Low Power Node

What Is a Low Power Node?

A Low Power Node (LPN) is a battery-powered Bluetooth Mesh device that minimizes radio activity by sleeping for extended periods and relying on an associated Friend Node to cache messages on its behalf. The LPN wakes periodically, polls its Friend for stored messages, processes them, and returns to sleep. This pattern enables mesh participation with coin-cell battery lifetimes measured in years.

Why LPNs Need Friends

In standard Bluetooth Mesh operation, every node must continuously scan the advertising channels to receive mesh messages -- a power-intensive activity that draws 5 -- 15 mA depending on the SoC. For a CR2032 coin cell (230 mAh), continuous scanning would drain the battery in under two days. The friendship mechanism offloads the continuous listening to a mains-powered Friend, letting the LPN duty-cycle its radio down to 1% or less.

LPN Lifecycle

The LPN goes through a defined lifecycle:

1. Provisioned -- the LPN joins the mesh via provisioning and receives its unicast address and NetKey.
2. Friend Discovery -- the LPN broadcasts a Friend Request on the advertising channels. Nearby Friend-capable nodes respond with Friend Offers.
3. Friendship Established -- the LPN selects the best Friend (strongest RSSI, largest cache) and confirms the relationship.
4. Sleep-Poll Cycle -- the LPN enters its main operating loop: sleep for the configured interval, wake, send a Friend Poll, receive cached messages, process, repeat.
5. Friendship Terminated -- if the LPN fails to poll within the Poll Timeout, or if the LPN or Friend explicitly terminates, the LPN returns to Friend Discovery.

Configuration Parameters

Power Budget Example

Consider an LPN using a Nordic nRF52840 SoC with a 10-second poll interval:

• Sleep current: 1.5 uA
• Active current during poll (TX + RX): 8 mA for ~5 ms per poll
• Average current: 1.5 uA + (8 mA x 5 ms / 10 s) = ~5.5 uA
• CR2032 (230 mAh) lifetime: ~4.8 years

This calculation assumes minimal application processing. Real-world lifetimes are typically 2 -- 4 years, accounting for sensor readings, self-test routines, and occasional retransmissions.

Message Delivery Latency

The trade-off for extreme power savings is increased message latency. A message arriving 1 second after the LPN polls will not be delivered until the next poll -- up to 10 seconds later in the example above. For applications like occupancy sensors or environmental monitors, this latency is acceptable. For time-critical applications (door locks, alarm actuators), LPNs are generally not suitable.

LPN Subscriptions

LPNs can subscribe to group addresses, and the Friend Node caches group-addressed messages alongside unicast messages. However, subscribing to many groups increases the Friend's cache pressure and may cause message eviction. Keep LPN subscriptions minimal -- typically one or two groups plus the unicast address.