Cell Tech service is not acceptable

After 2.5 years, my Samsung cellphone battery capacity reduced significantly. I took it to Cell Tech Glenfair for a replacement battery. I asked if it would be better to take my phone rather to a Samsung agency. I was told that Samsung will keep my phone for 7 days and they would not replace it with the original battery. So I handed it in with Cell Tech. When I got it back, it the battery life was worse than with the original 2.5 year old battery. I took it back and the manager said he would install another battery. This time the phone hardly lasted for 1.5 hours from a full charge. When I returned, I discovered to my shock that Cell Tech replaced my phone battery with an old battery from another used and damaged phone. Cell Tech took it again and replaced the battery. Again, the new battery life was worse than my original 2.5 year old battery. I asked Cell Tech to re-install my old battery. They said the old battery may have been damaged when they removed it from my phone and that they would only refund half my money. My argument was that I brought my phone to fix the problem with the reduced battery life. Cell Tech did not fix the problem, but wanted to keep half the money. Cell Tech refused to refund me in full and left me with the threat that if I get the old battery re-installed, the phone may not work at all. So, after a week's frustration without a phone, I now have a battery costing me R550, and performing worse than the one I wanted to replace in the first instance.