Chemical Batteries (Car Batteries)

[mearvk]

Why does the effectiveness of a car battery seem to drop off rather sharply when the voltage gets a bit below 12? I would think it would be more or less a linear decay of current ( I = V/R etc). Thus one would think that the current generated would be proportional to the voltage but anyone who's tried to start a car, listen to the radio etc on something like 11 volts knows it's pretty dead.


So what gives?

 

[jim hardy]

its electrolyte is getting depleted of SO₄ ions to migrate between electrodes,
and the electrodes get coated with PbSO₄
so its effective internal resistance increases and terminal voltage drops when loaded.

It can show 12 volts on a meter but be too weak to iluminate the dome light.

Here's a good writeup:
http://ecee.colorado.edu/~ecen4517/materials/Battery.pdf

anyone who's tried to start a car, listen to the radio etc on something like 11 volts knows it's pretty dead.

if you unload it voltage will drift back up to around 12.

That's also the reason you can grind it down to a nub with starter then wait a few minutes and get another burst, Ions in electrolyte migrate back to vicinity of electrodes.

 

[mearvk]

Thanks Jim.

So actually amperage is a better measure of the battery's health in some sense than voltage may be. It could have 12 volts but not enough of the electrolytes that makes the current so it wouldn't be able to deliver much in the way of power.