Powersports Battery Chemistry
Most commercially available motorcycle and powersports batteries are constructed using a lead acid technology. The following is a brief discussion of lead acid battery chemistry for a 12 volt battery. A 12V battery is made of six 2V cells in series. Each cell has positive and negative plates, with insulating "separators" between them. The plates are made by filling a lead gridwork with active material. The positive active material is lead dioxide, or PbO2, and the negative is "sponge lead", a porous lead mass providing lots of internal surface area.
- Simplification: H2SO4 (sulfuric acid) and water form an ionic solution, made of SO4 and H2 ions. When they meet the battery plates, the SO4 replaces the O2 on the positive (PbO2) plate, and they also replace an electron on the negative (Pb) plate, forming PbSO4 on each. The displaced electrons from the negative are the whole point to the battery's existence. The end result is an excess of electrons at the negative plate and thus a voltage with respect to the positive plate. The battery tries to achieve a chemical equilibrium where the degree of ionization balances the concentration of acid in the electrolyte (battery acid, or the mixture of Sulphuric acid and water in the battery). If it is "pushed" to a higher or lower voltage (i.e.; a higher or lower concentration of negative ions) the reaction will proceed in the correct direction to re-approach equilibrium (it will charge or discharge).
- If the electrons are allowed to travel (via an external load like a starter) to the positive plate, the SO4 from the acid replaces the O2 in the positive (PbO2) plate, forming PbSO4 (lead sulfate). The same thing happens on the negative plate, and an electron is replaced. The loss of the SO4s weakens the electrolyte. The remaining H2 ions in the electrolyte combine with the O2 ions which were displaced from the positive plate and form water, diluting the electrolyte further. Equilibrium will now occur at a lower concentration of ions, due to the lower concentration of H2SO4. You see this as a partially discharged battery having a lower voltage. When either plate has no more material which can be converted, or the acid is fully diluted, the battery is fully discharged.
Note: If the battery is left discharged too long, the lead sulphate will form large, hard crystals on the plates and will not be able to be forced back into the acid. These crystals are large enough to physically clog the pores in the plate surface. The whitish appearance of plates is this permanent PbSO4 which seals the plate surface off from the electrolyte, rendering it useless.
- During charging, the higher external voltage (from the charging system) forces electrons into the battery in the opposite direction, reversing the reaction. After all the SO4s have been forced back into the acid (battery is fully charged now), any further current electrolyzes the water in the electrolyte into hydrogen and oxygen, visible as bubbles rising to the surface. Batteries are not really fully charged until this happens. (Sealed batteries differ in this regard).
Caution! Lead acid batteries may vent hydrogen gas. This gas is a very explosive mixture! Safety glasses and protective clothing should be used when working with lead acid batteries.
