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Thread: Automotive Battery Rating

  1. #1 Automotive Battery Rating 
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    Can anyone tell me the approximate Ampere-Hour rating of a typical automotive battery? I'm guessing maybe 30 to 40? Reason to know is, my other forum interest is dedicated to vehicles. They are going around in circles often, trying to solve "battery-drain" issues. I believe typical "keep-alive" current for engine computers is about 5ma, another guy claims 20 to 50, which I expect is high. I have seen today's vehicles start easily after not running several months; surely 50ma will drain a battery in that length of time, won't it?

    Thanks. jocular


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    Quote Originally Posted by jocular View Post
    Can anyone tell me the approximate Ampere-Hour rating of a typical automotive battery? I'm guessing maybe 30 to 40? Reason to know is, my other forum interest is dedicated to vehicles. They are going around in circles often, trying to solve "battery-drain" issues. I believe typical "keep-alive" current for engine computers is about 5ma, another guy claims 20 to 50 . . .
    Both numbers are all over the place. Smaller cars have 10-20 amp hour batteries, larger cars and trucks up to 60. However (important) they can only deliver that a few times before the battery is destroyed. They are intended for short bursts of high power (for starting purposes) and degrade rapidly after that.

    I remember a famously bad car (one of the first with an ECU) that had a quiescent current draw of almost an amp; if you didn't drive the car for a few days the battery went dead. Nowadays it is in the 20 to 30ma range, with most going to inefficiencies in the buck converters driving the electronics rather than the electronics themselves. Also keep in mind that modern cars are often listening all the time for key transponders, cellphone communications etc and thus take more power.


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    Quote Originally Posted by billvon View Post
    Quote Originally Posted by jocular View Post
    Can anyone tell me the approximate Ampere-Hour rating of a typical automotive battery? I'm guessing maybe 30 to 40? Reason to know is, my other forum interest is dedicated to vehicles. They are going around in circles often, trying to solve "battery-drain" issues. I believe typical "keep-alive" current for engine computers is about 5ma, another guy claims 20 to 50 . . .
    Both numbers are all over the place. Smaller cars have 10-20 amp hour batteries, larger cars and trucks up to 60. However (important) they can only deliver that a few times before the battery is destroyed. They are intended for short bursts of high power (for starting purposes) and degrade rapidly after that.

    I remember a famously bad car (one of the first with an ECU) that had a quiescent current draw of almost an amp; if you didn't drive the car for a few days the battery went dead. Nowadays it is in the 20 to 30ma range, with most going to inefficiencies in the buck converters driving the electronics rather than the electronics themselves. Also keep in mind that modern cars are often listening all the time for key transponders, cellphone communications etc and thus take more power.
    I most certainly appreciate your input! 20 to 30ma is to me, too much, explainable, as you say, by all the extraneous shit being thrown in and not really needed. Very few on auto and truck forums seem able to comprehend when I reiterate, over and over, that the car battery is there for only ONE PURPOSE: start the engine. After that, no matter how much electrical load is imposed, blower, lights, cigarette lighter (ugh!), A/C compressor clutch (quite a load, actually!), etc., if the system was properly designed, the alternator at engine idle speed will maintain voltage at such level as to not require "make-up" by the battery. Given that, a battery can last quite a few years, assuming the engine always starts quickly. imp
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    Quote Originally Posted by jocular View Post
    I most certainly appreciate your input! 20 to 30ma is to me, too much, explainable, as you say, by all the extraneous shit being thrown in and not really needed. Very few on auto and truck forums seem able to comprehend when I reiterate, over and over, that the car battery is there for only ONE PURPOSE: start the engine. After that, no matter how much electrical load is imposed, blower, lights, cigarette lighter (ugh!), A/C compressor clutch (quite a load, actually!), etc., if the system was properly designed, the alternator at engine idle speed will maintain voltage at such level as to not require "make-up" by the battery.
    Agreed with one exception -

    In some cases, the initial load of an accessory (inrush on incandescent headlights, starting surge of DC motors) will be both too much and happen too fast for the alternator regulator to deal with. In those cases the battery provides a short burst of current until the alternator stabilizes and the load declines. I had a car in college that had a completely dead battery, but being a poor college student I couldn't afford a new one and thus jumped it whenever I needed to go somewhere. After a while the car would start doing odd things (stalling when I turned on the headlights) because the electrical system voltage was dropping too far without the battery to make up the power.
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    Quote Originally Posted by billvon View Post
    Quote Originally Posted by jocular View Post
    I most certainly appreciate your input! 20 to 30ma is to me, too much, explainable, as you say, by all the extraneous shit being thrown in and not really needed. Very few on auto and truck forums seem able to comprehend when I reiterate, over and over, that the car battery is there for only ONE PURPOSE: start the engine. After that, no matter how much electrical load is imposed, blower, lights, cigarette lighter (ugh!), A/C compressor clutch (quite a load, actually!), etc., if the system was properly designed, the alternator at engine idle speed will maintain voltage at such level as to not require "make-up" by the battery.
    Agreed with one exception -

