Question about setting the MPPT on BQ24650
That's not what it says.
With Vmpptset &lt; 1.2V, (but &gt; 175mV) it will try to maintain the input voltage at (some setpoint defined somewhere) by reducing the charge current. If even reducing the current to 0 isn't enough to maintain Vin, it will stay in that state (but fail to maintain Vin).
So what happens is that it will keep trying to charge, but at a lower current than the current setpoint, as long as Vin remains at the voltage setpoint. But if there isn't enough sunlight to maintain Vin even with 0 current, it will remain in that state but at 0 current, waiting for more light.
Or

A voltage above 175 mV on this pin enables charge

Charge is enabled, but what you get depends on the available current. But if there's not enough sunlight even to maintain Vin (at that setpoint) at 0 current, there's not a lot it can do.
This is not really MPPT as generally understood (the &quot;T&quot; means Tracking whereas this follows a pre-determined path): a poor man's MPPT if you will. I've seen a bunch of really cheap Chinese charge controllers that claim &quot;MPPT&quot; : presumably they use this chip or something similar.
Reading it a little further, the setpoint is defined by a voltage divider connected to MPPTSET. So if your solar panel claims its MPPT point is at (say) 16V at standard illumination, set the voltage divider to produce 1.2V from 16V.
Above 16V from the solar cell, you'll get constant current, to protect the battery.
Below 16V (1.2V at MPPset) a simple voltage divider will give you whatever current is available that keeps the cell producing close to 16V as long as Vin &gt; Vbatt.
You could manipulate MPPset to try and discover the real MPPT; that would require external electronics, but may increase charging current by a few % in poor light.
Also note discrepancy between &quot;To disable charge, pull MPPSET below 75 mV.&quot; on p.4 and 175mV on figure 8.2 p.12.
8.3.2 p.14 explains this as 100 mV hysteresis, 75mV to stop charging, 175 mV to restart.