Lead Acid Batteries

[Karen_Agusta]

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If you ever wondered what you battery actually does

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Lead-acid batteries, invented in 1859 by Gaston Planté, are the oldest type of rechargeable battery. Sulfation is one of the problems that kill batteries. Lead-sulfate is created when a battery is discharged. Then, when charged, in theory, the lead sulfate changes back to its component materials; lead, lead dioxide, and sulfuric acid.

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Thus as batteries age, hard lead-sulfate forms into crystals on the surface of the electrode plates . Heat build-up from sulfation and corrosion destroys the battery’s grid. It is the grid that holds the material that produces the chemical reaction that stores and releases electricity ... thus not only will you see a white build up ... you shall also see warpage on the grids.

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Your 12V Battery is actually 13.2 V ... it is only called a twelve volt battery. The issue presented to us here is the fact that the compounds that actually hold the charge are somewhat weak and brittle. They embed themselves amongst the mesh of the battery plates. This you can see in the form of a while powdery form of corrosion between the plates. You can at times shake at which point the calcium shall fall off and settle on the bottom.

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When the breaking away of the compounds and settling happens, one of 2 things happen. Either you end up shorting the plates with the build up on the bottom (the material "bridges" the gap) - a given cell in your battery then dies and therefore one of the 2.2 volt cells in your 13.2 volt battery is gone making it and 11 volt battery.

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Essentially, a lead-acid battery is a series of cells, or grids of both high and low antimony. This is due to the fact that a pure lead grid structure is not strong enough by iteself to stand vertically while supporting the active material and therefore other metals are alloyed with the lead for added strenght which also improves the electircal properties. The alloys added with either be antimony in a deep cycle battery or leaded-calcium batteries in which tin is added to the leaded-calcium grids to strengthen them.

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The negative plate is filled with grey oxide incorporating red lead oxide Barium Sulfate, and carbon black, while the positive plate is filled with grey oxide incorporating red lead oxide or litharge. Both plates are immersed into an electrolytic solution of diluted sulfuric acid. The red lead oxide plate is the positive pole and grey lead plate is the negative pole. In each cell, the anode is porous metallic lead and the cathode is made of lead dioxide. As a battery discharges, completing the circuit, electrons are released from the anode and the resulting Pb 2 + ions immediately react with SO 4 2 - ions precipitating out insoluble lead sulfate on the surface of the electrode. At the cathode, electrons from the external circuit reduce PbO 2 to water and Pb + ions, which also immediately react with sulfate ions to precipitate PbSO 4 on the electrode. The reactions are:

Anode: Pb(s)+SO 4 2 -PbSO 4(s)+2e-

Cathode: PbO 2 (s)+SO 4 2 - +4H + +2e-PbSO 4 (s)+2H 2 O(e)

Left Side Total: Pb(s)+PbO 2 +2SO 4 2 - (aq)+4H +

Both the cathode and the anode are largely converted to PbSO 4 (s) when the battery is fully discharged. By applying an opposite voltage to one cell, a reverse chemical reaction occurs and that cell will recharge. .

Lead-acid batteries undergo several charge/discharge/recharge cycles before they become inoperable and are recycled or enter the toxic waste stream. The reason for most batteries failing to operate properly is due to this electrochemical process, battery flaking of the PbSO 4 , or from an internal short circuit.

As the battery discharges, the positive plate reacts with the sulfuric acid to produce lead ions and water. The negative plate simply dissolves to form lead ions in the process, which finally leads to the battery's complete failure. While charging, the positive plate builds up a thicker coating of lead oxide, migrating water, and lead ions from the sulfuric acid. As it does so, the negative plate bubbles and releases hydrogen from the sulfuric acid as it builds up a crust of lead.

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It is the lead ions formed in the discharge cycle which cause problems. They agglomerate and fuse together with sulfate ions in sulfuric acid to form a highly insoluble lead sulfate. When this sulfate sediment builds a passive sediment layer on the electrodes, it reduces porous separator ionic conductivity by pore blocking and will eventually lead to cell shorting or self-discharge. The battery may well be serviceable in every other way, only the sulphation or calcification will stop the battery from operating.

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And those are a few notes on what your battery is doing.

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Rubber Side Down.

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Karen