Power supply

The HC-50 divides the tasks so that power is only needed where it is available: the gateway hangs on a power supply unit and listens continuously for radio signals. The nodes run on a battery and are very economical thanks to deep sleep. This allows measuring points to be operated even where there is no socket for miles around.

Gateway: mains operation

A gateway must be ready to receive packets from its nodes at any time. It therefore cannot sleep and is operated with a standard USB power supply (5 V). The power supply is connected to a socket at the installation site, see the gateway guide. It is intended for continuous outdoor operation.

Node: battery operation

The node gets by on a single battery because it sleeps almost all the time. A measuring cycle only lasts briefly:

1. The node wakes up.
2. It reads its sensors.
3. It sends the values to the gateway via LoRa.
4. It briefly waits for the acknowledgement.
5. It falls asleep again until the next cycle.

During the sleep phases the processor is practically off. The difference between the operating modes is enormous, as the reference values for the processor (ESP32 family) show:

The node uses deep sleep. The time between two measurements therefore uses almost no power. The ratio of sleep time to active time decides the runtime.

What the runtime depends on

The battery has 6000 mAh. How long that lasts depends on these factors:

• Measuring interval: The less often the node wakes up, the longer the battery lasts. This is the most important lever. In field use the interval is chosen considerably longer than in the preliminary version, which measures about every 20 seconds for quick testing.
• Sensors: Every sensor needs power while measuring. Some need a warm-up time or a higher current than others.
• Radio link: The node regulates its transmit power itself and only increases it when necessary. With a poor connection it transmits more often and more strongly. A good location of the gateway therefore saves battery, see LoRa radio technology.
• Temperature: The station is designed for −20 to 70 °C. In the cold, the usable capacity of any battery drops.
• Continuous operation: Functions that keep the node awake, e.g. the Paxcounter, greatly increase consumption. Where possible use them with a power supply instead of a battery, see the node guide.

Why a rechargeable LiFePO4 cell?

Disposable lithium cells such as the CR123 keep their 3 V voltage constant for a long time, have very low self-discharge and still work well at −20 °C. A Varta CR123 (3 V, 1700 mAh) even manages a standby operation of over five years in an economical device, depending on how often the device wakes up and how long it transmits.

For the HC-50 we nevertheless chose a rechargeable lithium iron phosphate cell:

• More capacity: 6000 mAh instead of 1700 mAh.
• Reusable: Around 1000 charge cycles instead of disposable cells, which saves cost and waste.
• Safe: The cell type is thermally stable and has no memory effect.
• Monitorable: Discharge and condition are measured and evaluated in the cloud.

The module is charged in the HC-50 CHRGR, all details are on the page HC-50 battery.

Solar

A solar module with integrated battery will be available soon. It recharges the battery during the day. At suitable locations a measuring station can then work completely autonomously, without batteries having to be replaced. The solar module is still in development.

Digression: radionuclide batteries

Radionuclide batteries would be extremely interesting for measuring stations at remote locations, because they supply power for decades. For good reason, however, they require a permit and are mainly reserved for military and scientific applications. If you have access to radionuclide batteries, we would be glad to develop a battery module for them too ;)

_{Further information: Radioisotope thermoelectric generator (Wikipedia)}