Ok, this is a common issue and the solution is relatively easy. The problem arises from the fact that you never let the phone run down below 25%, and never power it off. The phone's charging circuit needs to see the bottom range of the battery (10%), every once in a while in order to maintain accuracy with the battery level meter.
A battery is not like a car's gas tank where it always holds the same amount of gas with each fill up. A phone's battery ages and gets weaker over time. As a result, the amount of charge (current) it can hold when 100% full actually diminishes over time. After about 2 years a phone's battery will only hold about 80% of it's rated capacity when new, but will always display 100% when fully charged. The difference is it's displaying what percentage of the ACTUAL capacity it is charged to, not at what percentage of it's RATED capacity it's charged to.
For this reason, the phone must set and keep two "flags" (markers), updated to identify the battery's charged level and it's discharged level - essentially monitoring the battery's age or health. It's from these two flags along with monitoring the voltage during use that it calculates and displays the remaining charge for you. If the flags are not re-calibrated, over time the meter becomes miss-aligned or falls out of calibration with the battery's true capacity and charge level, and begins to display inaccurate levels.
Eventually it can result in what you are experiencing where it appears to have plenty of charge one minute, and then is completely dead a very short time later. If this out-of-calibration state is not corrected by the following procedure, it can eventually result in the phone becoming completely unresponsive to the power button, and eventually even to the charger.
In order to resolve this, about once every two or three months you should completely power off the phone, then connect it to the charger it came with and allow it to charge completely with the power off. To see the progress, you can briefly press and release either one of the volume controls, but NOT the Power button. In a couple seconds it should display a large battery icon and percentage of charge for about 10 to 15 seconds and then the display will go black again. Allow it to reach a full 100% before powering it back up again.
This initial 100% charge with power off sets the "charged" or "full" flag. It's important that this is done while powered off because while the phone is on it's using power. When charged with power on, even with the screen off the meter can get confused about exactly how much power the battery is pulling during the charge in order to determine when the battery is full. The meter can see a surge in power use by the phone such as while background syncing email or Facebook, and the subsequent drop-off in power consumption once the syncing stops and mistake it for the characteristic drop in current draw white charging that happens when a battery approaches 100% full. This can cause the charger to interrupt the charging process far too soon and this results in the meter setting the "full" flag at a much lower charge level than the battery's true capacity. That's the start of the disconnect or divergence between the battery's true capacity and level of charge versus what the meter believes it to be and displays as the level.
Ok, so then once fully charged to 100% (about 3 hours for the typical phone, or up to 5.5 hours for large capacity batteries such as the MAXX), remove it from the charger, power it up and use it normally, but in this first usage period after the full charge you want to allow the phone to reach a nearly complete discharge level. The phone will tell you when it is discharged and that you need to connect it to the charger. This is usually 10% - 15%, depending on the phone or operating system version. We DO NOT want the phone to reach 0% under almost any circumstances, as this can cause it to become completely unresponsive.
When it reaches that discharged state and the pop-up window telling you to charge appears, that is when the phone's meter sets the "discharged" flag. We're almost done. The last step is to repeat the first step where you powered it down and charged to 100% with the power off. Once you complete the second charge to 100% with the power off, you're done. If you perform these three steps in that order and as instructed, you will have successfully re-calibrated the phone's meter to the battery. You can now power the phone back up and use and charge in your normal patterns, but don't forget to perform this three step process again in about two to three months.
From there on, you shouldn't suffer the sudden and unexpected complete discharge and automatic shut-down you experienced, or the failure of the phone to respond to the power button or charger. In the future if you should ever find the phone won't respond to the power button, first try plugging it into the charger. If it begins charging and displays 0%, just let it remain on charge to 100% and follow the three steps above and you should be fine.
If the phone should ever fall to respond to the charger as you experienced, there are several ways to regain control...one of which is the hard-shutdown method you used by pressing and holding the power and volume down buttons together until it begins to boot. If you are forced to resort to that method of recovery, make sure to plug into the charger immediately upon the phone showing signs of life...you may only have seconds to react before the phone shuts down again and the next time it may not respond to the same recovery method.
Good luck!
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