Learn Hardcore German: Ich wechsle die Batterie im Auto.

die Batterie is the direct object of the verb wechsle, so it’s in the accusative case. Batterie is feminine, and the feminine definite article in both nominative and accusative is die.

Why is the definite article used in die Batterie instead of the indefinite eine Batterie?

We use the definite article because we’re referring to a specific battery—the one already installed in the car. eine Batterie would imply any (unspecified) battery, whereas die Batterie points to the known item.

What does im Auto mean, and why do we use im instead of in dem or in das?

im is a contraction of in + dem. The preposition in is two-way: with dative to indicate location (where something happens) and with accusative to indicate direction (into). Here we describe where you change the battery, so we need the dative case. Auto is neuter, so the dative definite article is dem, giving in dem Auto, which is shortened to im Auto.

Could we use in das Auto here instead of im Auto?

No. in das Auto (accusative) would mean moving into the car (e.g. “I get into the car”), which doesn’t fit. To express “inside the car” as the location of the action, you use the dative: in dem Auto → im Auto.

What is the difference between wechseln, auswechseln, austauschen, and ersetzen in this context?

All four can mean “to replace,” but with slight nuances:

wechseln: the general verb “to change/replace.”
auswechseln: separable; emphasizes the act of swapping out.
austauschen: “to exchange/swap,” often implying a one-to-one swap.
ersetzen: more formal “to substitute.”
For swapping a car battery, wechseln or auswechseln are by far the most common.

If I want to use auswechseln, how would the sentence look?

Because auswechseln is a separable verb, you put aus at the end in a main clause:
Ich wechsle die Batterie im Auto aus.

How do I say “I changed the battery in the car” in the past tense?

Two common options:
1) Perfect tense (colloquial/spoken): Ich habe die Batterie im Auto gewechselt.
– gewechselt is the past participle of wechseln; auxiliary haben.
2) Simple past (written/literary): Ich wechselte die Batterie im Auto.

Can I start the sentence with im Auto? How does the word order change?

Yes. German permits fronting an adverbial phrase. You must keep the finite verb in second position:
Im Auto wechsle ich die Batterie.
Here im Auto is first, wechsle stays in second position, and ich comes third.