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Just a few dumb questions... extra batteries

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  • #16
    Originally posted by Millstonemike View Post

    From Nissan's dealer service manual for my Maxima. There is a drop from the generator output to the battery under load (headlights, heater fan, etc.) through the (140 A) fuseable link. Nissan spec.'s that drop to be 0.2 V or less.

    Measured at my battery moments ago: High idle 14.35, low idle 14.2. With headlights and heater fan on high at 1500 rpms, 14.28. There's always tolerances in any car. But the nominal auto voltage is spec.'ed at 14.4 V.

    Though, I should have asked the OP were he tapped to measure the voltage.

    Fro the Nissan dealer service manual for my Maxima; measured at the "generator" on normal idle.

    Click image for larger version

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    I don't remember where my mechanic (friend) tapped into for the voltage gage, but I remember him saying it was a good place for reference.

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    • #17
      Originally posted by Millstonemike View Post

      From Nissan's dealer service manual for my Maxima. There is a drop from the generator output to the battery under load (headlights, heater fan, etc.) through the (140 A) fuseable link. Nissan spec.'s that drop to be 0.2 V or less.

      Measured at my battery moments ago: High idle 14.35, low idle 14.2. With headlights and heater fan on high at 1500 rpms, 14.28. There's always tolerances in any car. But the nominal auto voltage is spec.'ed at 14.4 V.

      Though, I should have asked the OP were he tapped to measure the voltage.

      Fro the Nissan dealer service manual for my Maxima; measured at the "generator" on normal idle.

      Click image for larger version

Name:	Volts.png
Views:	52
Size:	50.7 KB
ID:	1431898


      This helps illustrate the point I was trying to make, the output voltage and current ratings for an alternator are arbitrary values chosen by the manufacturer based on the conditions they believe the car will see and how fast they want to recharge the battery after starting. I had an 85 Maxima and the service manual for it shows 14.4 to 15V for the alternator with a nominal rating of 12V-70A. Just because the voltage under load is under the rated output voltage doesn't mean there's a problem or the car won't run. Look at how low the voltage is during starting. Fuel pump and engine management computer still work at well under 11V. Less than the rated alternator voltage means that it will take longer to recharge the battery as long as the voltage is above the float level.

      The nominal voltage for both vehicles is 12V, per the charts, not 14.4V. In the case of your maxima the midpoint or we can call it "nominal" alternator output voltage is 14.4V. In the case of my first gen maxima the midpoint for the alternator voltage was 14.7V. Could you swap alternators between the vehicles, yes. Could you swap batteries between them, yup. Would they both still run, yup. So why is there a difference in the alternator voltage spec for two cars that both have the same nominal operating voltage?

      The only thing magical about the 14.4V number and lead acid batteries is that it is the voltage that a lead acid battery will start outgassing. This wasn't a problem with batteries that you could do maintenance on. Obviously Nissan came to the conclusion that they were cooking their batteries with voltage regulators that would allow up to 15V. They could have just as easily chosen 13.8 or 14 as the maximum voltage the alternator would produce which would have caused longer recharge times.

      I think what's bugging me is the fact that you stated the OP had a problem with his voltages because when he cranked up the system it was 13.8 to 14V not 14.4V and wasn't keeping up with his system. As long the voltage at the battery is above the float voltage, about 13.25V it's not discharging and it's most certainly charging if it's at 13.8 to 14V albeit at a slower rate than if it was seeing 14.4V.


      Click image for larger version

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      • #18
        Originally posted by devnull View Post

        .... As long the voltage at the battery is above the float voltage, about 13.25V it's not discharging and it's most certainly charging if it's at 13.8 to 14V albeit at a slower rate than if it was seeing 14.4V.
        I agree with your assessment of battery voltages. But I'm talking about the alternator. The low voltage is an indicator the alternator is being overworked.

        The alternator is regulated with the windings putting out higher voltages that can be regulated to the desired output: 14 Volts whatever That's how it outputs a constant voltage over a wide range of rpms. When a load causes the voltage to drop below the regulated voltage, it's windings can't produce enough voltage - at that rpm. That can happen on idle and with lights and heater fan running at higher rpms.

        The OP should just run with the stock alternator and super-caps at the amp. If his "battery" light comes on - not charging - he needs a beefier alternator.


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