# Why are batteries modeled as ideal voltage sources?

Asked by itsnotmyfault1 (203) April 16th, 2013

When using batteries as your source in a physics/engineering problem, it’s considered as a voltage source. You’d say “a 9V battery”. However, looking at the battery as an electrochemical cell, I’d guess that it’s a current source. The reaction is going this fast and therefore provides this rate of electrons to the load. The only problem with me looking at it that way, is that I can also see it being a voltage source anyway: the ions diffuse through the electrolyte so fast that the total voltage across the battery is pretty much constant.

I still haven’t fully convinced myself that this chemistry based argument for being a voltage source really makes sense, but the question still remains: why are batteries modeled as voltage sources?

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Generally, we model a battery as a voltage source when the impedance of the load is much greater than the internal impedance of the battery. Example: a battery operated device that runs for years on a single AA battery.
We model the battery as a current source when we have high current loads that have impedances near or below the internal resistance of the battery. Examples: motors, heaters, and charging units.
Keep in mind you get the most power out of the system when the load impedance matches the battery’s internal impedance. (Power matching.)

In a full up simulation (MatLab) we have battery models that incorporate virtually all the characteristics of the particular battery.

When current draw is low, the voltage of a battery is a function of the chemistry, so is quite constant. As the battery is used, its internal resistance goes up so voltage goes down.

RocketGuy (12735)

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