У моего телефона слабый аккумулятор, и к вечеру он почти всегда разряжается.

Russian very often expresses possession with у + genitive, especially in everyday speech.

So У моего телефона слабый аккумулятор literally works like My phone has a weak battery or even more literally At my phone, there is a weak battery.

The version Мой телефон имеет слабый аккумулятор is grammatically possible, but иметь often sounds more formal, technical, or bookish in Russian. In normal conversation, у + genitive is much more natural.

Both words are in the genitive case because the preposition у requires the genitive.

Base form:

• мой телефон

After у:

• у моего телефона

So:

• мой → моего
• телефон → телефона

This is a very common pattern:

• у меня
• у брата
• у машины
• у моего телефона

Because in this kind of possession structure, the thing being possessed is usually the grammatical subject of the sentence, so it stays in the nominative.

In У моего телефона слабый аккумулятор:

• у моего телефона = the possessor
• слабый аккумулятор = the thing that exists / the subject

So Russian is structurally closer to:

• My phone has a weak battery than to
• Of my phone, of a weak battery

That is why аккумулятор is nominative singular, and слабый agrees with it.

Yes, but here слабый means something like poor, not very good, or not holding charge well, not physically weak in a literal sense.

So слабый аккумулятор suggests:

• the battery is bad
• the battery capacity is poor
• the battery does not last long

It does not usually mean that the battery is low right now.

If you want to say the battery is currently low or empty, Russian would more naturally use something like:

• аккумулятор разряжен
• батарея села
• телефон разрядился

Both can refer to a phone battery, but there is a nuance.

• аккумулятор is more specific and technical: a rechargeable battery
• батарея is more general and very common in everyday speech

So for a phone, both are possible:

• У телефона слабый аккумулятор
• У телефона слабая батарея

The first sounds a bit more technical or precise. The second is very natural in everyday speech.

К вечеру means by evening or toward evening. It focuses on the point in time by which something happens.

So:

• к вечеру он почти всегда разряжается = by evening it has almost always run down

But вечером means in the evening and just places the action during that time period.

Compare:

• К вечеру телефон разряжается = the phone is dead by evening
• Вечером телефон разряжается = the phone runs down in the evening

In this sentence, к вечеру fits better because the idea is that by that time, the battery is usually already gone.

It is dative singular because the preposition к requires the dative.

Base form:

• вечер

After к:

• к вечеру

This is a common time expression pattern:

• к утру = by morning
• к обеду = by lunchtime
• к вечеру = by evening
• к ночи = by nightfall

Grammatically, it could refer to either one, because both телефон and аккумулятор are masculine singular.

In real usage, many speakers will understand он here as телефон, because Russian often says:

• телефон разряжается = the phone is running out of battery / losing charge

But because аккумулятор is also nearby and also makes sense with разряжается, there is some ambiguity.

If you want to avoid that ambiguity, you could say:

• У моего телефона слабый аккумулятор, и к вечеру телефон почти всегда разряжается.

or

• У моего телефона слабый аккумулятор, и к вечеру аккумулятор почти всегда разряжается.

Because Russian often uses the present tense of the imperfective verb for things that happen regularly or habitually.

Here the key phrase is почти всегда = almost always. That tells you this is a repeated pattern, not a one-time future event.

So:

• он почти всегда разряжается = it almost always runs down / it almost always loses charge

This is a general habit:

• every day or on most days, by evening, that happens

If you wanted a one-time future meaning, you would use a different form, for example:

• разрядится = will run down, will become discharged

Because the sentence describes a repeated, usual process.

The imperfective разряжаться is the normal choice for:

• habitual actions
• repeated events
• general facts

That matches почти всегда very well.

The perfective partner разрядиться would be used more for a single completed event:

• К вечеру телефон разрядится = By evening, the phone will run out of charge

But in your sentence, the speaker is describing what generally happens, so разряжается is the right form.

Russian word order is flexible, and the beginning of the clause often highlights the time or setting.

So:

• и к вечеру он почти всегда разряжается

puts the time frame first: by evening.

You could also hear:

• и он почти всегда к вечеру разряжается

That is also understandable, but the original order sounds very natural because it sets up the time condition first.

Literally, разряжается means becomes discharged.

With phones and batteries, it commonly means:

• runs out of charge
• loses battery
• goes dead

It can be used with:

• аккумулятор разряжается
• батарея разряжается
• телефон разряжается

So Russian allows both the battery itself and the device to be described this way. In everyday speech, saying that the phone discharges is very normal.