see generally ACT/BEAM bont box battery is suppose to last 45 mins. so i think two reasons for a lesser backup, not getting enough power/time to charge or the battery is faulty. I am not sure how to he power situation rite now at your place. If what i remember bangalore was getting around 3-4 hours of power cut last month, if the situation is same and then it should last about 30-45 mins and the frequency of power cuts are more, then i think battery is not getting enough power. If you have an inventor at your place, you can offer ACT to provide prower from your house and they generally pay you rs 100 per month for the power drawn. This way you can have connectivity as long as your home has power. Also, the power consumption of bont equipment (i.e. the fiber terminator switch) is around 24-40 watts.
Incase the battery is faulty then its can only be solved with a replacement. better talk to your local service engineer once he is around and understand the cause of this.