Android smartphone battery suddenly drained twice as fast than usual

Question

I own an Android smartphone that I'm very little using (even to phone!), so since I got it (about one year) I use to be notified for charging it only once every other day.
(the only notable service permanently active is probably the wi-fi scanning)

But suddenly, four days ago, I was notified of low level battery about 24 hours after its previous complete charge. The next day I was notified after only 23 hours... and today after only 19 hours!

I'm pretty surprised, not only because this happens suddenly, without having changed anything in my way of using it, but also because looking at the Settings > Battery usage I find something like this:

• Screen: 15%
• Android system: 13%
• Android OS: 7%
• Idle phone: 3%

...and nothing else, so the total is only 38%!
I'm pretty surprised because in my mind, depending on how to understand the list meaning:

• If the list presents the consumed parts compared to the whole battery power the total should be equal to 100% - the reported battery state; in the current example, battery state was 13% so the total should be 87%
• Or the total should merely be 100% if the list presents the part of each component compared to the current consumption.

So here are two indissociable questions:

• What might be the cause of the sudden increase of consumption?
• What is the correct interpretation of the battery usage information?

If it'd be helpful: my smartphone is an Alcatel OneTouch Idol 3.
It was recently updated from Lollipop to Marsmallow (6.0.1), but this happened about 2 months before I suddenly face the issue related above.

Answer 1

Next day, seems that all works fine again, apparently due to one of these two simple actions (but I don't know which one, or both, are the real remedy!):

1. Just after the most recent charge I temporarily (see below) turned the device into Economy mode (using Settings > Battery | Top right menu > Economy mode).
2. Having looked at several related questions here in AE, I wanted to try clearing the system cache, but inadvertently missed the double press power+volume so I ended merely turning the device off then on, instead of processing clear cache; then I gave up for the moment.

The unexpected (but fortunate!) result was that the battery level remained > 90% during the next 12 hours, so I decided to deactivate the Economy mode and wait to see what happens.

Now, about 24 hours later, the battery level is still 62%, and the chart shows a prevision of 4 days availability before to be empty: four times better than when I noticed the issue, and even twice better than it used before the issue appears!

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EDIT: 10 days later, I can confirm that the improvement is real and persists. But there is more.

A very interesting (although unexplained!) point is that not only the battery stopped emptying in less than a day, but now it takes about 4-5 days before reaching 10%: in other words now the battery life cycle is more than 2x better than ever since I got the device!