the battery is down?

意思肯定就是电池没电了但可以说这种用法在书面上不是很普遍,不过在阅读中应该是可以理解的。
总结一些用法如下：
The battery has run down
The battery is running out
The battery is out of power
The battery is dead(查了一下,还真有)
The battery is flat
互相学习。