Мой телефон опять разрядился.

Because телефон is a masculine noun in Russian.

Possessive pronouns must agree with the noun they describe:

• мой = my (masculine)
• моя = my (feminine)
• моё = my (neuter)
• мои = my (plural)

So:

• мой телефон = my phone
• моя машина = my car
• моё устройство = my device

The -ся makes the verb reflexive/intransitive here.

Compare:

• разрядить телефон = to discharge the phone / drain the phone battery
• телефон разрядился = the phone discharged / the phone ran out of battery

So in this sentence, the phone is not something being acted on by another subject. Instead, the phone itself is presented as having lost its charge.

This is a very common Russian pattern:

• transitive verb without -ся
• intransitive/reflexive version with -ся

Разрядился is perfective past, which means the action is viewed as a completed event.

Here, the important idea is that the phone ended up dead / ran out of battery. That is a completed result, so perfective is natural.

• разрядился = discharged, ran out, died
• разряжался = was discharging / used to discharge / kept running down

In this sentence, you are reporting a fact/result, not describing an ongoing process.

In the past tense, Russian verbs agree with the subject in gender and number.

The subject here is телефон, which is masculine singular, so the verb is masculine singular too:

• разрядился = masculine
• разрядилась = feminine
• разрядилось = neuter
• разрядились = plural

Examples:

• Телефон разрядился.
• Батарея разрядилась.
• Устройство разрядилось.
• Наушники разрядились.

Опять means again.

So it shows that this is not the first time the phone has run out of battery.

In everyday speech, опять often carries a slight feeling of annoyance, surprise, or frustration, depending on context:

• Телефон опять разрядился. = The phone died again.

A related word is снова, which also means again. Very often they are interchangeable, but опять can sound a bit more like yet again in many contexts.

Yes. Russian word order is fairly flexible, and the basic meaning stays the same, but the focus can change.

Neutral order:

• Мой телефон опять разрядился.

Other possible orders:

• Опять мой телефон разрядился.
• Мой телефон разрядился опять.
• Разрядился опять мой телефон. — possible, but more marked

The original sentence sounds natural and neutral in everyday speech.

Russian does not use an auxiliary verb like English has in the past tense.

English might say:

• My phone has died again
• My phone died again
• My phone ran out of battery again

Russian simply uses the past tense form:

• телефон разрядился

So one Russian past-tense verb often covers what English expresses with either:

• simple past, or
• present perfect

The exact English translation depends on context.

No. It can be used for many things that lose electrical charge or run out of power.

Common examples:

• Телефон разрядился. = The phone died.
• Батарея разрядилась. = The battery ran out.
• Ноутбук разрядился. = The laptop died.
• Аккумулятор разрядился. = The battery discharged.

So it is a very useful verb for devices and batteries in general.

Yes, in colloquial Russian, сел is very common for a battery-powered device.

• Телефон сел. = The phone died.
• Батарейка села. = The battery died.

This is often more conversational than разрядился.

Roughly:

• разрядился = more literal/formal: discharged
• сел = very common everyday way to say the battery died

Both are natural, but разрядился is a very standard and clear choice.

It is in the nominative case, because it is the subject of the sentence.

The phone is the thing that ran out of battery, so it appears in nominative:

• телефон = nominative singular

You can tell this is the subject because the verb agrees with it:

• телефон разрядился

Russian has no articles.

So телефон can mean:

• a phone
• the phone
• simply phone, depending on context

Here, мой телефон already makes it definite enough, so English naturally uses my phone.

Russian relies on context, word order, and other clues instead of articles.