Bluetooth Low Energy was designed as the lowest possible power wireless technology that can be built.
Therefore, it’s important to optimize the parameters of a Bluetooth connection to avoid wasting energy. Two groups of parameters are involved: parameters during the connection and parameters to turn off the connection as often as possible. Indeed, the idea of BLE is to have the radio turned off as much as possible in order to save power.
Hence, the first parameter to consider is the connection interval. This defines how often the central device communicates with the peripheral. According to the BLE specification, the connection interval has a minimum value of 7.5ms and a max value of 4 seconds that can be programmed by multiples of 1.25ms. In a humidity sensor application, it is probably fine to be connected only every second unlike a motion tracking application, which will need to send as much data as possible and will choose a connection interval of 7.5ms. This parameter is really paramount as a ratio of 100 for the current consumption can be measured between the two options.
The second key parameter is the slave latency which is the number of connection intervals that the slave is allowed to skip. In an Android application, the connection interval is 7.5ms -- that is too much for many applications. If a slave wants to connect only every 30ms with an Android application, a slave latency of four should be programmed. Skipping four connection events decreases the total current consumption by roughly a ratio of four. Slave latency is also useful if you want to stay asleep, but then burst out data at a faster rate occasionally. Slave latency can be any value between 0 and 499, but cannot exceed ((SupervisionTimeout / conninterval) – 1). If supervision timer reaches the supervision timeout, the device considers the connection lost and exits the connection state. The connection supervision timeout is in the range of 100ms to 32s.
A good optimization during the connection saves very important µA too. The payload of each packet in BLE has a maximum of 20 bytes and a minimum of 0 bytes just to keep the connection alive. If only 10 bytes are sent instead of 20 bytes, the total average current decreases by around 10%.
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Bluetooth specifications accept up to 6 packets per connection interval. But if it is designed with iOS, Apple limits to 4 packets. Sending an extra packet, whatever the size is, increases current by several hundreds of µA. Due to overhead, it is much more efficient to send one packet with a payload of 20 bytes instead of 2 packets with a payload of 10 bytes. The 1st option is more efficient by several tens of percent.
Finally, the power output must be programmed to fit the desired distance. Some applications are just a few centimeters and not the default value of several tens of meters. Instead of an 8dBm output, -14 dBm can be enough and will save a significant amount of energy.
To conclude, correctly programming a few simple parameters dramatically changes the current consumption of a BLE design. BLE is used in many wearable applications powered by a tiny coin cell battery. Just a couple bad programming choices can drain the battery in a few days, instead of several years.