[Rusty]

'The ball was round' is not a passive voice sentence.
You simply stated a characteristic of the ball. The structure is 'ser' + 'subjective complement (formerly called 'predicate adjective')':
La pelota era redonda.

Characteristics in the past take the imperfect tense in Spanish.

The passive voice, very common in English, is when the subject receives the action of the verb. The agent (the thing that caused the action) may be present, or it may be omitted. The structure is 'ser' + 'past participle':
The house was destroyed. (no agent)
The house was destroyed by the tornado. (agent)

The passive voice is formed the same way in both languages, which makes it convenient for the learner, but the passive voice is not used nearly as often in Spanish. The 'passive se' construct is preferred.

The house was destroyed.

Passive 'se':
Se destruyó la casa.

Passive voice:
La casa fue destruida.