From then on the film will develop and thicken. If a battery is left disconnected, any internal charge will drain away slowly and eventually reach the critical point. īatteries also have a small amount of internal resistance that will discharge the battery even when it is disconnected. There are numerous other conditions that can lead to the same problem developing. This process of 'sulfation' takes time, so it only has a chance to build to significant levels if the battery is repeatedly discharged below this critical level. If the battery is stored or repeatedly operated in this partially charged state for an extended period, the film will slowly crystallize into a solid. If the battery is immediately recharged, the film will dissolve back into the acid.

At this point, the lead sulfate will begin to precipitate out of solution onto the battery plates, forming a spongy film. For instance, a 12V battery with a 100 ampere hour (Ah) capacity will reach this density when 25 Ah (300 Wh) or more have been drawn from the battery. In common designs, it reaches a critical density when discharged to about 75% depth of discharge, or below. A typical 12V battery consists of six individual 'cells' wired together in a single box, producing 12.66V when fully charged.Īs a battery is discharged the density of lead sulfate in solution increases. Each complete reaction produces about 2.11V. The two chemical processes continue as long as an external circuit is available to allow the electrons to flow back into the positive plates, but reaches equilibrium quickly when the battery is disconnected from the circuit. Lead reacts with the acid by taking in two electrons, leaving it negative while also producing lead sulfate.

Lead oxide reacts with the sulfur and oxygen in the acid to give up an electron, leaving the plate positively charged and producing lead sulfate.

Conventional lead–acid batteries consist of a number of plates of lead and lead dioxide suspended in a cell filled with weak sulfuric acid.

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