Learn Hardcore Croatian: Baterija na mobitelu je skoro prazna, trebam punjač. - The battery on my phone is almost empty, I need a charger.

na + locative often means on, on top of, on (a surface or device).
- na stolu – on the table
- na televizoru – on the TV
- na mobitelu – on the mobile phone

Here it really means the battery on the phone (the phone’s battery).

Why not the others?

u mobitelu = inside the phone. Grammatically correct, but it would sound like you are talking about something physically inside the phone, not the usual way to refer to the phone’s battery.
baterija mobitela (genitive) is also possible and means the phone’s battery, but it sounds a bit more like a written/technical phrase (the battery of the phone). Baterija na mobitelu is more everyday speech.

Why does mobitelu end in -u? What case is that?

Mobitelu is the locative singular form of mobitel (masculine noun).

In Croatian, the preposition na meaning on, on top of is normally followed by:

locative when it describes a location:
- na mobitelu – on the phone
- na stolu – on the table
- na televizoru – on the TV

For masculine nouns ending in a consonant (like mobitel), the locative singular often ends in -u:

na mobitelu
na poslu (at work)
na kompjuteru (on the computer)

Is there a reason baterija is feminine and prazna also ends in -a?

Yes. In Croatian, adjectives must agree with the noun in:

gender (masculine / feminine / neuter)
number (singular / plural)
case

Baterija is a feminine noun (it ends in -a in the nominative singular), so the adjective prazan (empty) must take the feminine nominative singular form:

masculine: prazan (e.g. prazan punjač – an empty charger)
feminine: prazna (e.g. prazna baterija)
neuter: prazno (e.g. prazno mjesto – an empty seat)

In the sentence, baterija is the subject in the nominative case, so the adjective is prazna.

What exactly does skoro mean here, and how is it different from gotovo or uskoro?

In this sentence, skoro means almost:

skoro prazna – almost empty

Skoro and gotovo can both mean almost, and in this context they’re interchangeable:

Baterija je skoro prazna.
Baterija je gotovo prazna.

Both are fine.

Be careful with uskoro, which means soon:

Vidimo se uskoro. – See you soon.

So:

skoro / gotovo = almost
uskoro = soon

In everyday speech, people sometimes use skoro loosely in a time sense (kind of like soon), but the standard, clear way to say soon is uskoro.

Why is there just je and not some longer verb phrase like in English (is getting empty, is almost dead)?

Croatian generally prefers simple structures here:

je is the 3rd person singular of biti (to be).
prazna is an adjective, empty.

So je prazna literally = is empty.

To express the idea is almost dead / running out, you typically just say:

Baterija je skoro prazna.
Baterija je pri kraju. (The battery is nearly at the end.)

Croatian often uses an adjective with biti instead of progressive forms (is getting / is becoming). There’s no progressive tense like in English; context and adverbs (skoro, već, sad) give the nuance.

Why is there a comma before trebam punjač instead of a conjunction like pa or jer?

You have two independent clauses:

Baterija na mobitelu je skoro prazna – The battery on the phone is almost empty.
trebam punjač – I need a charger.

In Croatian, it’s quite common in informal style to simply separate such clauses with a comma when the connection is obvious, especially when speaking:

Baterija na mobitelu je skoro prazna, trebam punjač.

You could also add a conjunction:

..., pa trebam punjač. – ..., so I need a charger.
..., zato trebam punjač. – ..., therefore I need a charger.
..., jer trebam punjač. – ..., because I need a charger. (This one changes the logic a bit.)

All are grammatically acceptable; the original is just a compact spoken style.

How does trebam work here? What’s the difference between trebam punjač, treba mi punjač, and punjač mi treba?

All three can express I need a charger, but the structure is different.

Trebam punjač.
- trebam = I need (1st person singular)
- punjač = direct object (accusative)
- Very direct: I need a charger.
Treba mi punjač.
- treba = 3rd person singular
- mi = to me (dative)
- Literally: A charger is needed to me.
- Very common and natural in speech.
Punjač mi treba.
- Same elements as 2, but with changed word order.
- Emphasis slightly more on punjač (the charger), like: It’s a charger that I need.

All are correct. The sentence you have uses the simple Trebam punjač, which is clear and neutral.

Why is punjač in this form? Shouldn’t it change in the accusative?

Punjač is a masculine inanimate noun.

For masculine inanimate nouns, the nominative and accusative singular forms are usually the same:

nominative: ovaj punjač – this charger (subject)
accusative: imam punjač – I have a charger (object)

By contrast, masculine animate nouns (especially people) change in the accusative:

nominative: vidim brat (wrong), brat – brother
accusative: vidim brata – I see (my) brother

So:

trebam punjač – punjač is accusative, but it looks the same as the nominative because it’s inanimate.

How do I know whether to understand baterija and punjač as the battery / the charger or a battery / a charger when there are no articles in Croatian?

Croatian doesn’t have articles (a / an / the), so the definiteness is inferred from context.

In this context:

You obviously mean the battery of your phone, not some random battery.
You mean a charger (any suitable charger), unless the context is specific (e.g. a particular charger you both know about).

Croatian just says:

Baterija na mobitelu je skoro prazna, trebam punjač.

In English you choose:

The battery on my phone is almost dead, I need a charger.

When translating into English, you add a / the / my based on what makes sense in that situation, not from anything explicitly present in the Croatian sentence.

Can the word order in the first clause be changed, like Na mobitelu je baterija skoro prazna?

Yes, Croatian has fairly flexible word order, and several variants are possible:

Baterija na mobitelu je skoro prazna. – neutral, very natural.
Na mobitelu je baterija skoro prazna. – also correct; slight focus on na mobitelu (on the phone).
Baterija je skoro prazna na mobitelu. – correct but sounds a bit awkward; we don’t usually push na mobitelu to the end here.

The original version is the most straightforward and idiomatic. Other permutations are possible, but they can change the emphasis or sound less natural in everyday speech.

Is mobitel the only correct word, or can I also say telefon or mobitelni telefon?

In modern everyday Croatian:

mobitel is the standard and most common word for mobile phone / cell phone.
telefon usually suggests a regular landline phone, but in casual speech some people also use it for mobiles.
mobilni telefon is more formal or technical (like saying mobile telephone).

So for everyday speech, mobitel is the safest and most natural choice:

Baterija na mobitelu je skoro prazna...

Is prazna the only way to say that the battery is low? Are there other common phrases?

There are several natural ways to express this idea. Besides:

Baterija na mobitelu je skoro prazna.

You might hear:

Baterija mi je pri kraju. – My battery is almost at the end.
Baterija je skoro na nuli. – The battery is almost at zero.
Mobitel mi je skoro crko. (very colloquial/slang) – My phone is almost dead.
Imam malo baterije. – I have little battery left.

All of these can be followed by something like:

Treba mi punjač. / Trebam punjač. – I need a charger.

Prazna literally means empty, but in this context it maps well to English dead / almost dead when talking about batteries.