TV and movies make it seem as if Breathalyzer tests are necessary for someone to be charged with DUI and convicted. After all, there is rarely a scene where someone faces charges without a test in the picture. However, some people do refuse the tests, never get them and end up convicted anyway.
The reasons for conviction may be multifold but often narrow down to a few top ones such as lack of good representation and the believability of the evidence presented.
Other types of evidence are admissible
You could be convicted based on evidence such as failing a field sobriety test. For example, if the arresting officer asked you to count backward from 59 to 45 and says you slurred your words and got the numbers wrong, these alleged errors could be evidence against you. In fact, the more words you speak and the more moving around you do, the more evidence police officers can amass. So, speak as little as possible, and move as little as possible.
Witnesses could also help lead to a conviction. For instance, the prosecution could bring in people to testify they saw you drinking alcohol constantly from 7 pm to 9 pm, when you claim you were not. There could also be photographic evidence and video evidence, perhaps from witnesses or from police equipment.
Lack of good representation
Often, a good defense attorney can undermine prosecution claims. With witnesses, a defense attorney can question their competence. Had they been drinking too? Why could the clear liquid not have been water, as you said it was?
Some field sobriety tests, such as countdowns, are not standardized, and there is a lot of room for a police officer's subjective interpretation. Even standardized tests such as the walk-and-turn test, where there are instructions, often lead to mistakes and instructions not being followed. However, a defendant who represents himself or herself might not know about these possibilities, and a public defender might not explore them as thoroughly as a private attorney might.