Hello Reddit. Kobo Libra Colour 32Gb 7" (N428) is a horrible device. It got shelved for a year+ and the battery deep discharged. Old phones can do better. No amount of re plugging the usb charger would fix the issue of device not taking any charging.
Your warranty is void and I'm not liable for your potential further losses.
Diagnosis - batterry discharged to exactly 2.0 volts. Nominal is around 3.8 volts. How did I find it out? With a multimeter, of course. There's two giant metal tabs on BMS, which is hidden behind paper insulator.
Undestanding - Every (useful) battery has BMS - Battery management system. Somehow combination of hardware and software works poorly in this device, making it so device discharges it way bellow a level BMS considers safe. As 2.0V is clearly not a safe voltage, indicating some kind of fault, the BMS refuses to charge.
Solution - THIS IS MILDLY DANGEROUS IF YOU DO NOT KNOW HOW TO HANDLE BATTERIES, ESPECIALLY LITHIUM - we charge the battery manually with extremely low current and voltage close to nominal. To do this, we need two wires and either lab power supply, or something like step-down converter module which I've used. My specific model is a red board with two tuning pots and three leds - XL4015 CC/CV Input 5V-38V Output 1.25V-36V 75W 5A .
What's critical to know is the rechargeable Li-ion Battery model EVE188595QH has 3.87V as nominal voltage, charging limit voltage of 4.45V. You also shouldn't attempt to puncture it, it will result in a fireball.
Execution - reference the photo, and marking on the board on top of the battery, which is hidden in paper cover. You can separate it, the whole device is plastic anyway. There's two giant tabs - marked B+ and B-. Get some thin wires and attach them with tape onto those tabs. Jumper cables from arduino kits work fine. Make sure they don't short out with each other.
Calibrate the power supply / step down power module to something reasonable - I started at 2.0v, so 3.0v at 20mA, followed with 3.6V 20mA sounded good. Crank down (anticlockwise) both pots. Then tune one of them (left?) while measuring with multimeter its output till you see 3V. Then switch into measuring mA mode (reattach multimeter cables to appropiate slots) and measure your PSU with the multimeter. Adjust the other pot as needed till you reach like 20mA. Safe values would be somewhere between 10mA and 100mA. Please note, 3.0V is kinda low still, so I needed to adjust the voltage eventually to 3.6V which is close enough to nominal voltage for ,,normal'' usb charging.
PSU good? E-reader cables good? Great. Check voltage across wires if needed.
Charging -Put the e-reader in glass container in case it decides to burn (the stains on mine are from dishwasher salt). Attach the wires, plus to plus, minus to minus. Connect the power supply. Turn it on, make sure nothing is getting terribly warm or smelling terrible. Wait a bit (15 minutes minimum, could be hours though). Re-check the voltage while the PSU is not plugged. If it's rising the battery may be healthy. Try to reach 3.6V.
Post-charging - Reconnect the battery connector to the device itself, detach all the charging stuff we used. Connect an USB power supply as normal. THE DEVICE WILL TURN ITSELF ON, you need to turn it off, hold the physical turn off button for a few seconds. I had about three boot loops before i managed to find the correct way, device tolerated this.
The device is happily taking 9.5W of charge from 5V 2A usb brick.
Aha, you're losing waterproofness. I'm suprised it was waterproofed to begin with.
I suspect the deep discharge is due to the feature of the device turning itself on automatically (and fully) every time it's plugged into a power supply.
Happy salvaging.