“I forgot to charge it.”

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.

forget to (do something)

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"!

charge (a battery or device)

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!