Workshop: Black Box

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Words Martin Pretorius

We might describe our offroaders as either petrol- or diesel powered, but that isn’t the whole truth: in reality, just about every passenger vehicle ever made is also powered by electricity. But, before you mutter that you don’t drive a hybrid, you need to remember that almost every engine does, to some extent, depend on a steady supply of power from the battery and the charging system.

We’re not even talking about electronically-controlled engines here, for even the most primitive diesel engine needs glow plugs to start, and old-school petrol engines need 12 volt on their ignition coils to work at all – and that’s discounting the convenience of the electric starter…

The modern era added a myriad of computers for everything from the instrument cluster, to the anti-lock braking system, to the list of electrical consumers; and a flood of aftermarket accessories such as portable fridges and powerful in-car entertainment systems are available for owners to burden the electrical systems of their vehicles even further.

At least today’s batteries are better than the old ones. While the basic principles of electrical storage batteries have remained unchanged over more than two centuries, modern design and materials have indeed improved their performance. Just as they did in their earliest years, batteries generate an electrical current by allowing a chemical reaction to create a voltage difference between their electrodes, but the chemicals involved in that reaction have been fine-tuned over the years to allow far greater efficiency than ever before, and new construction techniques enable modern batteries to contain far more energy in smaller packages.

Lead-acid batteries aren’t that simple anymore

While traditional lead-based batteries still dominate the automotive marketplace (excluding specialised applications such as some hybrids and fully electric vehicles), there are a number of different designs.

The oldest, simplest and least expensive type use “flooded” lead-acid cells, connected in series inside the battery casing – typically 6 cells, each delivering 2.1V for a total of 12.6V. These cells each have a lead anode (negative electrode) and a lead dioxide cathode (positive electrode) immersed in an electrolyte (in this case, sulphuric acid in about 38% concentration), and both electrodes react with the electrolyte.

At the negative electrode, the reaction releases electrons, while the reaction at the positive electrode consumes the electrons. This creates a potential difference (measured in volts) over the electrodes, and produces a current when the terminals are connected to a load.

There are two main problems with flooded-cell batteries, in addition to the obvious environmental issues related to the use of lead in anything. The first relates to the electrodes’ robustness and electrical efficiency: pure lead electrodes risk collapsing under their own weight because the base material is quite heavy, and their ability to take deep cycling is compromised as well.

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