I try to carry my cell phone everywhere, in case I fall down, or the car breaks down stranding me on I93, or you name the disaster. My children all informed me that venturing out of the house, or even out of bed without a cell phone was a life risking activity. It's a low end Sam Sung, model AM01. I bought it after Tracfone announced that they would no longer support the even lower end Tracfone I had been using. I got into the habit of plugging the SamSung into the charger when I went to bed in the evening. Smart phone would give a chirp and display battery charge, 80-90%, in short, using up 10-20% of battery power for just sitting in my shirt pocket, not making any calls.
The charger ran into trouble and stopped charging. I noticed the battery charge going down, reaching 7%. I messed around on the keyboard, settings has a "battery" option now (it did not back when I got the phone) and it offered a "max battery saving" option, which I took. I also had a good look at the charger, which has a strange internal connector which had become disconnected. Plugged the charger back together and it started to charge again.
After a day or so I noticed that the battery was at 100% when I plugged it into the charger in the evening. "Max battery saving" was doing some good, the phone was claiming 8 days and a few hours worth of battery charge. That is a goodness. I have had some lengthy power outages up here, but the worst was only six days. That was back before cell phones. So, on "max battery save" my cell phone ought to ride out the worst power failures I will ever suffer up here. The max battery save mode gains much of its saving from doing the screen white letters on a black background, instead of vice versa. That way the battery only has to light the few pixels in the letters, rather than every pixel on the whole screen. I find the screen easier to read the white on black background mode. It also claims to have shut down some applications doing who knows what behind my back. Anyhow, if you are running a low end Samsung phone, try the max battery save option.