgrade the factory wiring- be careful how you do it- as incorrectly run upgraded cables from the alternator can bypass the fusible links. AND the factory ammeter will not read correctly- I'd replace it with a voltmeter.

      And the thought of putting a battery under the gas tank sort of doesn't sound like a great idea...

      This made for a lot of extra room in my car-


    9. #9
      Join Date
      Jan 2014
      Location
      Austin, TX
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      Thanks for the input! I'm going to be gutting the entire electrical system and using an aftermarket harness this winter...to include a restoration of the factory gauge cluster and a swap to a voltmeter. Ill be going with a better alternator at that time as well.

      I had wondered about running two smaller batteries as opposed to one big one, simply because it frees up some more accessible mounting options, and more evenly distributes the weight.

      Would it be better to run two 6v batteries in series, or two smaller 12v batteries (Odyssey PC680) in parallel?

      Thanks!
      1972 C20 Suburban
      1964 Corvette Coupe

    10. #10
      Join Date
      Apr 2006
      Location
      Atlanta, GA
      Posts
      128
      Quote Originally Posted by FLYNAVY53 View Post
      Thanks for the input! I'm going to be gutting the entire electrical system and using an aftermarket harness this winter...to include a restoration of the factory gauge cluster and a swap to a voltmeter. Ill be going with a better alternator at that time as well.

      I had wondered about running two smaller batteries as opposed to one big one, simply because it frees up some more accessible mounting options, and more evenly distributes the weight.

      Would it be better to run two 6v batteries in series, or two smaller 12v batteries (Odyssey PC680) in parallel?

      Thanks!

      The best in your case would be the way to get higher amp hour rating but with a caveat-
      .

      Back when I was in the car stereo industry - I had many a car with a dual 12volt set up for the sound system- no isolator and they were just paralleled- no problems. However if one battery went bad - it would quickly wipe the other one out- the lower voltage "bad" one would discharge the good higher voltage one (socialism?!!!). I know the RV guys liked the 6v in a series as this was less a problem- look at your flashlight- the batteries are in a series - so with no load on them- they won't try to take voltage from one another ( unless they are government batteries!!!). The only issue is serviceability as 6V batteries are not everywhere.

      SO if the car is not a daily driver- I'd opt for the 6V series set up.

      Richard

    11. #11
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      Jan 2014
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      Austin, TX
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      Much appreciated Richard! BTW.....love the BMW motor in the C3 over on the Corvette forum....been following that build for years!
      1972 C20 Suburban
      1964 Corvette Coupe

    12. #12
      Join Date
      Sep 2013
      Location
      Houston, TX
      Posts
      246
      Country Flag: United States
      Flynavy53:
      I vote for the series connected 6volt batteries if your tight on space, but you'll be hard pressed to find replacements on a road trip (think, HR Power Tour). On the alternator, I suggest fitting a late-model CS130 style alternator for higher output, better low RPM output and an internal regulator. It requires a connector swap or an adaptor as the CS130 has a different connector. It will bolt in place of your 10/12si just need to check the clocking. Part number PN7935K-11G puts out 160A max with 80A at idle. HTH.
      Rodney
      Rodney Meyers
      72 Olds 442 Rest-mod clone

    13. #13
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      Jan 2014
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      Austin, TX
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      Thanks Rodney
      1972 C20 Suburban
      1964 Corvette Coupe

    14. #14
      Join Date
      Apr 2010
      Location
      Jersey Shore
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      I ran the PC680 in my '69 with a 150A alternator. It was fine for daily use and started my 11:1 350 no problem. If youre putting it in the spare tire area, make sure youve got a good, adequate sized cable going to the starter.
      -Chris
      '69 Corvette
      '55 Chevy Hardtop
      AutoWorks Middletown, NJ
      @autoworksnj for corvette and shop car pics
      https://www.pro-touring.com/showthre...e-Build-Thread

    15. #15
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      Jan 2014
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      Austin, TX
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      Good info, thanks Chris!
      1972 C20 Suburban
      1964 Corvette Coupe

    16. #16
      Join Date
      Jul 2016
      Posts
      341
      Country Flag: United States
      Here is a general BCI size guide chart:https://www.jegs.com/Sizecharts/bcigroup.html







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