Buyers guide to batteries

If power requirement is low, consider coin cells In this article, Newark looks at battery purchasing considerations, plus factors impacting the shipping of end devices.

Batteries vary by cell size and chemistry and are classified as primary or secondary. Primary batteries are fully charged when new but cannot be re-charged after they have discharged. Secondary batteries are rechargeable and include nickel metal-hydride, nickel cadmium and lithium ion.

Selecting the right battery requires weighing up many factors as outlined below.

Capacity: The current drain of the device will determine the chemistry. Alkaline batteries last longer than zinc, while lithium last longer than alkaline in high drain applications.

Voltage: electrical equipment is designed to function with a particular voltage and different battery types give different voltages. Once your design is complete you cannot change the voltage.

Discharge rate: Some types of battery discharge at a higher rate than others. Lead acid batteries can give very high current for short periods, whereas alkaline batteries are designed for long life.

Operating temperature: Temperature can dramatically impact battery performance. Lithium-based batteries might operate up to -40°C but the performance might drop. Likewise, operating some types of battery at too high a temperature can be dangerous.

Size: Common consumer sizes are AA, AAA and 9V, suitable for portable devices. Pouch format lithium batteries suit tablets and mobile phones. Packs of cells in series or parallel are used for laptops, e-bikes and power tools. Packs can be built to suit different application, voltage and amp/hour requirements. If power requirement is low, compact, low cost coin cells can be considered.

Cost: In most portable products the battery will be among the most expensive items on the BoM. Cost increases with performance. Things to consider include drain rate, whether the device needs to be rechargeable and whether the battery will be replaced by the consumer? In backup applications a high cost battery with longer life chemistry (such as Lithium thionyl chloride found in batteries from Saft, Eve and Tadiran) may be more economical by requiring less frequent maintenance.

Shelf life: Some batteries, such as lithium, have extremely low self-discharge rates, while some nickel metal hydride batteries can lose up to four per cent of their capacity every day. Shelf life is primarily a factor in primary batteries as the secondary batteries can be recharged.

For secondary batteries, their charge/discharged capability depends on their type and whether they are trickle charged or experience full discharge before re-charging. Batteries usually last much longer when used on a trickle charge.

How long a battery lasts depends on the chemistry and cell. It is normal for batteries of the same chemistry and cell from different manufacturers to perform similarly in application. The properties of cell chemistries and end device current requirements are more relevant in determining how well a battery will perform.

Newer chemistries containing lithium are subject to additional shipping requirements especially when using air freight. ICAO/IATA regulations governing the shipping of lithium batteries are strict and enforced rigorously. Compliance is the legal responsibility of the shipper and can require dangerous goods account approval.