بطارية هاتفي ضعيفة في المساء، ولذلك أشحنه في المكتب.

This is an iḍāfa construction, often called a genitive/possessive construction.

• بطارية = battery
• هاتفي = my phone

So literally it is battery of my phone.

A few important things about iḍāfa:

• The first noun usually does not take الـ.
• The second noun makes the whole phrase definite here, because هاتفي already means my phone.
• So بطارية هاتفي = the battery of my phone / my phone’s battery.

The -ي is the possessive suffix meaning my.

• هاتف = phone
• هاتفي = my phone

So in بطارية هاتفي, the second noun is possessed: my phone.

This suffix is very common in Arabic:

• كتابي = my book
• بيتي = my house
• سيارتي = my car

Because it describes بطارية, and بطارية is a feminine singular noun.

In Arabic, adjectives usually agree with the noun they describe in:

• gender
• number
• definiteness

So:

• بطارية = feminine singular
• ضعيفة = feminine singular

That is why you do not say ضعيف here.

In present-tense Arabic nominal sentences, the verb to be is usually not stated.

So:

• بطارية هاتفي ضعيفة
  literally: the battery of my phone weak
• natural English: My phone’s battery is weak

This is completely normal in Arabic. The implied meaning is is.

If you wanted the past, then Arabic would usually use a verb such as كانت:

• كانت بطارية هاتفي ضعيفة = My phone’s battery was weak

Because here ضعيفة is the predicate of the sentence, not an attributive adjective inside the noun phrase.

Compare:

1. بطارية هاتفي ضعيفة
   = My phone’s battery is weak
   Here ضعيفة is the predicate, so it is indefinite.

2. البطارية الضعيفة
   = the weak battery
   Here الضعيفة is a regular adjective directly modifying البطارية, so both are definite.

So in your sentence, ضعيفة is correct without الـ.

في المساء means in the evening.

• في = in
• المساء = the evening

Arabic often uses في with times just as English uses in:

• في الصباح = in the morning
• في الليل = at night / in the night
• في المساء = in the evening

So this phrase tells you when the battery is weak.

ولذلك means and therefore, so, or for that reason.

It is made of:

• و = and
• لذلك = therefore / for that reason

So the sentence structure is:

• The battery is weak in the evening
• and therefore / so I charge it in the office

It links the first statement to the result that follows.

Because the attached pronoun -ه refers to هاتفي (my phone), which is masculine.

• أشحنه = I charge it (masculine it)
• أشحنها = I charge it (feminine it)

Even though بطارية is feminine, the sentence is saying I charge my phone, not specifically I charge the battery.

So the logic is:

• weak thing mentioned: بطارية (battery, feminine)
• thing being charged: هاتفي (phone, masculine)

If the speaker wanted to say I charge the battery, then أشحنها would make sense.

أشحنه breaks down like this:

• أشحن = I charge
• -ه = him/it (masculine singular object)

So together:

• أشحنه = I charge it

The verb is in the imperfect (non-past), first person singular:

• أشحن = I charge / I am charging
• تشحن = you charge / she charges
• يشحن = he charges
• نشحن = we charge

This form can mean a habitual action here: I charge it / I usually charge it.

Arabic often uses attached object pronouns to avoid repetition.

So instead of saying:

• ... ولذلك أشحن هاتفي في المكتب
  = ... therefore I charge my phone in the office

the sentence says:

• ... ولذلك أشحنه في المكتب
  = ... therefore I charge it in the office

Both are grammatical. The pronoun version is simply smoother because هاتفي was already mentioned.

Because في المكتب expresses location, not movement.

• في المكتب = in the office / at the office
• إلى المكتب = to the office

In the sentence, the speaker is saying where they charge the phone, not that they are going somewhere.

So:

• أشحنه في المكتب = I charge it in the office / at the office

It contains both types of structure.

The first part is a nominal sentence:

• بطارية هاتفي ضعيفة في المساء
• literally: My phone’s battery weak in the evening

The second part is a verbal sentence:

• أشحنه في المكتب
• I charge it in the office

This is very common in Arabic: one clause can be nominal, and the next can be verbal.

Arabic offers more than one way to express possession-like relationships, but بطارية هاتفي ضعيفة is the most straightforward and natural here.

This pattern is especially common when one thing is literally a part of another:

• باب البيت = the door of the house
• شاشة الهاتف = the screen of the phone
• بطارية هاتفي = my phone’s battery

So for a learner, بطارية هاتفي is the best model to follow.

In normal modern Arabic writing, short case endings are usually not written.

So you see:

• بطارية هاتفي ضعيفة في المساء، ولذلك أشحنه في المكتب

In fully vowelled formal grammar style, the endings would show case more clearly, but in everyday printed Arabic they are usually omitted.

A learner should still know the underlying grammar:

• after في, the noun is logically in the genitive
• in an iḍāfa, the second noun is also logically genitive

But in ordinary text, these endings are typically not shown.