    In some cases, the initial load of an accessory (inrush on incandescent headlights, starting surge of DC motors) will be both too much and happen too fast for the alternator regulator to deal with. In those cases the battery provides a short burst of current until the alternator stabilizes and the load declines. I had a car in college that had a completely dead battery, but being a poor college student I couldn't afford a new one and thus jumped it whenever I needed to go somewhere. After a while the car would start doing odd things (stalling when I turned on the headlights) because the electrical system voltage was dropping too far without the battery to make up the power.
    A new battery is a good investment if you need a reliable car that starts every time you need it. I do remember the old days when hard to start cars were more common, but I refuse to put up with one now. I currently drive a 2006 Kia Optima and it starts up first time every time with no need to give it any gas. If I start having any start problems I don't waste any time getting a new battery.
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    Quote Originally Posted by billvon View Post
    Quote Originally Posted by jocular View Post
    I most certainly appreciate your input! 20 to 30ma is to me, too much, explainable, as you say, by all the extraneous shit being thrown in and not really needed. Very few on auto and truck forums seem able to comprehend when I reiterate, over and over, that the car battery is there for only ONE PURPOSE: start the engine. After that, no matter how much electrical load is imposed, blower, lights, cigarette lighter (ugh!), A/C compressor clutch (quite a load, actually!), etc., if the system was properly designed, the alternator at engine idle speed will maintain voltage at such level as to not require "make-up" by the battery.
    Agreed with one exception -

    In some cases, the initial load of an accessory (inrush on incandescent headlights, starting surge of DC motors) will be both too much and happen too fast for the alternator regulator to deal with. In those cases the battery provides a short burst of current until the alternator stabilizes and the load declines. I had a car in college that had a completely dead battery, but being a poor college student I couldn't afford a new one and thus jumped it whenever I needed to go somewhere. After a while the car would start doing odd things (stalling when I turned on the headlights) because the electrical system voltage was dropping too far without the battery to make up the power.
    Thanks for the input! I don't take exception to exceptions, for the sake of "staying ahead". However, today's voltage regulators, being electronic, are pretty damn fast. The old electromagnetic regulators were all we had way back when; the "reverse current cutout" was a relay contained in them, which had big heavy contacts which occasionally became welded "closed", with the result that when the engine was shut down, the battery drained itself back through the generator, which of course couldn't possibly rotate the engine, when thus made into a "motor"!

    The battery and alternator are essentially two voltage sources connected in parallel, the alternator providing the electrical energy needed once the engine is started and gets it spinning nice and fast. It IS possible to overload the alternator, either momentarily, or continuously; in either case, the battery makes up the "difference". Many guys instal large banks of high power lights on top of their truck's cab, below the bumpers, facing backwards, etc., not knowing that, especially with the older vehicles, they will wind up overtaxing their battery. jocular
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    The amp hours listed above are a bit low. My wife's new Honda Fit, which is a subcompact car has a 40 amp hr. My small pickup had a stock 110 amp-hr battery; It's on its 3rd battery (13 years old) and carries a 114 amp-hr battery now.
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    I've always bought the highest amps that would fit in my car so that it wouldn't drain away as quickly. I recommend this to anyone buying a battery for longer battery life. They might cost a little more but they are worth it. I have a 800 amp battery in my car now and it has been there 4 years with no signs of problems starting.
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    all you would ever want to know about batteries--- batteryuniversity.com
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    Quote Originally Posted by Lynx_Fox View Post
    The amp hours listed above are a bit low. My wife's new Honda Fit, which is a subcompact car has a 40 amp hr. My small pickup had a stock 110 amp-hr battery; It's on its 3rd battery (13 years old) and carries a 114 amp-hr battery now.
    IMO, "ampere-hour" ratings for batteries are more useful as P/R gimmicks than technical indicators of battery capacity. For example, we know that a typical starter motor may draw several hundred amperes while cranking an engine, especially a large one. However, looking at the battery spec. you call out above, the implication is that it is capable of delivering 114 amps for 1 hour. I would fall out of my chair, if that were shown to be true! A much more likely possibility would be that it MIGHT deliver 57 amps for 2 hours, that also being "114 ampere-hours". The decisive factor here is the amount of energy a battery can deliver before becoming "chemically exhausted". joc
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    The Faraday constant is the charge on one mole of electrons; approximately equal to 26.8 ampere-hours. It is used in electrochemical calculations.
    An ampere-hour is not a unit of energy. In a battery system, for example, accurate calculation of the energy delivered requires integration of the power delivered (product of instantaneous voltage and instantaneous current) over the discharge interval. Generally, the battery voltage varies during discharge; an average value or nominal value may be used to approximate the integration of power
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    Quote Originally Posted by fizzlooney View Post
    The Faraday constant is the charge on one mole of electrons; approximately equal to 26.8 ampere-hours. It is used in electrochemical calculations.
    An ampere-hour is not a unit of energy. In a battery system, for example, accurate calculation of the energy delivered requires integration of the power delivered (product of instantaneous voltage and instantaneous current) over the discharge interval. Generally, the battery voltage varies during discharge; an average value or nominal value may be used to approximate the integration of power
    Well-stated, and in a far more technically-descriptive way than I am capable of! jocular
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