Our resident lead-acid battery consultant, Mike McDonagh, unveils the mysteries of the curing process.
This series has been focused on the methods used to optimize the chemical and physical processes of converting the unreactive metal lead into a store and source of electrical energy. Most of the methods used have historic origins based on experience rather than scientific research. This is demonstrated perhaps more clearly by the proof the process of curing than any of the processes examined so far. As such it has been yet another source of variability and the cause of many mysterious ailments to afflict battery manufacturers over the years. However, as is the case with so much of the knowledge of lead acid technology, in depth research carried out over the last three decades has greatly enhanced our understanding of the chemical and physical processes which occur during this critical phase. Using this knowledge, it is possible to obtain reproducible, enhanced performance for all battery types and all market sectors
The process begins after the pasting and flash drying stages described in the previous article. The newly pasted plates are collected and stored before they are used, either in assembly of batteries for container formation, or are separately charged in vats for dry charged batteries. Although the surfaces of the plates are dry enough to stack after the flash drying process, the amount of residual moisture and unreacted or free lead left in the plates gives major problems in both the assembly and initial formation processes. A high free lead content increases the energy requirements for formation and the residual moisture can lead to mechanically soft active material as well as a variable water addition to a battery’s electrolyte.
However, it was found that plates left in stacks after flash drying would not only dry out but that they would also have lower free lead contents. The storage conditions which promoted this process were soon identified and the process of air curing was devised. In this, the residual moisture and heat content in the plates were identified as necessary parameters for the oxidation process. The humidity was maintained by using wet porous sacks placed on top of the pallet stacks. This had the dual purpose of maintaining sufficient moisture content to ensure completion of the lead oxidation and also to allow moisture to escape relatively slowly, due to the porosity of the cover.
The original purpose of curing was to reduce the free lead content of pastes manufactured with grey leady oxide. This was seen as a method of reducing the formation time due to the higher energy requirements of converting Pb to PbO2 in the positive, (see thermodynamic data in article 5). For once, in this whole sorry story of converting lead into a reactive substance, there was a beneficial consequence to the previous processes i.e. the residual moisture and heat content of the plates after pasting and flash drying.
Another benefit was the exothermic nature of the reaction, which serves the purpose of driving the reaction, speeding up the drying process and also providing a measure to indicate the completion of the free lead oxidation. For many years, the curing parameters were fixed by this natural process. Plates were stacked on pallets after flash drying in a manner which allowed air ingress. A wet sack was placed on top and these pallets were placed into rooms with a closed door for several days. The normal features were that stack temperatures reached around 55°C within a few hours, the oxidation reaction was completed within 15 to 20 hours and the drying portion would take around 36 – 48 hours. For most companies this was considered as a 72 hour process.
Kill or cure
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