Detecting battery degradation in voltage profile.
If you are lucky and the discharge profile is more or less constant, than yes for battery and maybe for capacitor. Just note that the measurement is not just voltage, it's voltage at certain time. Best is to build a table. In any case i would suggest to charge and discharge ten batteries for however many times it takes. Then it's possible to know the exact parameters to measure. 

By the way, one way is to count discharge cycles. It's easier and for most cases efficient enough.