bq25505 bq25570 using zinc air battery as the energy harvesting source

The bq255xx family can take any source having high impedance output, like a solar cell or TEG, to a medium or low impedance like a battery or other dc/dc converter.  With a lower impedance source, there is usually no need for MPPT because the source will not collapse when you pull current from it so all you need is a boost converter.  The bq25505 will work as a boost converter but its MPPT cannot be disabled so every 16s the boost will stop, sample the input voltage and then continue boosting.  If the VSTOR/VBAT equivalent capacitance is not large enough to handle the load during the 256ms sampling period every 16s, then the VSTOR/VBAT voltage might collapse.  The original bq25504's MPPT circuit can be disabled.

Since the maximum average input current for the bq255xx is 100mA, it will not be able to provide the desired output dc power (Pout = (0.8V-1.45V) *100mA *efficiency).  I recommend the TPS61200, which operates down to 0.3V but even it will not provide that much output power below 0.7V.