The watch's battery is worn down.
电池没电了。
1 battery 电池
replace the battery
更换电池
The battery is flat.
电池没电了。
a battery of..
一系列；一批；一大群
He faced a battery of questions.
他面临一系列问题。
2 worn down 磨损不堪的；疲惫不堪的..
My heels of my shoes have worn down.
我的鞋跟磨损了。
worn out
疲惫的
After the race,he is worn out.
赛后，他疲惫不堪。

A:What can I do for you?
有什么可以帮您的？
B:Yes.There is something wrong with my watch.It stopped several hours ago.
是的。我的表坏了。几个小时前它不走了。
A:Let me have a look.The watch's battery is worn down.
让我看看。电池没电了。
B:How much is a battery?
一个电池多少钱？
A:50 dollars.
50美元。
B:How long will it last?
能用多长时间？
A:About 2 years.
大约两年。