bakkeseo deiteoreul sseumyeon baeteoriga deo ppalli jureodeureo.

밖에서 means outside / outdoors (often implying away from home, e.g., not on Wi‑Fi).
-에서 marks the place where an action happens (쓰다 “to use” happens outside).
-에 is more for destination or existence (e.g., 밖에 있어요 = “I’m outside”).

Literally 쓰다 can mean “to write,” but it also commonly means to use.
So 데이터를 쓰다 means to use (mobile) data / cellular data. In real-life context, it usually implies mobile data usage, not just “data” in an abstract sense.

-를 is the object marker.
Here, 데이터 is what you’re using, so it takes -를: 데이터를 쓰다 = “use data.”

쓰면 = 쓰다 (to use) + -(으)면 (conditional).
It means “if/when (you) use…”. Depending on context, -(으)면 can feel like:

• if (condition)
• when (habit/general truth)

This sentence sounds like a general tendency: “When you use data outside, the battery drains faster.”

Yes. 배터리가 uses -가 to mark 배터리 as the subject of 줄어들어 (“decreases”).
It’s basically:
배터리 (subject) + 줄어들다 (to decrease)

Using -가 here is very natural because the sentence is describing what happens to the battery.

더 = “more” and 빨리 = “quickly/fast.”
Together 더 빨리 means “faster / more quickly”, i.e., compared to the usual situation.

줄어들다 means to decrease / to go down / to diminish.
For battery, it means the battery level goes down.

줄어들어 is the informal present style (plain casual). The dictionary form is 줄어들다.
Politer versions:

• 줄어들어요 (polite casual)
• 줄어듭니다 (formal)

Common natural pronunciation:

• 줄어들어 → sounds like 주러드러 (syllables get smoothed in fast speech)

Also:

• 밖에서 is often pronounced 바께서 because 밖 ends with a final consonant that affects the next sound.

Yes.

• 데이터를 쓰면 = more casual, very common in conversation
• 데이터를 사용하면 = more explicit/formal sounding (“if you use data”)

Both are correct; 쓰다 is typically the more natural everyday choice.

줄어들다 is fine and natural, especially meaning the percentage/amount goes down. Other very common choices:

• 배터리가 빨리 닳아. (닳다 = wear down; very common for batteries)
• 배터리가 빨리 소모돼. (소모되다 = be consumed; slightly more formal/technical)
• 배터리가 빨리 떨어져. (떨어지다 = drop/fall; common for battery level)

Each is similar; 닳다/떨어지다 often sound the most everyday.