Điện thoại của tôi gần hết pin, nên bây giờ tôi phải sạc ở văn phòng.

Yes. Both are possible.

• điện thoại của tôi = my phone
• điện thoại tôi = also my phone

The version with của is a very clear possessive structure:

• X của tôi = my X

In everyday speech, Vietnamese often drops của when the meaning is already clear:

• điện thoại tôi
• nhà tôi
• bạn tôi

So in casual speech, điện thoại tôi gần hết pin sounds very natural.
Using của is still completely correct; it can sound a little more explicit or careful.

Literally:

• gần = near / almost
• hết = finished, used up
• pin = battery

So gần hết pin means the battery is almost used up or the phone is almost out of battery.

This is a very common Vietnamese way to express almost out of battery.

You may also hear:

• sắp hết pin = about to run out of battery
• gần hết pin = nearly out of battery

Both are natural.
gần focuses on being close to that state, while sắp often suggests it will happen soon.

In Vietnamese, hết often works like a verb or predicate meaning to be gone / to be used up / to run out.

So:

• hết pin = out of battery
• hết tiền = out of money
• hết giờ = time is up

With phones, Vietnamese commonly says:

• Điện thoại hết pin rồi. = The phone is out of battery.

So gần hết pin follows that same pattern naturally.

nên means so / therefore / therefore I should / as a result depending on context.

In this sentence:

• Điện thoại của tôi gần hết pin, nên bây giờ tôi phải sạc ở văn phòng.
• My phone is almost out of battery, so now I have to charge it at the office.

It links the first clause to the consequence in the second clause.

A useful pattern is:

• A, nên B = A, so B

Examples:

• Trời mưa, nên tôi ở nhà. = It’s raining, so I stay home.
• Tôi mệt, nên tôi ngủ sớm. = I’m tired, so I sleep early.

In casual speech, people sometimes omit nên if the relationship is obvious, but keeping it makes the logic clear.

Yes, bây giờ means now / at this moment / right now, depending on context.

Here it adds the idea that the need is immediate:

• nên bây giờ tôi phải sạc... = so now I have to charge...

Without bây giờ, the sentence would still be grammatical:

• Điện thoại của tôi gần hết pin, nên tôi phải sạc ở văn phòng.

That version simply says:

• My phone is almost out of battery, so I have to charge it at the office.

Adding bây giờ emphasizes urgency or present necessity.

phải means must / have to.

So:

• tôi phải sạc = I have to charge it

It shows necessity, not just desire.

Compare:

• tôi phải sạc = I have to charge it
• tôi cần sạc = I need to charge it
• tôi muốn sạc = I want to charge it

In this sentence, phải makes sense because the battery is almost dead, so charging is necessary.

Both are possible.

• sạc = to charge
• sạc pin = to charge the battery

When the object is already obvious from context, Vietnamese often uses just sạc.

So:

• tôi phải sạc = I have to charge it
• tôi phải sạc pin = I have to charge the battery / charge my phone

In this sentence, since we already know the topic is the phone battery, sạc alone sounds natural.

Here ở văn phòng modifies the action sạc.

So the meaning is:

• I have to charge it at the office

It does not mean the office phone.

Breakdown:

• ở = at / in
• văn phòng = office

So:

• sạc ở văn phòng = charge it at the office

If Vietnamese wanted to say the office phone, it would need a different structure, such as:

• điện thoại ở văn phòng = the phone at the office / in the office

Yes, it could be omitted in some contexts, but repeating it is very normal and clear.

Full sentence:

• Điện thoại của tôi gần hết pin, nên bây giờ tôi phải sạc ở văn phòng.

Possible shorter version:

• Điện thoại của tôi gần hết pin, nên bây giờ phải sạc ở văn phòng.

That shortened version can work in casual speech, especially when the subject is obvious. But Vietnamese often repeats the subject for clarity, especially in complete written sentences.

So the repeated tôi is not strange at all.

It is helpful because the sentence has two clauses:

1. Điện thoại của tôi gần hết pin
2. nên bây giờ tôi phải sạc ở văn phòng

The comma marks a pause and makes the cause-and-result relationship easier to read.

In informal texting, people may leave out punctuation, but in normal writing the comma is a good choice here.

Yes, it is natural. A native speaker might also say it in slightly different ways depending on style.

Some natural variants:

• Điện thoại tôi gần hết pin, nên giờ tôi phải sạc ở văn phòng.
• Điện thoại tôi sắp hết pin rồi, nên giờ phải sạc ở văn phòng.
• Điện thoại tôi gần hết pin rồi, nên tôi phải sạc ở văn phòng.

Notes:

• giờ is a shorter, more conversational form of bây giờ
• rồi often adds the sense that the situation has already reached a certain point, like already
• dropping của and sometimes the second tôi is common in speech

So the original sentence is correct and natural, just a bit fuller and clearer.

A few parts that learners often want help with:

• Điện thoại
  
  • điện has a heavy/falling tone
  • thoại also has a heavy/falling tone
• gần
  
  • has a rising broken tone in Northern pronunciation
• hết
  
  • has a sharp rising tone
• phải
  
  • has the dipping-then-rising hỏi tone
• sạc
  
  • has a heavy low tone
• văn phòng
  
  • phòng has a falling tone

A rough phrase grouping for pronunciation is:

• Điện thoại của tôi / gần hết pin, / nên bây giờ tôi phải sạc / ở văn phòng.

Learners usually find it easier if they practice in chunks rather than word by word.