Vital but heavy, and liability in an accident, old-fashioned batteries simply do not pass motor racing's acid test. But then some bright spark in the aerospace industry solved the problem - with Kitty Litter! Keith Howard explains
A recurring theme of this series is that new comp introduced into motorsport often have their origins in the aircraft industry. This is not surprising: aircraft components and technologies components have to be lightweight, yet capable of demonstrating excellent reliability under operating conditions where they are exposed to high levels of vibration and extremes of temperature. Race car requirements are much the same.
In the case of the Varley Green Top battery, still more similarities came into play. Conventional lead-acid batteries are bad news in aircraft because if you turn them upside down they leak sulphuric acid. For the same reason, they're not very welcome in a racing car because of what might happen in a shunt, acid burns being every bit as nasty as those caused by fire. Aircraft — piston or jet — also need a battery that can produce high short-term currents for engine starting, another requirement mirrored in many motorsport applications. There was an inevitability about aircraft battery technology making the transition to the racetrack, which it eventually did in the form of the famous Green Top, adapted from Vadey's own aircraft products. Varley, a British company owned by Berec (the British Ever Ready Company), had begun making aircraft batteries during WWII. Its design combined two important features.
First, it used a larger number of thinner plates than a conventional lead-acid battery, which gave it improved cranking performance for the weight. As a rule of thumb, cranking performance was typically two to two-and-a-half times better than that of a conventional battery with the same capacity.
Second, the Varley design immobilised the liquid electrolyte using plate separators that contained fuller's earth. Plates and separators were compressed together to form a solid but porous block, into which the sulphuric acid electrolyte was absorbed. It would eventually leak out if you inverted the battery for a protracted period, but it was vastly safer than the conventional alternative. For readers who've only encountered fuller's earth in cat litter, a few words of explanation.
A natural deposit, it comprises mostly hydrated aluminium silicates plus metal ions such as magnesium, sodium and calcium, locked within their structure. Originally used by textile workers (fullers) to clean wool fleeces, whence it gets its name, it is widely employed in industry because of its high absorbency for oil and grease. Unlike clay, which it otherwise resembles, it crumbles rather than forming a plastic solid when exposed to water.
So to use it to absorb battery acid, it has to be supported somehow — a technology Varley mastered in its aircraft batteries. When exactly the Green Top was first offered for sale to the motor-racing community I haven't been able to determine, but it was certainly in use in F1 by the early '60s. The Lotus 25, for example, used a Varley and exploited the chance to place the battery wherever was most convenient for weight distribution, access or packaging. Ahead of the driver's seat, under his knees, became a standard position — unthinkable with a battery that might leak in a roll-over.
Modem F1 cars don't have on-board engine start, of course, but the Varley Green Top enjoyed a long career in the premier formula before off-board starting became the norm. Ferrari, which resisted the change longer than any other team, was still using a Green Top to fire up the 14's flat-12 in 1979. Ironically, the Varley bowed out of F1 just before the major technical change that gave birth to the Red Top, an even safer battery. In the early 1980s, Varley was bought by the American battery manufacturer Gates Energy Products. Gates had invented the truly sealed lead-acid battery, and it quickly melded its know-how with Varley's to produce the SBS — sealed battery system — launched in 1983. Previous lead-acid batteries had had to incorporate vents in their casing because of what happens when they are overcharged. Once the battery can accept no more charge, the charging current begins to electrolyse the water within the electrolyte, producing hydrogen and oxygen gas. To prevent a dangerous build-up of pressure, these have to be vented to the outside.
This is why old types of leadacid battery had to be topped up occasionally, because gassing reduced the volume of electrolyte. Even the Green Top could be topped up, although it was a somewhat arcane procedure: extra electrolyte was added, the battery was allowed to stand while the fuller's absorbed it, and any excess was carefully poured off. What Gates had developed was a means of remixing the hydrogen and oxygen within the battery to reform water, a process known as gas recombination. Again, it was the separators between the plates which were the key to achieving this. Oxygen is evolved from the positive plate before hydrogen is produced from the negative plate, so the essence of the technique is to transport the oxygen across to the negative plate before hydrogen is released, the recombination taking place within the spongy lead on the plate surface. In practice, a recombination efficiency of greater than 95 per cent is achievable this way, which means the battery case can be sealed, save for a pressure relief valve that limits overpressure.
Gates' neat AGM (absorptive glass mat) separator did this by using thin borosilicate glass fibres. As well as absorbing the electrolyte to their surface — mimicking the function of the fuller's earth in the original Varley design — these also provided a path for the transport of oxygen. Gates was taken over by Hawker, the world's est manufacturer of industrial batteries, in 1993. The Varley name is no longer used but the battery lives on, under the name of Red Top, manufactured by Hawker Energy Products for DMS Technologies in Hampshire. Today's product range is wider than ever, encompassing capacities from 8 to 51 amphours, and the batteries are still used in high-level competition. Prodrive and Ralliart, for instance, fit them in their Subaru and Mitsubishi WRC cars.