AGS Battery Backup Reduced

AoA... Last May,2014 I bought New AGS 175 battery and installed it with Homage UPS, UPS charging rate is set on below 15 amp.

It gave me 5 hr backup on 02 fans and 03 energy savers During hot weather.

When winter started, its utilization reduced to 03 energy savers only and in day time I use to keep UPS switched off.

Now when hot weather again Starts, I shocked to experience that this Battery giving me 1.5 hr backup on 02 fans.

Gravity is normal as it should be,

Battery Mechanic did not check the battery but told me that plates need to be replaced Which will cost me worth 6000/=

Dont know whats wrong with my battery.

Can ny friend know or guide me how to diagnose or detect the actual problem ???