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AGM BATTERY CHARGING - ALTERNATOR VOLTAGE BOOSTER

Discussion in '2nd Gen. Tacomas (2005-2015)' started by 05Taco4x4, May 24, 2019.

  1. May 24, 2019 at 5:19 PM
    #1
    05Taco4x4

    05Taco4x4 [OP] ToyotaHubs

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    A buddy gifted me an Optima Red Top after I boosted my engine. A few weeks later I came across a post by @hx989 in the nefarious "What Have You Done To Your 2nd Gen Tacoma Today?" thread, explaining how he added a diode to his Alt-S circuit to charge his AGM battery. o_O I wasn't aware my alternator wasn't sufficiently charging my battery, I certainly didn't know how adding a diode would make it do so, and what's a "Alt-S"? Apparently, AGM batteries require higher voltages than lead-acid batteries to charge and our alternators don't put out enough. Placing a diode in place of the Alt-S (alternator sensing) circuit fuse tricks the alternator into seeing less voltage than the battery actually has, raising the alternator voltage output.


    Littlefuse Mini Diode Fuse
    1 amp
    02400113P
    02400113XP
    1.5 amp
    02400120P

    Remove this bit of plastic
    02400113P.jpg
    Note:
    Replace with fuse while reprogramming ECU
     
    Last edited: Aug 16, 2019
  2. May 24, 2019 at 6:38 PM
    #2
    vwbuggsy

    vwbuggsy Well-Known Member

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    I get the idea here. I have an agm battery and have never made this change. Normal charging voltage is ~13.5 volts I think, and battery resting is 12.5 ish. So does this cause a ~14.7 charging voltage, or allow the battery to charge to a resting charge of ~13.7... Or both? And what is the downside of not doing this because my agm seems to have been doing fine without it for a couple years now.
     
  3. May 24, 2019 at 7:00 PM
    #3
    6 gearT444E

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  4. May 24, 2019 at 10:12 PM
    #4
    Roostfactor

    Roostfactor Well-Known Member

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    My diode is about .5 to .7 volts higher than stock.

    That is why I stated a better way to install it in the system is to add it in series while leaving the stock fuse in place.
    You get the voltage drop and the protection that way.
     
  5. May 24, 2019 at 10:18 PM
    #5
    crashnburn80

    crashnburn80 Vehicle Design Engineer

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  6. May 24, 2019 at 10:24 PM
    #6
    Roostfactor

    Roostfactor Well-Known Member

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    crashnburn80 likes this.
  7. May 25, 2019 at 7:36 AM
    #7
    vwbuggsy

    vwbuggsy Well-Known Member

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  8. May 25, 2019 at 12:17 PM
    #8
    Jeffch

    Jeffch Well-Known Member

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  9. May 27, 2019 at 12:07 PM
    #9
    MolonLabeTaco

    MolonLabeTaco Well-Known Member

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    I ran a 27F AGM in my '97 FZJ80 for over 4 years & never had an issue. Is this really necessary?
     
  10. May 27, 2019 at 6:36 PM
    #10
    6 gearT444E

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    No, but it will extend the life of the battery and if you are using it for a setup where you need the capacity, it will never be fully charged even from the start.
     
    Redsyota likes this.
  11. May 27, 2019 at 6:51 PM
    #11
    MolonLabeTaco

    MolonLabeTaco Well-Known Member

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    It ran my fridge 24/7 & started every time. Not once was it ever so dead it wouldn't start.
     
  12. May 27, 2019 at 7:02 PM
    #12
    6 gearT444E

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    I never said it wouldn't, but you will never get the battery fully charged with the lower voltage meant for a standard flooded lead acid battery. I take my AGM batteries and put them on an AGM charger when the truck is in the garage to make sure they are topped off.
     
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  13. Jun 1, 2019 at 2:00 PM
    #13
    Labbi85

    Labbi85 Well-Known Member

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    Did this mod today and it kinda works. Unfortunately the diode I ordered (02400113P) has voltage drop of about 0.58V (measured with a fluke multimeter) and this rises my voltage from 13.9V to around 14.5V. This is of course better then nothing but not quite at the 14.7V I was hoping for. Unfortunately I can't find any data sheet for these diodes which include a voltage diagram.
     
  14. Jun 5, 2019 at 1:49 AM
    #14
    xxmagpulxx

    xxmagpulxx Well-Known Member

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    So apparently the HKB Diode wont work in a 2016 or newer Tacoma because the 2016 or newer Tacoma doesn't have an ALT-S circuit. After waiting 3 plus weeks for the adjustable diode, ask me how I know.

    While we are on this subject, I have a brand new, never been used diode for sale. Since I couldn't use it, Ill sell it for what I paid for it, and you don't have to wait forever to get it from Australia. Its located in Nevada.
     
    Redsyota likes this.
  15. Jun 6, 2019 at 5:20 PM
    #15
    SamuelJ

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    I know Im late to chime in here but I bought the hbk one about a year ago too and it works like a champ. I have two yellow tops and no problems thus far. The hbk one doesn't ixnay the voltage protection either.
     
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  16. Jun 7, 2019 at 12:31 AM
    #16
    crashnburn80

    crashnburn80 Vehicle Design Engineer

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    That has already been covered in depth here. Including many working on how to reprogram the ECU with TechStream or use a tune to recalibrate the ECU to modify 3rd gen voltage charging, since 3rd Gen voltage is ECU controlled and not alternator controlled like earlier gens that can then be modified with a diode.
     
  17. Jun 7, 2019 at 6:24 AM
    #17
    Jacked Burton

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  18. Jun 7, 2019 at 6:47 AM
    #18
    maineah

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    Typical voltage drop across a diode is .7 volts. I think before going to all the effort I would actually check the charge voltage, a drawn down AGM would like to see 14.6-14.8 dropping to 13.6-13.8 well with in the realm of a stock alternator.
     
  19. Jun 7, 2019 at 6:53 AM
    #19
    Grossomotto

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  20. Jun 10, 2019 at 5:19 PM
    #20
    MacMedic

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    I too have a brand new, never been used diode for sale. Since I couldn't use it, Ill sell it for what I paid for it, and you don't have to wait forever to get it from Australia. Its located in Connecticut.
     

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