During this period the battery is also trickle charged to continue reducing the amount of lead sulfur in solution. ĭepending on the size of the battery, the desulfation process can take from 48 hours to weeks to complete. There are some websites which advertise "battery desulfators" running at megahertz frequencies. However, the electrochemical processes found in batteries have time constants on the order of seconds and will not be affected by megahertz frequencies. This process is sometimes called "ringing". These will resonate with each other, and something the size of a battery will usually resonate at a few megahertz. Īny metal structure, such as a battery, will have some parasitic inductance and some parasitic capacitance. Some battery conditioners use short pulses of high voltage, too short to cause significant heating, but long enough to reverse the crystallization process. Normally, running high voltage into a battery will cause it to rapidly heat and potentially cause thermal runaway, which may cause it to explode. The lead sulfate layer can be dissolved back into solution by applying much higher voltages. To the user, it appears that the battery is dying. In this case the battery charger indicates the charge cycle is complete, but the battery actually holds very little energy. As the charging process continues, such a battery will reach the charger's preset cut-off more rapidly, long before it has had time to accept a complete charge. As related by Ohm's law, current is the ratio of voltage to resistance, so a sulfated battery will have lower current flow. Ī sulfated battery has higher electrical resistance than an unsulfated battery of identical construction. Eventually the charger will turn off when the current drops below a pre-set threshold. As the battery fills, its internal voltage rises towards the fixed voltage being supplied to it, and the rate of current flow slows. In this mode the charger holds a steady voltage slightly above that of a full battery, in order to push current into the cells. Common to almost all chargers, including non-switched models, is the middle stage, normally known as "absorption". This is the reason batteries will be found to charge poorly or not at all if left in storage for a long period of time.Ĭonventional battery chargers use a one-, two-, or three-stage process to recharge the battery, with a switched-mode power supply including more stages in order to fill the battery more rapidly and completely. 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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