COVER STORY
Specifying a battery – the five key questions
As demand for batteries has grown across industrial and consumer applications, battery manufacturers have responded with a wealth of new products, including completely new chemistries. Whilst on the whole this is a good thing, there are now so many battery chemistries available including the associated options for intelligent battery management systems (BMS) and protection circuits, specifying the right battery for an application can give engineers a real headache.
To help, Brian Newby, technical marketing engineer at Anglia, has set out the five key questions that you should be asking yourself when choosing a battery, leading to a list of the eleven technical parameters you will need to specify.
fully charged Li-ion battery will have a nominal voltage of around 4.2V dropping down to as low as 2.5V when fully discharged. The specification of the BMS needs to take this into account during the design phase.
What are the size, weight and form factor constraints?
Rechargeable or not?
One of the first considerations will be does the application need a rechargeable (secondary) battery or can it be powered from a non-rechargeable (primary) battery? The answer to this question is very application dependant. For example, a sensor node which is installed in a remote location with no fixed power source or convenient access to power for regular recharging, the most viable option will be a primary battery of a suitable capacity to provide the required run time. However, it is not always that simple, taking the same application but where the sensor unit now has some form of energy harvesting integrated, such as a photovoltaic solar element, then a secondary
battery could be more suitable and allow for a much longer run time and reduced servicing cycles.
What is the demand profile?
Some applications demand high or peak current drain for short durations (High Power) such as the operating conditions of a power tool. Others have a lower current drain over longer durations (High Energy) such as the profile of a portable measuring instrument. This will also influence the type and chemistry of battery specified. For example, there are Li-ion cells which are designed specifically for Power or Energy drain applications, this is also true with other chemistries. It is particularly important to specify the correct type as not all cells are the same.
How much energy is required? The third question to consider is how long does the equipment need to run for before the batteries are replaced or recharged? At this point you will usually define what output voltage and current you need from the battery pack to power the device. Before you can fully answer this you need to know the application power consumption, its low power state or sleep cycles and the expected run time. Once known the watt hour (Wh) capacity required can be calculated. Designers should also note the output voltage of the battery will change depending on its state of charge (SoC). For example, a
8 April 2022 Components in Electronics
www.cieonline.co.uk
It is not uncommon to see a product design where there is not enough space available for a battery of suitable capacity. This leads to compromises on the run time or cycle life of the equipment later down the line. It is so important to think about the battery early in the design process to avoid this issue, as it will have a fundamental impact on the user’s experience of the product. The key parameter here is battery energy density. Energy density is the measure of a battery’s watt-hours divided by its weight or volume. Higher is always best. Selecting a battery with higher energy density can provide a solution where space is constrained. Most battery chemistries in common use today are considered to be mature
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