“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 like this.
I forgot to charge it.
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":
A: Is it charged?
Charging a battery that has run out of energy can also be called "recharging" it:
I need to recharge my phone.
When you "forget to do" something, it means that you wanted to do something, but you didn't do it because you forgot:
Oh, I forgot to lock the door. Hold on just a second.
Oh shoot. Remy called me yesterday and I forgot to call him back.
Notice that there is a different phrase for when you do something, but then later forget that you did it. That phrase is "I forgot that (clause)":
I forgot that she was a year younger than us. I thought we were all the same age.