Learn Hardcore German: Das Solargerät lädt den Akku schnell auf. - The solar device charges the battery quickly.

Solargerät is a neuter noun in German, which is why it takes the definite article das. Compounds ending in -gerät (“device”) are almost always neuter (e.g. das Küchengerät, “the kitchen appliance”).

How is Solargerät formed as a compound noun?

It’s a combination of Solar (“solar”) + Gerät (“device”). In German, two (or more) nouns can be joined into one word, and the final element (Gerät) determines the gender and basic meaning.

What case is den Akku in, and how do we know?

den Akku is in the accusative case (singular masculine). It’s the direct object of the verb aufladen (“to charge up”). Masculine nouns have den in the singular accusative.

Why is it den Akku and not dem Akku?

Den marks the accusative case, which we use for direct objects. Dem Akku would be dative (indirect object), but here the battery is what’s being charged, so it’s accusative.

Why does auf appear at the end of the sentence?

Because aufladen is a separable-prefix verb. In the present tense, the prefix (auf) detaches and moves to the end, while the stem (lädt) stays in second position: 1) infinitive: aufladen
2) conjugated: Er lädt … auf.

Why is schnell placed between den Akku and auf?

Adverbs of manner (like schnell, “quickly”) typically go in the middle field, after the object but before the separated prefix. The order is: 1) Subject (Das Solargerät)
2) Verb (lädt)
3) Object (den Akku)
4) Adverb (schnell)
5) Separated prefix (auf)

Is schnell here an adjective or an adverb?

It’s an adverb modifying the verb aufladen. In German, many words look identical as adjectives and adverbs; here, schnell tells us how the action is done (“quickly”), so it’s an adverb.

What’s the difference between laden and aufladen?

laden by itself can mean “to load” (e.g. loading cargo).
aufladen (with the separable prefix auf-) specifically means “to charge” (e.g. charging a battery). The prefix adds the sense of filling up or replenishing.