Rezervna baterija i prijenosni punjač dio su naše zalihe.

Because the subject has two nouns joined by i:

• rezervna baterija
• prijenosni punjač

Together they form a plural subject, so Croatian uses su, the 3rd person plural of biti = are.

Because dio means part, and the idea is that these two items together make up one part of the supply/stock.

This works much like English:

• The battery and charger are part of our supplies.

So:

• su is plural because the subject is plural
• dio stays singular because the predicate is part

If you wanted to emphasize separate pieces, you could use dijelovi = parts.

Here zalihe is genitive singular of zaliha.

That is because dio normally takes the genitive:

• dio čega? = part of what?

So:

• zaliha = stock, supply, reserve
• zalihe = of the stock/supply

A learner may notice that zalihe can also look like a plural form. That is true, but in this sentence the grammar shows it is genitive singular, because it depends on dio.

Yes. That is also correct.

Croatian word order is more flexible than English. The given sentence places dio before su:

• Rezervna baterija i prijenosni punjač dio su naše zalihe.

A very natural alternative is:

• Rezervna baterija i prijenosni punjač su dio naše zalihe.

Both mean the same thing. The version with dio su sounds a bit more stylistic or formal.

Because adjectives in Croatian must agree with the noun in gender, number, and case.

Here:

• baterija is feminine singular nominative, so you get rezervna
• punjač is masculine singular nominative, so you get prijenosni

So the endings are different because the nouns are different genders.

Because possessive words in Croatian also agree with the noun they describe.

The basic word is naš = our, but it changes form depending on gender, number, and case:

• naš
• naša
• naše
• naši
• naših, etc.

Here it goes with zalihe, which in this sentence is the genitive singular form of zaliha, so naše is the correct agreement form.

Zaliha can mean stock, reserve, supply, or stored amount, depending on context.

In this sentence, it means something like:

• our supply
• our stock
• our reserve

It is a useful word for things kept for later use: food, equipment, medicine, spare items, and so on.

Also note this nuance:

• dio naše zalihe = part of our stock/supply
• dio naših zaliha = part of our supplies/stocks

Both are possible in Croatian, but they are slightly different in meaning.

The dictionary forms are:

• baterija — feminine
• punjač — masculine
• dio — masculine
• zaliha — feminine

This matters because gender affects adjective endings, possessives, and some case forms.

Because Croatian does not have articles.

So Croatian does not use separate words corresponding directly to English a/an and the. Definiteness is usually understood from context.

In this sentence, naše already helps make the meaning specific, so Croatian does not need an article.

Yes, very normal.

• rezervan / rezervna / rezervno means spare, reserve, or backup
• baterija means battery

So rezervna baterija is a standard way to say spare battery. Depending on context, you might also hear expressions with dodatna = additional, but rezervna baterija is the straightforward match for spare battery.