Si la bateria del teu mòbil s'acaba a classe, pots portar un carregador petit.

Si means if. It introduces a condition:

• Si la bateria del teu mòbil s'acaba a classe... = If your phone battery runs out in class...

This is a very common way to make if-clauses in Catalan.

Catalan often expresses this idea literally as the battery of your phone:

• la bateria = the battery
• del teu mòbil = of your phone

So:

• la bateria del teu mòbil = your phone battery

This structure is very normal in Catalan.

Because del is a contraction of de + el.

In Catalan, possessives usually go with a definite article:

• el teu mòbil = your phone

So:

• de + el teu mòbil → del teu mòbil

This is exactly like other common contractions in Catalan, such as:

• a + el → al
• de + els → dels

That is a key difference between English and Catalan.

In standard Catalan, possessives normally take the definite article:

• el teu mòbil = your phone
• la teva bateria = your battery
• els meus llibres = my books

So although English does not use the here, Catalan usually does.

Yes. Mòbil means mobile phone / cell phone, and it is the normal everyday word in Catalan.

Examples:

• No trobo el mòbil. = I can't find my phone.
• El meu mòbil no té bateria. = My phone has no battery.

Here s'acaba means something like:

• runs out
• dies
• comes to an end

With battery, Catalan often uses acabar-se:

• La bateria s'acaba. = The battery runs out / dies.

So in this sentence:

• Si la bateria del teu mòbil s'acaba... = If your phone battery runs out...

The apostrophe shows that es has been shortened before a vowel.

The full form is:

• es acaba

But before a vowel sound, Catalan usually contracts it:

• es acaba → s'acaba

This es is the pronoun used with the pronominal verb acabar-se.

Because acabar-se is very common when something gets used up or runs out.

Compare:

• acabar = to finish
• acabar-se = to be finished / to run out

So:

• La classe acaba a les cinc. = Class ends at five.
• La bateria s'acaba. = The battery runs out.

In your sentence, the pronominal form sounds natural because the battery is being depleted.

Because Catalan often uses present + present for a real or likely condition.

So:

• Si la bateria del teu mòbil s'acaba a classe, pots portar un carregador petit.

This means something like:

• If your phone battery runs out in class, you can bring a small charger.

English can do something similar:

• If it happens, you can...

This is a normal structure in Catalan for general advice or real situations.

A classe means in class or during class.

So:

• a classe = in class / at school during a lesson

Catalan often uses a where English uses in.

Examples:

• Som a casa. = We are at home.
• És a classe. = He/She is in class.

So s'acaba a classe means the battery runs out while you are in class.

Pots is the you form of poder (to be able to / can).

• pots = you can

portar usually means to bring, to carry, or to take along, depending on context.

So:

• pots portar un carregador petit = you can bring a small charger

In this sentence, bring is the most natural translation.

Because in Catalan, adjectives often come after the noun.

• un carregador petit = a small charger
• un llibre interessant = an interesting book
• una casa gran = a big house

Some adjectives can go before the noun, but the normal basic position is often after it.

Carregador means charger.

By itself, it can mean any kind of charger, but in this sentence it is clearly understood as a phone charger because of the context:

• la bateria del teu mòbil
• un carregador petit

So you do not need to repeat de mòbil unless you want to be extra specific.

A rough pronunciation guide is:

• Si ≈ see
• la bateria ≈ lah bah-teh-REE-ah
• del teu mòbil ≈ del TEH-oo MOH-beel
• s'acaba ≈ suh-KAH-buh
• a classe ≈ uh KLAH-suh
• pots portar ≈ pots por-TAR
• un carregador petit ≈ oon kuh-rreh-guh-DOR puh-TEET

A couple of useful notes:

• ò in mòbil is an open o
• ll in carregador is not there, but rr is a strong rolled sound
• final consonants are usually pronounced more clearly than in English

Yes, but the meaning changes slightly.

• mòbil = mobile phone / cell phone
• telèfon = telephone / phone more generally

If you specifically mean a cell phone, mòbil is the most natural choice here. So el teu mòbil is better than el teu telèfon in this sentence.