You may already know all this, so apologies in advance for the long post. Maybe it will same someone some effort.
As previously pointed out, the CN3065 is constant voltage charger. You may want to switch to a CN3791 MPPT charger instead.
Regarding the low voltage/depleted startup issue:
IMHO, the easiest way to avoid this is to simply keep from dropping below the low voltage threshold. You can use a variable power supply instead of the battery to establish when the voltage is too low. Just turn down the voltage until your radio quits working. This voltage plus a little margin is your target minimum voltage for your battery. Size your battery so your charger can recharge it enough so voltage never goes below the minimum in worst case scenarios…
Regarding the effectiveness of your charger:
You can characterize your charger by determining the solar cell voltage at which the charger begins to charge. Do this by using a variable power supply in place of the solar input to your charger and make note of the voltage at which the charger starts working. Once it starts charging turn the voltage down until the charger stops charging. This value may be considerably less than the start voltage. For example the RAK Green Power board starts at 5.3v and stops at 2v.
This start-stop voltage window is important because it is the effective useful range of the solar cell output. In the case of the Green Power board, if the cell never reaches 5.3v, the charging will never happen. Working down to 2v doesn’t matter if it can’t start.
Also, some solar panels intended for USB charging have an internal 5v regulator. This means the charger by design, will never reach 5.3v and will never work through a Green Power board or other charger through a similar start voltage. An unregulated solar panel is preferred.
BTW, MPPT controller chargers are designed to work for a specific voltage range. For example, a 6v MPPT controller must be used with a 6v panel.
The “T” in MPPT means tracking… good controllers can work over a huge range
Only the cheap boards set a static MPPT voltage using a resistor or a voltage divider. Some of them have a poti to set it by hand.
Charge controllers like the BQ25185 (super cheap around 1-2€) are trying to get the maximum current out of a solar cell by staying slightly above the breakdown voltage. TI calls this feature VINDPM.
Not trying to be contrary & I’m not an MPPT expert, but the cheap MPPT controllers I’ve used (CN3791 based) have to be ordered for 5v, 6v, 9v, or 12v solar input voltage. Yes MPPT tracks, but I think the boards are designed for a voltage range.
My comment was just a warning to pay attention to what you order since the various voltage models look alike.
