Learn Hardcore Portuguese (Brazil): A bateria é nova.

a nova bateria: often “the new/different battery” (new to the owner, a replacement, <a class="border-b border-dotted border-blue-400 dark:border-blue-500 cursor-pointer text-inherit no-underline" href="/grammar/portuguese-brazil/determiners/outro">another one).
a bateria nova: emphasizes that it’s brand-new/unused. In practice, both are common; context clarifies the intended nuance.

It’s the feminine singular definite article in Portuguese, equivalent to English “the.” So A bateria = “the battery” (or “the drum kit,” depending on context).

Why is bateria feminine, and how can I tell?

Grammatical gender is a property of each noun. Many nouns ending in -a are feminine, and bateria is one of them. You learn the gender with the noun: a bateria (feminine), not “o bateria.” The matching indefinite article is uma bateria (“a battery”).

Does bateria mean “battery” or “drums”?

Both, depending on context:

Battery (rechargeable pack): a bateria do celular (the phone battery), a bateria do carro (the car battery).
Drum kit / drum section: ele toca bateria (he plays drums), a bateria da escola de samba (the samba school’s drum section). For small disposable cells (AA/AAA), Brazilians usually say pilha, not bateria.

What exactly is é, and how is it conjugated?

É is the 3rd person singular of the verb ser (“to be”) in the present tense.

Present of ser: eu sou, você/ele/ela é, nós somos, vocês/eles/elas são. In the sentence, é = “is.”