Battery Technology
A designer’s guide to building smart batteries into electronic applications
Rob Phillips guides design engineers through the process of embedding batteries in medical, military or automation applications
W
hen integrating a rechargeable battery into a professional application, especially into a
mission critical medical or military device, the first things that come to mind are reliability and safety. However, there are many other factors designers need to consider throughout the various stages of custom battery design.
Many design engineers believe that, as its power source, the battery is the very heart of the product. However, I would go even deeper into the core of the battery and argue that it is the cell that is the nucleus of the battery, determining the performance, lifecycle and durability of the application.
life or a specific battery size. In other words, finding out exactly what you want the battery to do for your piece of equipment is crucial to the success of the design.
Cell selection As a result, the first thing the OEM and battery developer should consider in each case is cell selection. There are always a number of stakeholders in any OEM device design and it is important for everyone to be involved early in the development cycle, if the optimal battery solution is to be created. To select the most appropriate cell types one first needs to determine the power consumption of the application, the runtime requirements and environmental
have dominated product development due to their high energy density and excellent safety record, but the older Nickel chemistries do still offer superior performance in certain applications. One fundamental guideline is to decide whether the battery has to withstand extreme conditions such as freezing temperatures, scorching heats, humidity or dirt. In order to guarantee efficiency, we select only the type of cells that are suitable to these conditions. In these instances, the electronics need to be programmed differently to cope with the various environments, whilst the physical characteristics of the battery need to be robust enough for the given extremes. Intelligent cell selection can also help
ensure the future availability of the battery, which means the cell can be replaced if a superior version becomes available. This ensures a longer life-cycle for your battery and helps alleviate obsolescence related issues.
Electronics
The design of the electronics embedded within the system is another key issue for the OEM and its battery partner to address. The battery itself is an integral part of something that Accutronics refers to as the power management ‘triangle’. This consists of the actual device on one side and the battery and the charger on each of the other two sides. One might define a good power management system as these three elements working together in harmony.
There is no such thing as a typical battery design project. This is partly because bespoke power sources are used everywhere from hospitals to manufacturing plants and military operations. Some customers might be looking for high energy capacity, whilst others might need low temperature performance, high availability, a longer cycle
16 April 2012
operating conditions. Feeding into this is the weight and volume budget and a requirement for the battery to meet specific cost targets.
The battery developer has a number of cell types available to them including Nickel Cadmium, Nickel-Metal Hydride, Lithium ion and Lithium ion Polymer. In recent years it is the Lithium ion chemistries that
Components in Electronics
Needless to say, every portable device in the consumer world, from satellite navigation units and laptops, to media players and mobile phones, has some sort of electronic power management system. The same applies in the specialist worlds of medical, military and industrial electronic device design. The difference is that in these sectors the operation of the battery is often mission critical. The best way to ensure that the power management triangle principle is observed, is by using a common communication system – such as the Smart Battery System (SBS) – to determine the methods of communication between smart batteries, smart chargers and system devices.
A battery that has been designed to meet the SBS standards, controls how it’s going to be charged by communicating with a smart charger and requesting the voltage and current it needs. This is the safest and most efficient way of charging the battery because it is always the battery itself that remains in control rather than having a charge regime imposed upon it. The physical characteristics of a battery, such as its size, durability and weight, also play a crucial part in its design. Similarly, performance characteristics such as efficiency, reliability and availability are also crucial. For instance, portable medical devices might require a high tolerance to vibration, so that they can be used while transporting patients by helicopter. Achieving this is a matter of managing the physical and performance characteristics simultaneously, which is no mean feat. In military products, such as night vision goggles or rugged portable computers, which are often used in extreme environmental surroundings, the battery needs to be able to cope with those conditions and also be small and lightweight for ease of carriage. For instance, it might need to be watertight or feature a sufficient level of ingress protection to keep it secure against sand and dirt.
Material selection
Material choice is crucial in ensuring these requirements are met. Magnesium for instance is very low weight, although expensive. If cost is a significant factor, which it very often is, a designer might instead work on the general shape and size of the battery and insulation techniques to help meet the client’s expectations.
Another important factor to consider, and one that applies to the integration of all sub-assembled components, is the availability of parts and their potential obsolescence. For this reason, engineers should avoid designing-in off the shelf consumer batteries. Our experience has shown that these are poorly supported technically and very likely to become obsolete quickly.
For example, if you are manufacturing a portable satellite navigation system for
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