Learn Hardcore Japanese: sumaho no zyuuden ga mada tarinai node, mou sukosi matimasu. - My phone still isn’t charged enough, so I’ll wait a little longer.

Breakdown of sumaho no juuden ga mada tarinai node, mou sukoshi machimasu.

Why is の used in スマホの充電?

Here, の links two nouns and works a bit like of or an apostrophe-s in English.

So スマホの充電 means the smartphone’s charge or the charging of the smartphone.

In this sentence, it refers to the phone’s battery charge level, not the act of plugging it in as the main focus.

What does 充電 mean here: charging or battery charge?

It can mean either depending on context, but in this sentence it means the amount of charge the phone has.

Because of 足りない (not enough), the sentence is talking about the battery level being insufficient:

So this is closer to my phone isn’t charged enough yet than simply the phone is charging.

Why is the particle が used after 充電?

が marks 充電 as the thing being described by 足りない.

With 足りない, the thing that is lacking is often marked with が.

You may also see patterns like:

So が is natural here because charge is the thing that is not enough.