You can measure minimum capacity of the battery-

1> What exactly is minimum capacity in your Perspective?

a>whether the nodes alive for long duration without any activity having X mah capacity b> whether the node required  X mah capacity to do some operation before it is useless

2> For what type of applications your are using sensors?

I have implement some sleep algorithm in my controller. controller needs minimum of 1.8V for its operation. i am designing this for healthcare application

Dear Prakash Rajaiya; u need to evaluate the battery life time in turns of ROUNDS.

For hw many longer round of event detection and forwarded to the BS the n/w is alive.

Same u can prolong through some sleep mode (which is must for a WSN) , enc-operate some TDMA and CSMA algo for efficiency.

these will lead yr N/W life time. 

Prakash Rajaiya

U need to look at power required for sensing, communication, data collection and processing. For that you need to known how many nodes are in active state, idle state , sleep state because your application is for health care it could include several nodes.

Is the minimum requirement only to initialize the network .