I also had the same issue in my Aqua (2012) in November 2017. I have consulted many people and finally arrived at the decision to replace cells. To make a long story short, did the repair with Nishantha electricals (Attidiya) and i got the job done with a 6 month warranty for less than 60k. Replaced 7 cells.Job was done by a German tech qualified guy and he carefully did the job.
You can temporally delete the error message from Dash board but it will appear after few hours or days. Replacing a battery is also tricky. According to Nishantha (I know this guy for a long time), brand new batteries can be found in the market from 3.5 lacks but still without warranty.
Japs have been very careful when designing the battery pack and they have not mentioned the year of manufacture of the battery. It can be identified by a CODE.
I've been running my car without any issues for close to two months now and hope it will last at least an year. I checked the matter with TOYOTA, and they said battery issues can be found in cars which has done over 100,000km, (True in my case!).
Nishantha recommend to replace the battery if more than 10 cells are weak. For aqua it comes with 20 cells and prius has 28. Price of a cell can be around Rs.6500 to Rs.8000. I got mine done for Rs.7000 per cell.
This was the only issue i had for last two years from my car and i guess its still worth it considering the fuel consumption. Before the repair it did about 16 in moderate traffic and now easily does about 18/19 p/l.
I saw someone has shared his number in this thread before but sharing it again here for help 0777799953