Specifying a battery – the five key questions

Question 1: 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. 

Question 2: 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.

Question 3: 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 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.

Question 4: What are the size, weight and form factor constraints?

                         

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 batterieswatt-hours divided by its weight or volume. Higher is always best. Selecting a battery with higher energy density can provide a solution wherespace is constrained. Most battery chemistries in common use today are considered to be mature meaning it is unlikely they will dramatically increase in energy density as subsequent generations are released. You cannot rely on these next generation cells to come to the rescue. If size and weight are a critical factor in your design then make sure you are thinking about this early in the design process. As a rule of thumb, I would recommend Li-ion which provides one of the highest Wh/Kg energy densities.

When specifying the space required in the equipment for the battery further considerations may be needed depending on the chemistry and battery housing used. For example, a Li-ion pack based on a cylindrical metal case cell, such as the common 18650, has a rigid case which should not swell or deform over the lifetime of the battery under normal conditions. The same Li-ion chemistry but packaged in a soft polymer pouch can expand over the life of the battery due to the aging process that results in the creation of gas referred to as outgassing. This needs to be carefully accounted for to ensure it does not cause the casing of the equipment to swell, or even worse the cell could rupture on a sharp surface inside the enclosure as it expands.

Removable, embedded and any shape, size or form factor can be accommodated when designing a battery pack, however we would always recommend specifying commonly available prismatic or cylindrical cells. This will help keep costs within defined budgets and prevent possible issues with availability that can occur when using less common cells. Lead-wires with and without connectors or surface mount spring contacts can be specified on the pack design to suit the application too, there are some common connectors and contacts that we can advise on the best options for.

Question 5: Which safety approvals are needed?

The approvals required for a battery pack will depend on where it will be deployed. Each geographical region has its own set of regulations, and specific safety critical applications such as medical have additional requirements. It is important to understand these requirements at the outset so they can be factored in. Approvals can have a significant impact on the overall cost of the pack.

In addition to region specific regulations and safety approvals, nearly all lithium-based batteries are required to pass UN 38.3 (UN Transportation Testing) to ensure they are safe for transportation. This is because Lithium batteries have a high energy density which means they can be susceptible to overheating and become a fire hazard if not designed or handled correctly. Lithium or Li-ion batteries are generally considered safe when designed, manufactured and used properly. However, if the batteries use low-quality materials, are poorly assembled, used or charged incorrectly, damaged, or have design defects, they can pose a serious fire risk.

Any significant change to the battery pack which could affect safety such as the cells or ICs used in the BMS or protection circuit after the product has been released to the market may mean certification has to be performed again leading to significant additional costs. Whilst it would be impossible to guarantee a cell or IC will not go end of life during the life of a product, by following careful design considerations and partnering with Anglia who can offer support at all stages of the design, a lot of these risk factors can be mitigated.

Conclusion: the technical specifications

By now you should have enough information to draw up the technical specifications of the battery. Some of these will be dependent on the application whilst others will be defined by the environment. Here is a list of the typical parameters you will need to specify.

 1.      Battery type: Rechargeable or non-rechargeable and chemistry

 2.      Nominal rating: Voltage and Current

 3.      Nominal capacity: Specified in mAh

 4.      Discharge Current: Standard and Peak

 5.      Charge Current: Standard and fast charge and the charging method

 6.      Operating and Storage conditions: Min and max ambient operating and storage temperature and humidity conditions.

 7.      Mechanical Properties: Dimensions & weight

 8.      Expected cycle life: Number of times the battery pack can be discharged and recharged

 9.      Cell protection: As a minimum over charge/discharge detection, overcurrent and short circuit detection plus any specific requirements

 10.     Environmental protection: Hard or soft housing, waterproof and/or impact resistance             

 11.     Approvals: For specific regions or applications (i.e., Medical)

 

Design support

Anglia offers support for customer designs with free evaluation kits, demonstration boards and samples via the EZYsample service which is available to all registered Anglia Live account customers.

Anglia have a wealth of experience specifying and designing battery packs across a wide range of industries and applications, they can offer advice and support at cell, BMS and protection circuit system level and can provide a bespoke battery pack design service. This expertise is available to assist customers with all aspects of the product design and component selection, providing direct support and access to additional comprehensive resources including technical application notes and reference designs from our partner suppliers.

 Visit www.anglia-live.com to see the full range of products available from Anglia.