Using lead-acid batteries, we are commonly asked why, in certain circumstances, we'll recommend the use of a cyclic duty battery; why won't an ordinary car battery do, it's much cheaper?

It helps to have a basic understanding of the principles at work when answering this question. A battery is an array of lead plates immersed in an electrolyte; sulphuric acid. For a given amount of space, we can have a large number of thin plates, a small number of thick plates or various combinations in between.

From the simplified assumption that the battery electrical to chemical energy conversion takes place on the surface of the plates, it is fairly obvious that the larger the plate surface area, the greater the cumulative rate of reaction. This equates to the available current, hence a greater plate area will provide a higher maximum battery current – a large number of thin plates will have a greater surface area than a small number of thick plates.

During discharge, plate material is transported in solution between the plates, with the process being reversed on re-charge. The length of time for which discharge can be maintained depends on the total volume of available plate material; more material equates to more capacity.

To summarise, the ability of a battery to deliver high current is determined by its plate area, and the time for which discharge can be sustained, by its total plate volume. One aspect of battery design is optimising these two requirements whilst keeping the battery size & weight to a minimum & battery life to a maximum. With this basic understanding in mind, the differences between engine starting & cyclic battery design are easier to understand:

A starter battery intended for engine cranking is designed to give the highest possible current for a short time in the smallest possible volume at the lowest possible cost; so uses a large number of thin plates. The battery’s capacity is rarely used; the engine usually starts within 2 or 3 seconds – typically using less than 1 amp-hour. Once the alternator is spinning, it is normally able to support the vehicle loads and no further demands are made on the cranking battery. The problem with thin plates if the full capacity is repeatedly used, is that the material transported during charge and discharge does not re-deposit in exactly the same physical form each time, leading to gradual degeneration of the battery. Starting an engine uses such a small part of the actual amp-hour capacity that the plate degeneration mechanism is not relevant, but if the same battery is used for a different purpose, it can be very quickly worn out; just 50 full charge discharge cycles would not be unusual for such a battery.

A cyclic battery has a different internal construction where the design is optimised to maintain plate structure when cycled, which generally means thicker material. This results in a lower surface area (for a given size) hence a lower charge/discharge current capability. Cyclic batteries deliver from around 250 charge-discharge cycles, up to several thousand, before wearing out. Being a more complex construction & containing more lead, they consequently cost more to manufacture.

Much effort has gone into getting the best of both worlds, with different plate shapes and structures, special separators which allow more plates to be packed in, and starved electrolyte systems. Each variation is a compromise designed to extract maximum performance for a particular type of use. The key is to select the right type for any given application. The right battery may be more expensive, but it will last longer, turning out to be cheaper in the long run.

Rules of thumb are as follows:

For engine starting, a "Starting Lighting and Ignition" (SLI) battery is cost optimal, ie a standard “car battery”.

For light loads where the battery is being continually charged & discharged, consider a "leisure battery". Many such batteries are rated for a maximum charge or discharge current of C/10 – this is because it uses thick plates to get long life, but this results in a small active surface area. Flooded type leisure batteries may require frequent “topping up”, as they do not necessarily have a built-in reservoir of electrolyte like some “maintenance free” starter batteries.

Sealed batteries offer maintenance free performance by employing a construction which recombines any released gases back into water, & for this reason are also known as recombinant types. These batteries are not actually sealed, but employ a low pressure relief seal which prevents spillage. They are ideally suited to cyclic applications & their construction offers both high current and long life. There are two main designs which differ in the way in which the liquid electrolyte is held captive within each cell; these are the AGM (absorbed glass mat) & GEL (gelled electrolyte) constructions. Each has advantages & drawbacks, & selection is determined by the application. A simplified comparison follows:

This is only a brief and simplified overview to introduce the concepts, if you need any help with selecting batteries or designing a power system please give one of our engineers a call or email us.