zhè kuài diànchí yǐjīng yòng le hěn jiǔ, nánguài diànnǎo zǒngshì méidiàn.

Chinese normally needs a measure word / classifier after a demonstrative like 这 or 那.

So the basic pattern is:

• 这 + measure word + noun
• 那 + measure word + noun

That is why you get 这块电池, not just 这电池.

About 块 specifically:

• 块 is a classifier for pieces, chunks, or single units.
• With 电池, classifier choice can vary by context.
• 节 is very common for cylindrical batteries like AA batteries.
• 块 can sound natural for a battery as a single unit, especially a battery pack or non-cylindrical battery.

You may also hear the more general 这个电池 in everyday speech, but 块 is more specific here.

已经 means already.

It goes before the verb phrase and shows that the situation has reached a certain point by now:

• 这块电池已经用了很久
• This battery has already been used for a long time

So 已经 gives the sentence a sense of by this point / as of now.

Here, 了 is a particle attached to the verb 用.

In this pattern, verb + 了 + duration, it often shows that an action or state has continued for some amount of time up to now.

So:

• 用了很久 = has been used for a long time

This does not mean a simple past like English used in every case. In many sentences like this, it feels closer to an English present perfect idea.

A similar pattern is:

• 我学了三年中文。
• I have studied Chinese for three years.

They often work together.

A very common Chinese pattern is:

• 已经 ... 了

This gives a strong sense of already or by now.

In this sentence:

• 已经 is the adverb meaning already
• 了 is the particle marking the realized/ongoing-up-to-now situation

You can sometimes omit one of them in other sentences, but using both together is extremely common and natural.

That is a very common thing in Chinese.

In English, we might expect something like:

• Someone has used this battery for a long time or
• This battery has been used for a long time

Chinese often lets the thing affected become the topic or subject, while the person doing the action is left unstated if it is obvious or unimportant.

So:

• 这块电池已经用了很久

naturally means:

• This battery has been in use for a long time
• This battery has been used for a long time

The user of the battery is not important here, so Chinese simply leaves that out.

In 很久, the phrase as a whole means a long time.

So:

• 很久 = a long time
• 用了很久 = has been used for a long time

Here, 很 does not need to be translated as a strong very. It is just part of the natural expression.

If you translate too literally as very long, it can sound odd in English. In this sentence, the best natural meaning is simply for a long time.

难怪 means no wonder or that explains why.

It introduces a conclusion based on what was just said.

So the logic is:

• This battery has been used for a long time
• No wonder the computer is always out of power

It is a very common word when you suddenly understand the reason for something.

总是 literally means always, but in many everyday sentences it can also mean:

• constantly
• again and again
• keeps
• all the time

So:

• 电脑总是没电

can mean:

• The computer is always out of power
• The computer keeps running out of power

It does not have to mean every single moment without exception. It often just describes a repeated annoying pattern.

Both are grammatical, but 没电 is the common, compact, everyday way to say:

• out of power
• has no charge
• dead battery

So:

• 电脑没电 = The computer is out of power

没有电 is more explicit and slightly more formal or literal sounding. In conversation, 没电 is extremely common.

Because this is about absence/lack, not a regular negation of an action.

• 没电 means there is no power / no charge
• 没 here is short for 没有

You cannot say 不电.

A useful idea is:

• 不 often negates habits, intentions, or general statements
• 没(有) often means not have, there isn’t, or did not

Since 电 is something the computer lacks, 没电 is the correct expression.

Yes, it is very natural.

In Chinese, 没电 is the normal way to say a device has run out of battery or power.

So depending on context, 电脑没电 can mean:

• the battery is empty
• the computer has no charge
• the computer has run out of power

Chinese often says this very directly, without repeating battery.

Chinese often connects related clauses more directly than English does.

Here the comma links:

• 这块电池已经用了很久
• 难怪电脑总是没电

And 难怪 itself already does a lot of the connecting work, because it means no wonder.

So the whole sentence flows naturally as:

• This battery has been used for a long time, no wonder the computer is always out of power.

You do not need an extra 所以 here.

Yes. 怪不得 and 难怪 both commonly mean no wonder.

So you could also say:

• 这块电池已经用了很久，怪不得电脑总是没电。

Both are natural.
难怪 is a bit shorter, and 怪不得 is also very common in speech.