Battery current draw

[swx1042 10+ year member]

After starting your car, how much current does the battery draw from the alternator to charge itself?

I don't have a big stereo or any other equipment, i've just always wondered.

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[n2audio 5,000+ posts]

In good health I bet it only draws a few amps (3-5?) for a minute or two to top back off then about an amp continuously.

 

[swx1042 10+ year member]

so, as it ages, it might start drawing more and more until it becomes a great load like 20-30 amps or so?

 

[dman4486 5,000+ posts]

so, as it ages, it might start drawing more and more until it becomes a great load like 20-30 amps or so?

I don't think it would draw that much unless it had a bad cell and was pulling the charging system vdc down......

 

[swx1042 10+ year member]

so the battery's pull will always pretty reasonable for most of its life?

so if you had two batteries, would the current pull just double?,

(i.e. if one battery pulls 3-5 for charging, then two would pull 6-10?)

 

[SPLEclipse 10+ year member]

so if you had two batteries, would the current pull just double?, (i.e. if one battery pulls 3-5 for charging, then two would pull 6-10?)

no. thats like saying if you had two hearts they would both pump as hard/often with the same vascular system.

you have to think about things the other way. your car needs amperage, so it "tells" the battery (or batteries) to provide it. it (they) does. the alt outputs the same amount of current based on RPM no matter what. it will charge whatever needs to be charged in the system, be it lights, the headunit, a capacitor, or battery. when the battery can not provide the car with what it needs, the alt compinsates as best as it can to a limit. when it can't, the battery (or batteries) provide the juice and "wait" until the required draw drops so it (they) can be brought back up by the alt.

technically if your system was performing fine and maintaining correct voltage, you could add 50 extra batteries and not notice a difference (assuming they are all healthy and ignoring resistance, etc.).

 

[swx1042 10+ year member]

no. thats like saying if you had two hearts they would both pump as hard/often with the same vascular system.
you have to think about things the other way. your car needs amperage, so it "tells" the battery (or batteries) to provide it. it (they) does. the alt outputs the same amount of current based on RPM no matter what. it will charge whatever needs to be charged in the system, be it lights, the headunit, a capacitor, or battery. when the battery can not provide the car with what it needs, the alt compinsates as best as it can to a limit. when it can't, the battery (or batteries) provide the juice and "wait" until the required draw drops so it (they) can be brought back up by the alt.

technically if your system was performing fine and maintaining correct voltage, you could add 50 extra batteries and not notice a difference (assuming they are all healthy and ignoring resistance, etc.).

I'm not talking about the batteries compensating for a small alternator though,

I mean more like after starting a car on a cold start or using electronics while the engine has been off.

 

[Rashaddd 5,000+ posts]

Batteries will pull as much as they can at a given time in a given environment. A fully charged battery will pull nothing. A fully depleted (to reasonable limits keeping it still "alive" and not permanently damaged) battery will pull as much as it can handle internally or as much as your alt can supply it.

 

[swx1042 10+ year member]

gotcha!