How to disable charging of a battery while still powering my application?
Two tricks...


"The charge management controller can be disabled by
allowing the PROG input to float." (Sec 3.5 in the datasheet)
Add a diode to power the step down directly from the supply when the supply is present. 


The resulting circuit looks like this....



Anytime the +5V supply is present, it will be higher voltage than the battery voltage which will be maximum of about 4.2V even when charging. This reverse biases the diode, so the step down (or any downstream load) is powered directly from the input supply rather than the battery and charger. 

"Charge Enable" now enables the PROG pin to drain though the PROG resistor. Without "Charge Enable", the PROG pin will float and the battery charger will not charge the battery (use this when temp is out of range).