Faulty Battery

I purchased a new battery on the 18th August 2014. At the end of January 2015 the battery went flat, it was recharged and every 2 and 1/2 week after it was charged it went flat. My mechanic told my father that the battery is to weak to hold the charge. I went back to the company, they checked and told us that they would recharge the battery and if anything was wrong it would be replaced. When we collected the battery they told me that there was nothing wrong with the battery. But every two week the battery went flat. My mechanic decided to charge the battery themselves and told me that the battery was not holding the charge. I ask want does that mean and he said that there was a dead cell in the battery. He took it back to the shop, they did a check in front of him and said that they need to charge the battery. He told them that they could see that the one cell was dead why must it be charged and they insisted that it must be. When he collected the battery a few days later they told him that it is working and he wanted to know what they did. According to them they added acid to the cell and charge it and I would need to come back in 2 weeks' time for them to check.