Battery life issue

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Greets,
I'm having a problem with my Prophet.
Sometimes the battery drains very very quickly.
For example, I charged the phone yesterday at 18:00, and today at 16:00 I saw that it was turned off. When I turned it on, it was with 0% battery remaining.
It is strange because generally I get 2 days of battery time, not less than a day.
I did not used the phone since I take it off the charge.
What can be the problem?

I've had this happen too a few times, but never tracked it down.

It seemed to be when the battery got to 40% it dropped very quickly, but I've also had drop to 0% from full in less than a day with very little use.