Your mobile phone doesn't work because the power is low. You want to make a call, so you ask your friend to borrow his phone. He asks why you can't use your own phone. You answer:
I forgot to charge it.
This is a simple phrase to use when there's an action you're supposed to do, but you forget and don't do it. Remember to use "to"!
To "charge" a device means to plug it in and fill the battery. You can use the word "charge" with "battery" or you can use it with the device itself:
Don't forget to charge the camera!
When the battery is full, you say that the device "is charged".
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