Battery
noun
A pond for electricity? May seem weird at first, but if you think of something that's gathering all the electricity into one pond... that thing is like a battery (and is a battery).
でんち
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
スマホの電池が切れた。
The battery of the smartphone has run out.
このラジオ、電池が入ってないよ。
There is no battery in this radio.
この電池は大きすぎます。
This battery is too big.