Learn Spanish (Latin America): La batería es más resistente, pero la calidad es peor. - The battery is more resistant, but the quality is worse.

Peor is an irregular comparative that already means "worse" (or "more bad"). Because the "more" is built right into the word, you never add más in front of it. Saying más peor sounds like "more worse" in English, and más mala is incorrect for comparisons.

Why does "more resistant" need más (más resistente), but "worse" doesn't?

Most adjectives in Spanish form comparisons by putting más or menos in front of them, just like adding "more" or "less" in English. However, a few common adjectives have special, single-word comparative forms. Peor (worse) is one of these irregulars, alongside mejor (better), mayor (older/larger), and menor (younger/smaller).

Why does this sentence use es instead of está? Can't a battery's resistance change?

We use es (from the verb ser) because it describes the inherent specifications or built-in traits of a product. Even though a specific battery might drain over time, its overall design—being resistant or having bad quality—is considered a fundamental characteristic, so we use ser.

Is there a trick to remembering that calidad is feminine (la calidad)?