Battery lab testing and the limits of the datasheet: Page 2 of 3

September 09, 2013 //By Achim Loesch
Battery lab testing and the limits of the datasheet
Achim Loesch of Varta Microbattery explains how the engineer can infer a certain amount of information about the expected performance of a battery system in a real application from specifications contained in the cell’s datasheet.
– the number of charge/discharge cycles that the cell can sustain – is also shown, but again at a single constant-current output and at a constant 25°C.

These data are useful for comparing one cell with another, and making a relative judgement about different cells’ performance. For instance, if the designer judges that discharge capacity is an important parameter in an application that will normally operate at an average ambient temperature of 5°C, a comparison showing that cell A has a greater discharge capacity than cell B at both 0°C and 25°C is useful, and suggests that cell A will probably also have a greater capacity at 5°C.

This kind of relative judgement is important in component selection. But to know how the selected cell will actually perform in an end product, absolute performance data are required – and here, the designer runs up against the limits of the datasheet. True, by applying common sense the engineer can extrapolate a certain amount from the datasheet specifications. Such extrapolation is not, however, normally supported by de-rating models supplied by the cell manufacturer.

It is impossible in any case to extrapolate from datasheet specifications

for usage conditions that vary markedly from the datasheet conditions. And in practice, this will apply to many different kinds of end product.

For example, a pedelec (electrically assisted bicycle) typically has a discharge profile that is completely different from the datasheet’s neat, constant-current output. On a trip through hilly terrain, the rider might draw the peak power output when climbing a hill, then switch off the electric motor as the pedelec freewheels downhill, in a repeating pattern of high-discharge/zero-discharge episodes.

In many cases, the temperature inside the battery pack’s housing will be much higher than 25°C, as high currents flow through the power circuit and generate waste heat.

Equally, however, the pedelec might also need to be rated for operation in cold northern climates in which operation

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