Some Technical Details on Lead Acid Batteries

The Chemistry of Sulfation, and Why Pulsing Helps

Here is a basic look at the chemistry of conventional lead-acid batteries under charge and discharge. This data is taken from the technical manual for Power Sonic batteries. The manual is worth looking at in its own right: Click here for the PDF.

The basic electrochemical reaction equation in a lead-acid battery can be written as follows:

Pb + 2H2SO4 + PbO2      <---- Charging        + PbSO4 + 2H2O + PbSO4
                      Discharging---->

Or, in words:


porous lead      +    sulfuric acid   +    porous lead dioxide
active material        electrolyte         active material
negative plate                             positive plate


                 ^    charging   ^
 Becomes         ----------------
                 V discharging V


lead sulfate     +      water         +    lead sulfate
active material       electrolyte          active material
negative plate                             positive plate

Discharge

During the discharge portion of the reaction, lead dioxide (positive plate) and lead (negative plate) react with sulfuric acid to create lead sulfate, water and energy.

Charge

During the recharge phase of the reaction, the cycle is reversed: the lead sulfate and water are electro-chemically converted to lead, lead oxide and sulfuric acid by an external electrical charging source.

This is how things work when the battery is new and clean. There are several effects that come in as the battery is used. One is the process called sulfation, which is of central concern to this effort. Here is Richard Perez, from Home Power magazine #29, page 44:

The biggest problem in lead-acid cells is sulfation due to chronic undercharging. Here the sulfate ions have entered into deep bonds with the lead on the cell's plates. The sulfate ions can bond with the lead at three successively deeper energy levels. Level One is the bond we use when we normally charge and discharge the cell. After a month or so at Level One, some of the bonds form Level Two bonds which require more electric power to break. After several months of being at Level Two bond, the sulfate ions really cozy up to the lead and form Level Three bonds. Level Three bonds are not accessible electrically. No amount of recharging will break Level Three bonds. The longer the lead sulfate bond stays at a level the more likely it is to form a closer acquaintance and enter the next deeper level. This is why it is so important to fully, regularly, and completely, recharge lead-acid cells.

Equalization Charges

If the loss in capacity is due to Level Two bonding, then a repeated series of equalizing charges will break the Level Two bonds. Under equalization the Level Two bonds will first be transformed into Level One bonds, and then the sulfate ion can be kicked loose of the lead entirely and reenter the electrolyte solution. If your lead-acid cells have lost capacity, then a regime of equalizing charges is the first procedure to try. An equalization charge is a controlled overcharge of an already fully recharged cell. First recharge the cell and then continue to charge the cell at a C/20 rate for five to seven hours. During equalization charges, the cell voltage will become very high, about 2.7 VDC per cell. This overcharge contains the necessary power to break the Level Two bonds and force them to Level One. Once they reach Level One, the bond is easily broken and the sulfate ions reenter into solution in the electrolyte.

In the above, the Level One, Two, or Three bonds refer to progressively larger and insoluable crystals of lead sulfate. Like most crystal formation, it is a slow process. So the question is, How could pulse charging affect this situation? According to conventional wisdom, not at all. Here is the party line, from a product applications manager at Trojan Battery Co. on pulsing:
The active material in the positive electrode....is lead dioxide. This molecule is a relative of rust, it is a corrosion product. When you charge a lead acid battery, one of the things you attempt to accomplish is the repair or reformation of the corrosion layer of the positive plate. If you don't properly charge the battery, the corrosion layer begins to break down in the acid environment and the voltage characteristic of the battery changes for the worse.

Pulse charging does not influence or improve the corrosion layer of the positive electrode and therefore, does not permanently or properly improve the performance of the battery. It is only through the electrochemical process of corroding the positive electrode that you optimize the battery's performance. To properly corrode the positive electrode the battery voltage has to reach and then excced the gassing potential of the battery. In a deep cycle battery, not gassing the cells will result in stratified electrolyte, ineffective corrosion of the positive plate, reduced performance and shortened life.
My opinions on the matter, subject to further learning are:

that pulsing does affect the Level Two bonds by constantly applying an overpotential above the gassing point, at a kilohertz rate. Thus, even if the battery is not fully charged, it keeps the Level One bonds from turning into Level Two bonds. Since the process is slow and continuous, it does not liberate significant amounts of gasses, as they are able to dissolve into the electrolyte.
that pulsing can also affect the Level Three bonds by virtue of the high rise time of the pulses. Since the Level Three bond crystals are insoluable and electrically inactive to DC current, they act as a dielectric. This forms a capacitive connection between the deeper layer of the plate and the electrolyte that will pass transients of current with low impedance. This then allows for enough energy to be transferred to the material to allow for the slow breaking apart of the crystals.
The main factors are: fast rise time, high peak amperage, and moderate repetition rate.
There are a number of other factors that can degrade batteries, to site a few:

Cycling of the battery causes "shedding", or loss of material from the plates. If you see a battery that has reached its limit, you might find chunks of lead on the bottom of the cell, and actual holes in the plates. Nothing's to be done in this case. Excessive charge rate will also cause this to occur.
Electrolyte stratification occurs when the batteries do not receive sufficient charge rate to cause bubbles to rise and stir the liquid. The heavier electrolyte sinks to the bottom of the cell. This renders the lower portion of the plates inactive, and that is where sulfation gets started. Some large batteries use pumps to force the electrolyte to turn over. While pulsing can repair the harm done from prolonged stratification, it does not alleviate the situation. Only sufficiently high charge rate for the size of the battery will cause the needed convection currents. As one pulse charging patent states, (US Patent #3,963,976) high peak amperage is needed to overcome this factor.
Electrolyte contamination happens everytime a (dirty) hydrometer is inserted in the cells, or less than good distilled water is used. This can lead to a mineral layer on the plates, which might be removed by pulsing, or by EDTA.
Shorted cells can be caused by sulfation. It seems that the crystal formation expands and warps the plate material. This can cause the case to bulge, or to push adjacent plates together. If you are lucky, desulfation can relieve this situation, but I have not seen it myself.
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