The 30-year-old struggled to shine after joining the Blues on a free transfer from German giants Bayern Munich last summer.
Ballack said: “It was pretty much as I expected it to be — difficult.
“I went in search of a new challenge and I found one.
“Highs and lows are part of it but it’s good because they help me develop.
“I have not lost any of my strength. In fact, I have developed further while in England.”