Leanna Taylor, ex-wife of Ross Harris, began her testimony on Monday, the 17th day of testimony in the Ross Harris hot car death trial in Brunswick, Ga.

Taylor broke down in tears as she described her son, Cooper Harris, the 22-month-old who died in the back seat of Ross Harris' SUV in a Vinings, Ga., office park.

"Cooper was the sweetest little boy," she said. "He had so much life in him. He was everything to me."

For much of the afternoon Monday, lead defense attorney Maddox Kilgore showed video clips of young Cooper, and had Taylor to describe the family dynamic between she, Ross and Cooper. The video clips included images of Cooper playing and eating. Taylor broke down in tears at each of the videos.

Kilgore asked Taylor if Ross loved Cooper.

"Yes, he did," Taylor replied. "Very much."

During cross-examination, prosecutors asked Taylor more about the day of Cooper's death, and Ross's actions. There were also questions regarding Ross' infidelity and his "double life."

"I didn't think he would cheat on me," she said. "I can't say that I'm shocked that he'd gone out of our marriage. (It was) the degree."

Cross-examination of Taylor will continue on Tuesday.

Monday morning began with Taylor detailing sexual dysfunction within their marriage, and whether or not she was aware of Harris' infidelity.

When Taylor took the stand, Ross Harris refused to look at his ex-wife, turning away while she was on the stand.

Ross Harris faces a number of charges, including malice murder and felony murder in the June 2014 death of Cooper. The defense claims the death was the result of a tragic accident.

The defense began to build their case last Friday afternoon by calling Roger Webb, a man that leased a home to the Harris’, and Cobb County detective Shaun Murphy to the stand.

Murphy worked with Phil Stoddard on the case, and the defense pointed out his mistake in measuring the height of the SUV Cooper died in was critical to the case.

RELATED | Official jury video of Ross Harris SUV released by court officials

After questioning concluded for the day on Friday, the defense asked for a second mistrial in the case. They claimed the prosecution failed to prove their case and only offered misinformation and lies in their presentation. Judge Staley one again denied their mistrial request, only one day after their previous request for a mistrial.

PHOTOS | Leanna Taylor takes the stand in the Ross Harris trial

PHOTOS | Ross Harris Hot Car Death Trial - Day 16