Learn Hardcore Japanese: densya de sumaho no zyuuden ga kireta node, zyuudenki wo karita. - My phone ran out of charge on the train, so I borrowed a charger.

It marks the place where the event happened: “on the train.” So 電車で here means “while on the train.” The same で can also mark “by means of,” like 電車で行く (go by train), but context makes it clear this one is location. If you want to be extra explicit, you can say 電車の中で (“inside the train”).

Why is が used with 充電 (充電が切れた) instead of は?

が marks the subject of an intransitive event and introduces it as new information: “the charge ran out.”
は would topicalize or contrast: 充電は切れたけど… (“As for the charge, it ran out, but …”). In a neutral report of what happened, が is most natural.

Why use 切れる to mean “run out”? Isn’t 切れる “to cut/break”?

切れる is an intransitive verb that often means “to be cut off / run out / expire.” It’s used idiomatically with resources or connections:

電池/バッテリーが切れる (the battery dies)
電波が切れる (signal drops)
期限が切れる (expire)
息が切れる (run out of breath) Here, 充電が切れた = “the charge ran out.”

What’s the difference among 充電, バッテリー, 電池, and 充電器?

充電: “charge; charging (the state/amount of charge).” 充電がない/切れる = no charge/charge ran out.
バッテリー: the battery (esp. in phones/laptops). Very common in speech: バッテリーが切れた.
電池: battery cell; used a lot for replaceable batteries, but people also say スマホの電池が切れた in everyday talk.
充電器: charger (the device you borrow here).

Can I say 充電を借りた (“I borrowed charge”)?

That sounds unnatural. You borrow the device, not the charge. Say:

充電器を借りた (I borrowed a charger), or
充電させてもらった／充電させていただいた (I was allowed to charge), which is common when you use someone’s outlet or port.

How do I indicate who I borrowed the charger from?

Use the lender with に or から (both are natural):

友だちに／から充電器を借りた。
Polite: 駅員さんに充電器をお借りしました。 Using から emphasizes “from,” but に is very common with 借りる/貸す.

Can I replace ので with から or だから? What’s the nuance?

ので: “because/since,” softer and a bit more objective or explanatory. Good in polite contexts.
から: also “because,” a bit more direct/subjective.
だから starts a new sentence: 充電が切れた。だから、充電器を借りた。 All are grammatical; ので sounds slightly softer.

How do I make the sentence more polite?

A common polite version:

電車でスマホの充電が切れたので、充電器を借りました。 More polite/humble (talking to the lender):
電車でスマホの充電が切れたので、充電器をお借りしました。 Very formal reason:
…充電が切れてしまったため、充電器をお借りしました。

Can I link the two parts with the て-form instead of ので?

Yes. 電車でスマホの充電が切れて、充電器を借りた。 The て-form can imply sequence or cause; it’s more colloquial/neutral than clearly causal ので. Colloquial regret/emphasis:

…切れちゃって、充電器を借りた。

Is 電車で ambiguous between “on the train” and “by train”? How do I avoid confusion?

It can be either, depending on the verb. With an event like 充電が切れた, it’s read as location (“on the train”). To be explicit, use 電車の中で. With motion verbs (行く, 来る), 電車で is “by train.”

Why are there spaces between words? Do Japanese normally write like that?

No. Spaces were likely added for learners. Normally it’s written:

電車でスマホの充電が切れたので、充電器を借りた。

How do you pronounce the sentence?

Kana reading: でんしゃでスマホのじゅうでんがきれたので、じゅうでんきをかりた。
Romaji: Densha de sumaho no juuden ga kireta node, juudenki o karita. Note: the particle を is pronounced “o.”

What’s the transitive/intransitive relationship with 切る and 切れる?

切る (transitive): someone cuts/turns something off. Ex: ロープを切る, 電源を切る, 電話を切る (hang up).
切れる (intransitive): something gets cut/is cut off/runs out. Ex: ロープが切れる, 電源が切れる, 充電が切れる. You wouldn’t normally say 充電を切る.

What if the battery was low but not completely dead?

Use:

バッテリーが少ない／少なくなった。
残り20％になった。
充電が切れかけている。 (about to die)
バッテリーが危ない。 (colloquial: it’s about to die)

Is スマホ the best word here? What about 携帯/ケータイ?

スマホ specifically means smartphone; very common.
携帯／ケータイ can mean “mobile phone” in general; many people still say it even for smartphones. If you want to be precise, スマホ is perfect; 携帯 is fine in casual conversation.

Can I say 切れたんで instead of 切れたので?

Yes. んで is a casual contraction of ので after verbs/adjectives: 切れたんで、… For nouns/na-adjectives, なので → なんで (e.g., 雨なんで), but note なんで also means “why,” so be aware of context in writing.