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The principle of the lead acid cell can be demonstrated with simple sheet lead plates for the two electrodes. However such a construction would only produce around an amp for roughly postcard sized plates, and it would not produce such a current for more than a few minutes.

A plate consists of a rectangular lead plate alloyed with a little antimony to improve the mechanical characteristics. The plate is in fact a grid with rectangular holes in it, the lead forming thin walls to the holes. The holes are filled with a mixture of red lead and 33% dilute sulphuric acid (Different manufacturers have modified the mixture). The paste is pressed into the holes in the plates, which are slightly tapered on both sides to assist in retention of the paste. This paste remains porous and allows the acid to react with the lead inside the plate increasing the surface area many fold. At this stage the positive and negative plates are identical. Once dry the plates are then stacked together with suitable separators and inserted in the battery container. An odd number of plates are always used, with one more negative plate than positive. Each alternate plate is connected together. After the acid has been added to the cell, the cell is given its first forming charge. The positive plates gradually turn the chocolate brown color of Lead Dioxide, and the negative turn the slate gray of 'spongy' lead. Such a cell is ready to be used.

One of the problems with the plates in a lead-acid battery is that the plates change size as the battery charges and discharges, the plates increasing in size as the active material absorbs sulphate from the acid during discharge, and decreasing as they give up the sulphate during charging. This causes the plates to gradually shed the paste during their life. It is important that there is plenty of room underneath the plates to catch this shed material. If this material reaches the plates a shorted cell will occur.

The grid structure in both pasted and tubular plate batteries is made from a lead alloy. A pure lead grid structure is not strong enough by itself to stand vertically while supporting the active material. Other metals in small quantities are alloyed with lead for added strength and improved electrical properties. The most commonly alloyed metals are antimony, calcium, tin, and selenium.

The two most common alloys used today to harden the grid are antimony and calcium. Batteries with these types of grids are sometimes called “lead-antimony” and & “lead-calcium” batteries. Tin is added to lead-calcium grids to improve cyclability. The major differences between batteries with lead-antimony and lead-calcium grids are as follows:

Lead-antimony batteries can be deep cycled more times than lead-calcium batteries.

Flooded lead-antimony batteries require more frequent maintenance as they near end-of-life since they use an increasing amount of water and require periodic equalization charges.

Lead-calcium batteries have lower self-discharge rates as shown in the illustration below and therefore, will draw less current while on float charge than lead-antimony batteries.

Lead-calcium positive plates may grow in length and width because of grid oxidation at the grain boundaries. This oxidation is usually caused by long-term overcharging, which is common to UPS and other batteries on constant-float changing. Grids may grow in size sufficiently to cause buckling or rupture of their containers.


Another type of grid alloy is lead-selenium. In reality, this battery is actually a low lead-antimony grid with a slight amount of selenium. Lead-selenium has characteristics that fall somewhere between lead-calcium and lead-antimony.

When pure lead is mixed with an alloy there may be undesirable characteristics introduced in the performance of the battery.



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Last Updated: Monday, December 03, 2007 - 5:53 AM Eastern Time.