I had the issue in this topic, and got it solved.
Here are forums where I have found similar case and solution.
I have a particular case. My phone is a HTC One M8, but I have protected it with a special case with a battery extension, home made. I have assembled myself the batteries, chargers and boosters. My booster has a small issue and does not enter safe mode when battery is low. It warns about low batt with a LED, but still tries to deliver 5V. The booster is connected to the phone via a flat cable, stolen from a wireless charger; those chargers have the smallest USB plus I could find ever, and, I found this plug only in these decvices.
When the main battery of the phone is empty, I switch the booster on, and recharge the phone with the backpack. I have to look at the LED; when it's red, I have to switch it off. Everynight, everything get charged on the same single plug. One plug to charge both batts.
The advantages of my solution are:
- phone can last 18h with heavy work (wifi, bluetooth, GPS, and screen on, continuously)
- heavy case to protect my phone, plus tampered glass
- one plug to charge everything
- the micro-micro USB plug is ALWAYS in the phone. This means, the female plug of phone can not get dirty, or filled with dust. I never unplug it, so it does not age. I never plug a classic USB cable directly in the phone, so, no chance to break it. I have broken 3 or 4 times the back charger plug; if I am in hurry, I can exceptionally charge the phone directly; next day home, I can fix it.
This night, I was using my phone, and switched the backpack on. I could not plug my wall charger, because it's home made, has an issue, and makes the virtual keyboard erratic. I fell asleep with the phone in my hand, and the backpack went red ... for 4 hours. During 4 hours, it was trying to deliver 5V, and failed at so. Since my battery was delivering an unstable +5V, the phone was trying to charge, was staying on, but could not properly charge. In the end, all batteries got flat.
This morning, when I woke up, I plugged the stuff; only the back-batt started charging. Phone itself refused to charge; screen was black, and charging LED stayed off. So, I fell in what we call the "phone refuses to charge".
But, because of my set-up, I can *NOT* be in the dirty or broken plug case. Just impossible. And because of context, I just knew I had a flatt battery. This is technically called "deep discharged". And it contributes to aging of batt. Like overcharging, this heavily affects the total capacity.
I knew I could fix the issue by opening the phone, and putting a LiPo charger directly on the batt lines. I have loads of standalone chargers. I had a standalone wall charger for my previous HTC Sensation. But with the M8, we can not just open the case, and take the batt out. It's much more complicated. So, I have been Googling for more than half an hour. 95% results were about "remove dust of plug" and "bring it to the shop to change your plug". Some people also mention "a bug in the charger chipset".
I have plugged the phone on a classic USB charger, but via an USB meter monitor:
dx.com/p/382581 (dispite description, the device can accept up to 13V on input - this detail is critical when you use real QC2 chargers able to deliver 9 and 12V, like this one:
The USB meter was indicating 5V 0.03 A (or 9V 0.01 A, depending on source). So, the phone was using a bit of energy, but clearly not charging. I have left it 4h. Just in case, can not harm.
Then, I pressed together power, and volume up. The phone started to boot after 55 or 65s. About one minute; but clearly way longer than 42 or 47s found on forums. It started directly to some rescue mode, then, started to boot normally, without doing anything. Just released buttons during the first white screen. After booting, I have a normal phone. Did not need to perform any action. During boot, the phone had 2% batt, and was charging normaly. But, the charger LED remained off; I waited 5mn, untill I got 4%, unplugged the charger, replugged, and then got the orange LED.
Remember that your M8 can accept up to 9V on input. QC2 Chargers are really faster: with 9V 1.5A, you can charge your M8 from 0% to 100% in 95mn, even if you are intensively using it (gaming, wifi, GPS). Next gen phone will accept 12V. Even if manufacturers ignore that. HTC buys chips, and don't fully read specs. Aukey chargers are more powerfull and better reliable than official HTC ones.
Remember that many "slow charging" issues are in fact due to bad cable: charger is good, phone is good, but cable in between has high gauge copper, too thin wires, and produces voltage drop between plugs. HTC phones require 4.4V at the plug.
Remember that M8 and most modern phones (same for Samsung) are now MHL (wrongly called OTG): they are true USB hosts; you can plug any device on your phone; just buy an MHL (OTG) cable for 4 euros, and then a USB hub, and then any device: mouse, keyboard, USB monitor, ethernet dongle, SATA adapter, CF-uSD adapter ... and for 12 euros you have HDMI converters. Modern smartphones are real computers. Use them.