John - K5AWO
I am not a chemist but I have read this does something that pulls sulphites out of the grid structures and deposits it on the surfaces of the grids not unlike boiling water leaves solids in the bottom of the pan. This eventually crosses over the plates and hence the discharge current begins.
I think this will answer the question..... :{
How a battery works, long...
Lead Storage Battery
In this battery, lead serves as the anode, and lead coated with lead dioxide serves as the cathode. Both electrodes dip onto an electrolyte solution of sulfuric acid. The reactions are as follows:
Anode reaction: Pb + HSO4- ---> PbSO4 + H+ + 2e-
Cathode reaction: PbO2 + HSO4- + 3H+ + 2e- ---> PbSO4 + 2H20
-----------------
Cell Reaction: Pb(s) + PbO2(s) + 2H+(aq) + 2HSO4-(aq) ---> 2PbSO4(s) + 2H2O(l)
A typical auto lead storage battery has six cells connected in series. Each cell has multiple electrodes in the form of grids and produces approximately 2v, giving the total charge of ~= 12. 0v. In this reaction, the sulfuric acid is consumed as the battery discharges, producing lead sulfate, hydrogen, and 2 electrons, thus lowering the density of the electrolyte solution from the initial value of about 1. 28 g/cm^3 to some lower value, dependant upon the resultant charge. A fully discharged battery will have an electrolyte solution density around 1. 00 g/cm^3, which is the density of H2O(l)
This is how the condition of a battery can be measured. The floating green ball has a respective density slightly less to that of a fully charged battery. When the charge cells in the battery drops, the density of the aqueous solution drops. The solution no longer has sufficient density to float the ball, and the ball sinks, giving the appearance of a black image in the inspection hole.
The lead sulfate formed in the cell reaction during discharge adheres (though the process of electrolysis) to the grid surfaces of the electrodes. The battery is recharged by forcing current through the battery in the opposite direction to reverse the cell reaction. In theory, a lead acid battery can be charged an infinite number of times.
But... .
Over time, this process becomes weakened and not an entire reversal of the reaction occurs, thus leaving some lead sulfate on the grid surfaces. This process can be calculated by finding the equilibrium constant for the reaction. This is referred to as K.
K...
This gets deep. Taking the cell reaction: Pb + PbO2 + 2H+ + 2HSO4- ---> 2PbSO4 + 2H2O... .
K would become.....
[PbSO4]^2 [H2O]^2
K=---------------------------------------
[Pb] [PbO2] [H+]^2 [HSO4-]^2
Substitute the Molarity (I do not have these numbers memorized) and do the simple math. This resultant number will tell you how efficient the reaction will be for a relevant temperature. This process is known as the Haber Process.
The reaction written in the reverse order leads to the equilibrium expression:
[Pb] [PbO2] [H+]^2 [HSO4-]^2
K'=---------------------------------------
[PbSO4]^2 [H2O]^2
We can easily see

that K and K' are reciprocals of each other. Therefore:
[Pb] [PbO2] [H+]^2 [HSO4-]^2 1
K'=--------------------------------------- = --
[PbSO4]^2 [H2O]^2 K
At this point, we can use the law of Mass reduction: Oo.
[PbSO4]^2 [H2O]^2
K"=------------------------------------------------ =
[Pb]^(1/2) [PbO2]^(1/2) [H+] [HSO4-]
| [ [PbSO4]^4 [H2O]^2]^4 | ^(1/2)
| --------------------------------- |
| [PbSO4]^2 [H2O]^2 |
Therefore...
= K" = K^(1/2)
=Knew = (Koriginal)^n, where n = #of occurrences of reaction.
This all means that the reaction is never 100% efficient, and the process charging process will eventually reach some stopping point because of lead sulfate build up on the cell grid. No more charging cycles will occur, because the solution will no longer support reverse separation.
The road jarring of the battery during driving can knock the lead sulfate off the grid, where it collects on the bottom of the batter. If enough collects in the bottom of the battery, it will short out the grid surfaces, resulting in a dead cell. Each dead cell reduces the battery capacity by ~= 2v.
As we see, the batteries usefulness can quickly diminish.
In the end, it is usually not the charging/discharging that will kill a lead/acid battery, but jarring the battery on the road which kills.
Also...
Traditional storage require periodic "topping off" of distilled water in the electrolyte solution. This solution is depleted by the electrolysis that accompanies the charging process. This is the:
Anode reaction: Pb + HSO4- ---> PbSO4 + H+ + 2e-
Sulfuric acid dissociating and producing hydrogen ions. Very flammable.
The sealed types of batteries use an electrode made of an alloy of calcium and lead that inhibits the electrolysis of water, Yea Right . Therefore, these types of batteries require no additional water.
These batteries will eventually boil out the electrolyte solution, and require more H2O(l).
The use of non-distilled water will cause chemical side-reactions accelerating the failure of the battery.
NOTE. .
Most of this information is coming from my memory of chemistry, so I do believe it to be correct. It has been al long time, so there might be a mistake with the chemical reaction of the anode and cathode.
Just my $0. 02
-Rich