Smaller rechargeable Li-ion button cells for wearables

Varta-CoinPower-CP7840

“The provision of energy in a tiny space is playing a growing role in an increasing number of aspects of modern mobility and health,” said the firm. “This is the case with hearables, which are worn on or in the ear. They not only facilitate hearing, but also offer a wide range of other functions, such as listening to music, hands-free telephony, voice amplification or simultaneous translation. As well as classic hearing aids, hearables include over-ear headphones and wireless telephony earbuds.”

The additions are:

  • CP0854 – 8mm dia, 5.4mm thick, 25mAh
  • CP9440 – 9.4mm dia, 4mm thick, 25mAh
  • CP7840 – 7.8mm dia, 4mm thick, 16mAh

Nominal operating voltage is 3.7V for all.

Varta-CoinPower-rangeCoinPower cells are said by the firm to have a ‘silicon-dominated’ anode, partly responsible for the capacity values

As well as hearables, application is foreseen in fitness wristbands, smart car keys and medical equipment.

“In this segment, we are benefiting from the shift in technology away from cylindrical battery cells to coin format batteries,” said Varta general manager Philipp Miehlich. “We have been supporting this change in technology for wireless headphones with our technology.”

A3 versions of certain CoinPower cells (CP0854 and CP9440 are specified) can be charged using three different safe profiles, including an unusual two-step procedure that is only applicable between 15°C and 45°C – the other two more conventional techniques can be used over 0°C to 45°C.

Dubbed ‘rapid charge’, a constant current of 2C (where C is a current equivalent to the amp-hour capacity of the cell) can be applied up to 4.0V terminal voltage, when charge current is dropped to 0.5C until 4.2V (all voltages within ±50mV). At this point ~93% of capacity has been filled in under 1hour 15mins. Further constant voltage charging tops-up the cell to 100% in a total of 1hr 40mins. Varta warns: “For this charging procedure, the performance stated in the datasheet regarding capacity and cycle life may decrease.”

Details of charging and much other information is available in this clearly-written handbook.

The CoinPower series is produced in Ellwangen Germany.