Learn Hardcore Serbian: Mislim da znam rešenje: moraš da napuniš bateriju. - I think I know the solution: you have to charge the battery.

In Serbian, after verbs like mislim (I think), znam (I know), želim (I want), and many others, it is very common to use da + present tense instead of an infinitive.

So:

Mislim da znam rešenje. = I think I know the solution.

This structure is extremely normal in everyday Serbian.
A literal infinitive-style structure like Mislim znati... would sound wrong.

A useful pattern to remember is:

mislim da... = I think that...
znam da... = I know that...
moram da... = I must...

Serbian does have infinitives, but in many common situations, especially in modern speech, da + present is preferred.

Why is there another da in moraš da napuniš?

This is the same kind of pattern: da + present tense after a modal or similar verb.

moraš da napuniš = you have to charge
literally: you must that you charge

In natural English we do not say it that way, but in Serbian this is standard.

So the sentence contains two different da clauses:

Mislim da znam rešenje = I think that I know the solution
moraš da napuniš bateriju =