Unlike many other kinds of electronic component, a battery pack is highly sensitive to changes in operating conditions. In most cases, the battery pecifications that design engineers pay close attention to include cycle life, capacity and peak power output. The values of all these parameters vary with variations in operating conditions such as ambient temperature, discharge profile and charging rate.
In many cases the actual usage conditions will deviate substantially from the ideal conditions specified in a cell datasheet. This means that, if a design engineer needs to be certain about the way a battery pack will perform in the field, additional methods of measuring or calculating expected performance will be required.
The value of the datasheet
When designing or specifying a battery pack, components such as the charging circuit, protection circuit and housing all have a measurable effect on the performance of the system. But the primary limiting factor on the electrical performance of the battery pack is the performance of the cell or cells. This is why the datasheet of a cell is – or at least should be – an important resource for the battery pack designer.
In practice, however, a typical cell datasheet tells the engineer about the performance of the cell only under a specific set of conditions. Let us take as an example a widely used product, the NCR18650, a cylindrical lithium-ion cell supplied by Panasonic. The product’s datasheet (dated February 2010) specifies
a minimum nominal capacity of 2,750mAh.
This specification, however, only applies at a constant discharge current of 550mA and at a temperature of 25°C. The same datasheet provides discharge curves, showing the voltage and total power output at three discharge rates – but again at constant current and 25°C. The effect of variations in temperature
on output voltage and discharge capacity is shown in a separate graph, but now only at a single – again constant – output current. Cycle life