Battery design
With especial emphasis on the “Porvic” separator
When a motorist expounds on the mechanical excellence of his new vehicle he never mentions the battery, although he is quick to curse it when it fails him. If the majority of enthusiasts realised how complex is the assembly lying within that uninspiring black box they might be prompted to take a little more care of it. The purpose of this article is to discuss battery design in general and, in particular, the “Porvic” separator as used in Dagenite batteries, which are supplied as original equipment for Rolls-Royce cars.
Firstly, there are the plates, which consist of intricate skeleton castings in an antimonial lead alloy filled with the active materials in the form of a paste. Apart from the mechanical design of the grids, the purity and composition of the alloy is of great importance, whilst the pastes, positive and negative, must be made from carefully selected raw materials by complex methods which have been developed and proved by research over the last 80 years. The aim of the conscientious manufacturer is to improve both the performance and the life of the battery, and the methods which must be used to arrive at both these ends are extraordinarily subtle and complex.
Good plates, however, are of little avail without good separators. Again, many motorists probably have little idea what a battery separator is or what it does, but this particular component is so vital to the performance and life of the battery that money has been poured into development all over the world to produce separators having a long life and good performance.
The separators in a battery have to carry out a number of functions. They have to keep the plates apart so that there can be no electrical contact between positive and negative plates except through the acid. At the same time they have to act as a porous reservoir for acid with as many minute acid paths as possible traversing their thickness so that they can carry the heavy starting currents without undue resistance, which would waste power uselessly in heating the battery. Then they must be mechanically strong and resilient, and they must not lose their strength or become eroded in service although they are continually immersed in strong acid and subjected to intense oxidation during the charging of the battery.
The traditional separator of the past, which was still in the majority up to World War ll, was made from wood – not just any wood – one particular type of wood known as as Post Oxford Cedar was universally conceded to be the best. The wood was chemically treated to remove most of the resinous material, thus increasing the porosity, but it was then very weak and it could not be dried without shrinking, buckling and splitting. Consequently it had to remain wet thereafter, which meant that an assembled battery could be stored only for a limited period before it was filled with acid or otherwise undesirable changes would occur in the plates or the separators.
In service, although capable of a high level of performance, the wood separator had a limited life since it was gradually eaten away by oxidation, apart from being inherently mechanically weak and liable to split. As a result it was almost inevitably the first component of the battery to become unserviceable, breaking down, perforating, and allowing active-material to bridge across from positive to negative plates, to cause an electrical short which prevented the battery holding a charge.
These deficiencies of the wood separator led many people to search for something better and the quest had proceeded for many years when the advent of new thermoplastic materials helped Pritchett and Gold, makers of Dagenite batteries, to find a successful solution in the later years of the war.
Today, at Dagenham, Pritchett and Gold have a large and modern “factory within a factory” turning out more than a hundred million “Porvic” battery separators each year. The “Porvic” separator is made from unplasticised Polyvinyl Chloride, and it has a performance equal to the best wood separator whilst possessing superior mechanical properties and virtually unlimited life. It is “microporous,” that is, it is not visibly porous, although in fact about 87% of its volume consists of minute cavities each communicating by ports with all the adjoioing cavities. The microporosity is given to the Polyvinyl Chloride (P.V.C. for short) by an ingenious process which is fully patented. Starch is used as a temporary filler to produce these pores because in starch nature has obligingly provided a range of products having uniform and fine granules. Pritchett and Gold use maize starch to the tune of about 60 tons a week. The starch is first “flash dried” in a pneumatic dryer in a current of hot air and it is then incorporated into a plastic dough with P.V.C. and an organic solvent using a large battery of steam-heated dough-mixers. The dough is next extruded from powerful hydraulic presses designed and made by Pritchett and Gold, the moving blanket of dough passing through rolls, where it is given the ribbed profile necessary in the final separator. (Actually the strip is extruded and rolled two separators wide.) It is then festooned on racks before traversing tunnel ovens 80 ft. long where the solvent is extracted and taken away for recovery in a stream of hot air. The solvent recovery plant, vitally necessary for the economics of the manufacture, is a mass of air coolers, absorption columns, stripping columns, fractionating columns, heat exchangers and condensers.
After the solvent has been removed the strip is hard and brittle. It proceeds to a wet treatment plant which is an inferno of tanks of boiling water and boiling sulphuric acid. Here the strip is first swollen to a greatly enlarged size, an essential part of the process, and then the starch in it is converted by the boiling acid into sugar, which dissolves away leaving the cavities the starch granules occupied, the strip shrinking back to its original dimensions. Incidentally, the sugar solution is not wasted but is sold for further chemical manufactures.
All is not finished yet. The strip is cut into pieces about a yard long and carried on a stainless steel belt through a tunnel filled with live steam. This is to relax mechanical strains and to make it thermally stable. It is then impregnated with a special wetting agent to give it the very necessary property of acid absorbency, and, when it has been passed by the laboratory, it is dried in large conveyorised ovens before passing on to the somewhat involved cutting operationsc where the hundreds of different sizes of separator are produced. Finally it is given a stringent visual inspection before passing out for use.
Truly, few motorists know that the separator is such a critical component of the car battery and fewer still have any suspicion that battery separators are the products of such a complex and unique process.