Wireless Systems Design

Calculating battery lifetime

Figures are for ANT 2.4 GHz practical wireless networking protocol running on Nordic Semiconductor nRF24AP1.

Assumptions:

· Heart rate belt communicating with sportswatch.

· One-hour operation per day (operation cycle 24 hours).

· Messaging period: one message per second (1 Hz).

· CR2032 battery: nominal voltage = 3 V, operating voltage = 2 V, capacity = 220 mAh.

· nRF24AP1: suspend (sleep) current = 2 µA, active base current = 35 µA, peak current Rx = 22 mA, peak current Tx = 16 mA (at 0 dBm).

· Communication method: bi-directional bit synch interface.

· Time of maximum current duration: Rx = 600 µs, Tx = 400 µs.

Rx energy required = power x message duration = 2 V x 22 mA x 600 µs = 26 µAs

Tx energy required = 2 V x 16 mA x 400 µs = 12.8 µAs

Battery life (hr) = battery capacity x operational cycle

active time x (base current + (energy x message rate)) + suspend time x suspend current

Tx battery life = 220 mAh x 24 h

1 h x (35 µA + (12.8 µAh x 1 Hz)) + 23 h x 2 µA = 6.4 years

Rx battery life = 220 mAh x 24 h

1 h x (35 µA + (26 µAh x 1 Hz)) + 23 h x 2 µA = 5.6 years

The Web site www.thisisant.com includes a handy power estimator that calculates battery lifetimes.

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