Dear Sathyanathan, ageing of batteris can be basically observed by the output voltage and/or the solution's concentration. For instance in lead batteries used in vehicles, rated voltage per element is 2V, while acid concentration should be above 1,15 gr/cm3. When the concentration falls down to 1,12gr/cm3 or the voltage below to 1,75V the battery needs to be replaced. I think is easier to observe the voltage instead of the concentration (although a more sophsticated system with both parameters can be designed). Moreover you have to measure the current. As time goes on, battery's internal resistance is increased, thus the producing current is getting lower. So you could monitor the current (e.g with a lamp, a diode, an ammeter etc...). 

Thank u... Any idea about accelerated ageing test.