From November 2023, all future Hawk boards will have a switch to allow the user to switch between high or low charge rates. The low charge rate option is required to allow the charging circuit to work with smaller solar panels like the 1W panel tested here.

Keeping the switch “On” will select the 400mA charge rate. Switching it to “Off” will set the battery charge rate to 85mA.

This switch is present on boards with the serial number 886406 or later.

Below is a graph of Battery Voltage vs External Voltage (Solar Panel) over time for a Hawk configured to work with a 1W panel.

The period in the red rectangle is interesting. In this time period, the panel is in weak, indirect sunlight and generating voltage. However, there is not enough power being generated to charge the battery. The behaviour of the charging circuit in this time period of low light is outlined below.

Low Light Charging Behaviour

• The charger is initially off. No load on the panel therefore its voltage is high.
• The charger sees high input voltage and turns on.
• Charging the battery is a big load, so the panel voltage drops.
• The charger detects the voltage drop and stops charging. 
• The charger times out for 7s.
• With less load applied, the panel voltage increases again.
• Repeat when the 7s timeout is completed.

For more information about connecting a Hawk to a solar panel please see Connect a Solar Panel to the Hawk.

Notes on the Hawk Charge Circuit:

• The Hawk uses a “constant current” charge circuit commonly used to charge LiPo battery packs. 
• You do get more advanced (and much more expensive) solar chargers that use a technology called “maximum power point” management where they adjust the amount of current used to charge the battery based on the power the solar panel can output. 
• For cost and complexity reasons we use the more common constant current charge circuit.