Learn Hardcore Croatian: Ako ne napunimo mobitele, baterija će se isprazniti usred razgovora. - If we don’t charge our phones, the battery will run out in the middle of the conversation.

This is not the standard way to form a neutral conditional.
Nećemo napuniti in the ako-clause would usually sound odd or would be understood as:
- “If we refuse / don’t want to charge the phones…” (emphasis on won’t as a choice).
For a normal, neutral “If we don’t charge the phones…”, you should stick with:
Ako ne napunimo mobitele…

Why do we say napunimo and not punimo?

This is about aspect (perfective vs. imperfective verbs):

puniti – imperfective: “to be charging, to charge (as a process/habit)”
napuniti – perfective: “to charge up, to fill completely (one finished action)”

In your sentence:

Ako ne napunimo mobitele…
→ focuses on one complete charging that should happen before the conversation.

If you said:

Ako ne punimo mobitele…

it would sound more like:

“If we don’t (generally) charge our phones / if we don’t keep charging our phones…”

So napunimo is chosen because we’re talking about failing to do one necessary complete charge, not about an ongoing habit.

What exactly is going on in baterija će se isprazniti? What are će, se, and isprazniti?

The phrase baterija će se isprazniti breaks down like this:

baterija – “the battery”, subject, nominative singular
će – future tense auxiliary (from htjeti, but here just a future marker)
se – reflexive clitic pronoun (unstressed, always sticks near the beginning of the clause)
isprazniti – infinitive, perfective verb, “to empty”

Together:

baterija će se isprazniti = “the battery will empty (itself) / will run out”

So grammatically it is:

[subject] + će + se + infinitive = 3rd person singular future tense of the reflexive verb isprazniti se.

Why do we need the reflexive se in baterija će se isprazniti? Could I just say baterija će isprazniti?

You need se here because the meaning is “the battery becomes empty,” not “the battery empties something else.”

isprazniti nešto (without se) = “to empty something” (transitive)
- e.g. Isprazniti čašu – “to empty the glass”
isprazniti se = “to become empty, to get emptied” (reflexive / intransitive)
- e.g. Baterija se ispraznila. – “The battery ran out.”

So:

baterija će isprazniti is incomplete: it would mean “the battery will empty … (what?)”
Correct for “the battery will run out” is baterija će se isprazniti.

Why is it mobitele here, and what case is that?

Mobitele is in the accusative plural:

Base form (nominative singular): mobitel – “mobile phone”
Accusative plural: mobitele

We use the accusative because:

napuniti (što?) – “to charge (what?)”
The thing being charged is the direct object, which goes into the accusative.

So in Ako ne napunimo mobitele…, mobitele is simply “(the) phones” as the direct object of napunimo.

Why is baterija singular while mobitele is plural? Shouldn’t it be plural baterije to match?

There is no requirement for baterija (subject) to match mobitele (object) in number.

baterija – subject of the second clause (what will run out)
mobitele – object of the first clause (what we charge)

In context, there are a few possible interpretations:

It could mean “the battery” of whichever phone we’re talking on at that moment.
It can sound general: “the battery” (as a typical phone battery) will run out.

You could say baterije će se isprazniti if you want to stress that all the batteries (of multiple phones) will run out, for example:

Ako ne napunimo mobitele, baterije će se isprazniti usred razgovora.

Both are grammatically fine; the choice is about what you want to emphasize.

What does usred mean grammatically, and which case does it require in usred razgovora?

Usred is a preposition meaning:

“in the (very) middle of”, “in the midst of”

Grammatically:

usred always takes the genitive case.

So:

Base noun: razgovor – “conversation” (nominative)
Genitive singular: razgovora

Therefore:

usred razgovora = “in the middle of the conversation”
with razgovora in the genitive because of usred.

Why is it razgovora and not razgovor?

Because usred requires the genitive case.

Nominative: razgovor – used as the subject, or in dictionary form
Genitive: razgovora – used after usred, and also after prepositions like tijekom, poslije, prije, etc.

So:

usred razgovora (genitive) – correct
usred razgovor (nominative) – incorrect

Can the word order be changed, for example to Baterija će se usred razgovora isprazniti ako ne napunimo mobitele? Does the meaning change?

Yes, Croatian allows fairly flexible word order. Some natural variants are:

Ako ne napunimo mobitele, baterija će se isprazniti usred razgovora.
Ako ne napunimo mobitele, usred razgovora će se baterija isprazniti.
Baterija će se usred razgovora isprazniti ako ne napunimo mobitele.

The basic meaning stays the same; what changes is emphasis:

Putting usred razgovora earlier highlights when it will happen.
Putting baterija at the beginning highlights the battery.
The original order is the most neutral and textbook-like.

What you generally cannot change is the internal order of clitics like se and će in an arbitrary way; for example:

baterija se će isprazniti – sounds wrong.
se wants to be very early in the clause (usually in second position).

What is the difference between ako and kad here? Could I say Kad ne napunimo mobitele, baterija će se isprazniti usred razgovora?

Both ako and kad can introduce a clause that looks like a condition, but they are not the same:

ako = “if” (real condition, uncertain whether it will happen)
kad = “when” (time; often implies that it does happen, or will certainly happen)

In your sentence:

Ako ne napunimo mobitele…
→ “If we don’t charge the phones…” (maybe we will, maybe we won’t)
Kad ne napunimo mobitele…
→ tends to sound like “When we don’t charge the phones…”
(as in, whenever it happens / on those occasions)

You can say it with kad, but it then feels more like a repeated / typical situation, not a single hypothetical one. For the usual one-time warning, ako is the natural choice.

How would I say the opposite idea: “If we charge the phones, the battery will not run out in the middle of the conversation”?

You can say:

Ako napunimo mobitele, baterija se neće isprazniti usred razgovora.

Points to notice:

neće is the negative of će.
With a reflexive verb in the future, se still stays very early in the clause:
- baterija se neće isprazniti (natural)
- baterija neće se isprazniti – sounds wrong or at least very unnatural.

So the pattern is:

[Subject] + se + neće + infinitive
→ Baterija se neće isprazniti.

Could I use the singular mobitel instead of plural mobitele, and would that sound natural?

Yes, but the meaning shifts a bit.

Ako ne napunimo mobitel, baterija će se isprazniti usred razgovora.

This suggests:

There is one particular phone that we are talking about (for example, a shared phone).

The plural in the original:

Ako ne napunimo mobitele…

sounds more like:

“our phones” in general (yours and mine, or everyone’s phones).

Both forms are grammatically fine; choose singular or plural depending on whether you mean one specific phone or phones in general / several phones.

Where exactly does ne go in the first part: why ne napunimo mobitele and not something like napunimo ne mobitele?

In simple verb forms, ne goes directly in front of the conjugated verb:

napunimo → ne napunimo
punimo → ne punimo
ćemo napuniti → nećemo napuniti

So:

Ako ne napunimo mobitele… – correct
Ako napunimo ne mobitele… – incorrect; ne cannot be placed between the verb and its object like that.

In compound tenses with clitics, ne attaches to the auxiliary (neću, neće, etc.), but the basic rule you see here is: put ne immediately before the finite (conjugated) verb form.