When 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. Rob Phillips, managing director of Accutronics, guides design engineers through the process of embedding batteries in medical, military or automation applications.
Many design engineers believe that, as its power source, the battery is the very heart of the product. I, however, 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.
There is no such thing as a typical battery design project, partly because bespoke power sources are used everywhere – from hospitals to manufacturing plants and military operations. While some customers might be looking for high energy capacity, others might need low temperature performance, high availability, a longer cycle life or a specific battery size. Therefore finding out exactly what you want the battery to do for your piece of equipment is crucial to the success of the design.
Considerations
As a result, the first thing the OEM and battery developer should consider 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 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 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 – and to guarantee efficiency we select only the type of cells that are suitable for such conditions. In these instances, the electronics need to be programmed differently to cope with the environment, whilst the physical characteristics of the battery also need to be robust enough.
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 battery life-cycle and helps alleviate obsolescence related issues.
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’, which 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.
Battery characteristics
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; as do performance characteristics such as efficiency, reliability and availability. 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. It might also need to be watertight or feature a sufficient level of ingress protection to keep it secure against sand and dirt.
Material choice is crucial in ensuring that these requirements are met. Magnesium, for instance, is very low weight, although expensive. If cost is a significant factor, 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 likely to become obsolete quickly.
For example, if you are manufacturing a portable satellite navigation system for military use, you may only make a couple of hundred units a year. If, to help cut costs, you opt for a battery intended for a consumer DVD player, you put yourself in very serious danger of being sent a last order notice from your supplier sooner than you might expect! If the original battery manufacturer stops making the product, you will be faced with a situation in which the only way you can get spare batteries is by reverse engineering your own!
This is a particular issue for medical and military products, which usually have a long life cycle, frequently spanning more than a decade. Because of this, a good cell manufacturer will always work closely with the customer, and possibly the end user, to design cells that have a clear road map to guide the OEM through the future availability of the product. This means that even if new technology renders the original plan obsolete, the manufacturer would still be able to provide the battery or an exact equivalent, and it will still work to the customer’s specification.
Choosing a customised industrial or professional battery, designed with your application in mind, guarantees you a lifetime of support.
Certification
Once a battery prototype has been developed, the final stage is to certify that product for use, from both safety and performance perspectives, and Accutronics normally manages the entire approval process on behalf of its customers. Each battery design undergoes the appropriate safety testing from, for example, the IEC (International Electrotechnical Commission), the UL (Underwriters Laboratories), as well as the IATA (International Air Transportation Authority) and military standards testing from MIL (The US Department of Defence Test Method Standard for Environmental Engineering Considerations and Laboratory Tests), if relevant.
At the heart of devicesComing back to my original hypotheses, it becomes clear why the battery is the very heart of many military, medical and industrial electronic devices. We might also argue that it is the quality of the design process which ensures the heart will continue to
beat during the lifespan of the OEM’s device. As long as this happens and the battery provides power in an efficient and precisely monitored way, the device itself will do the rest.
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