When power to the device is insufficient to power the device correctly, (i.e. the battery goes flat), the device will enter an Under Voltage Lock Out (UVLO) state to prevent reboot loops, and damage to any components. 

The UVLO behaviour has been designed to cater for different power setups e.g. 

• Battery + Solar Panel
• Battery + stable mains power
• Battery Only

Summary of behaviour:

1. The device resets and enters the UVLO state when the battery goes flat. 
2. For a Battery + Solar Panel setup, the logs will show the below
   
   The count is a safety measure in cases where the device resets a lot due to UVLO. The idea is that upon Power On, if the battery is below 3.7V, this count is incremented. Once the count reaches 5, the device will sleep for 5 mins if there is no battery connected or charge for 15 mins if there is a battery connected.
3. The device will then wait for external power to be detected
4. Once detected, it will take around 30s to determine if the external power is stable
5. Otherwise, repeat from step 3

The above ensures that on top of avoiding reboot loops, we also avoid entering/exiting UVLO repeatedly in the case of a solar panel outputting a low current trying to charge a flat battery. 

Some points to note when bench testing/provisioning the Hawk

1. To avoid getting stuck in the UVLO state use a well-charged battery (3.8V+), or connect a stable external power source (e.g. 12V bench power supply)
2. You can exit the UVLO state by pressing the button on the board for around 200ms. The LED will flash to indicate success.

On the Hawk Lite, the UVLO behaviour is simplified.

Simply, if UVLO is detected, the device will sleep for 3 hours to allow the batteries to recover.

The device will then wake up and attempt to reconnect.

To avoid a UVLO event, ensure you use high-quality, new batteries.

In addition, ensure that your sensor's power needs are served by the Hawk Lites lower energy budget.

If in doubt, test with a Joulsescope or use the Hawk Pro with LTC batteries or a solar panel.

There is an unusual edge case that customers may experience when using solar panels with the Hawk.

Two conditions need to be true for this edge case to occur.

1.  The Hawk LiPo battery must be quite low (~3.5V)
2. The Hawk must be connected to a solar panel which provides a low amount of power. For example, a 5W panel in overcast conditions.

In these conditions, the Hawk attempts to charge its battery from the solar panel, but the panel cannot provide enough power.

However, the panel is providing some voltage, which activates some internal circuitry between the battery and the board. This circuitry causes the battery voltage to drop by ~0.4V. In cases where the battery is already low, this causes the battery voltage to drop too low to power the Hawk, and the device will brownout.