@bradnz So you want to tell, if the voltage of the LiPo battery is above 2.5 V. That should be done using the ADC, for various reasons.
a) The value at which a digital input switches from HIGH to LOW is not well defined and not stable. It is somewhere between 0.75 * Vdd and 0.25 * Vdd
b) You have to check the value of the variable charger anyhow, so instead of doing that you could also read an ADC value. If you want to have that done in an interrupt fashion, you can set-up a timer interrup which reads the ADC,