Toyota Aqua Battery last for at least 200000 KM, so don't just jump to battery fault directly. I have mentioned some points in previous comment. follow them and for battery test Charge your battery to full and shift your car to EV mode drive in EV mode and see how much distance it covers, if that distance is less than 1KM than there could be an issue with Battery.
I have Toyota Aqua S 2015, Giving Average of 24 KM/L on Highway, In City it depends, in peak rush hours of summer it will give between 14-18 KM/L, But in normal traffic and medium temperature outside it can go upto 22 KM/L. if your speed of car is between 30-40 KM/h and short trips of 3-4 KM it very difficult for AQUA to give you avg above 15 KM/L. so don't get tensed about battery.