[Teclast X98 Air 3G] motoi_bogdan:
So... like most of you, i have at least one problem with my teclast x98 air tablet, version C9J8, running only windows (8.1 pro/10 preview).

At this time i've constantly encountered the following problems:

- not turning on after being shut down. Pressing the power button doesn't do nothing. Sometimes it does this while charging, at the end of the charge, othertimes it... simply won't power on.

- huge battery drain in stand by. I've flashed ALL (yes... all) dual boot/single boot air/air 2 BIOS files i could get, in all versions. If it's got a .bin extension... yes, i've flashed that too. No change AT ALL!

- huge batterty drain... when powered off !?!?! Yes, that's the next level of awsomeness. You know your tablet is special when it sometimes discharges faster when turned off compared with it turned on. (@XDA, guys .. can you please add some facepalm smiley/emoji.. i wanted to use it about 30 times since i got this tablet)

- sudden shutdowns. Like when you use your tablet it simply dies in your hands with no apparent reason.

- battery meter stuck at 7% and only 1 cell reported. NOT fixed by the methods already known (flashing BIOS and letting it discharge then recharge with tablet turned off).

So.. i'm pissed off. I've disassembled the damn thing in search for some answers. I'll by posting some photos with the guts of this thing (c9j8 version).

First of all, I wanted to check the power draw directly from the battery, so i've desoldered the positive wire from the battery and inserted an amps meter to check the current flow.

For example, the stock charger will supply around 1.5-1.9 amps to the battery when charging (tablet off). For comparison a small 5V 1A, samsung charger supplied 1.1 amps in the same setup. Some other interesting facts, when on and booted to windows - the tablet draws about 1.1-2.2 amps (mostly depending on screen brigtness and cpu load). That's a total of 4-11 watts. If you lower the brigtness from full to low (bottom third of the slider) you effectively half the power consumption. As usual the display consumes more then 50% of the total power being drawn. Those who complain about huge power drain on standby will be surprised to know that the tablet draws 0.3-0.8 amps (it fluctuates) in standby. That’s HUGE. It should be 0.03-0.05 amps at MAXIMUM. 0.3 amps multiplied to a 3.8volts cell is 1.14Watts draw per hour at minimum in my case.

Leaving that aside, let's return to the above problems. The battery is connected to the motherboard by using a 3 wire connection (positive, ground and data bus/i2c or similar). The motherboard itself doesn't feature ANY protection/power management chip aside from a single ROHM controller located under a metal shield. Even if some data is passed between the battery and motherboard, you can simply decouple the battery and power the tablet with regular 18650 lithium cells or 3 AA alkaline batteries in series. The tablet is stupid enough not to notice any difference.

Let's go more deep in the start-up sequence.

When you press the power button, a half a second 500mA ramping to 800mA load is registered. The power management chip measures the voltage drop under that load and if it deems it to be "acceptable" it passes power to the rest of the motherboard. BIOS/firmware takes over from there but does a measurement of its own. If it results in an "ok" the boot sequence can the follow. If not, the BIOS would then power off the tablet. Here lies the first problem. The power management chip and BIOS thresholds for a "low voltage" battery are different. Very different. The chip itself considers the battery voltage to be ok if it's above about 3.45-3.5 volts and not dropping lower then 3.3v under a 500-800mA load. The BIOS/firmware (or whatever software part does this) won't accept a measurement below 3.65v. volts. So, when you pass the BIOS stage and boot to windows, the data you get when checking your battery comes from the power management chip. If you fully discharge the battery in windows (down to 2-3% or similar) and you are able to shutdown the tablet by yourself (it doesn’t cut power by itself) you could find that it cannot power back again because even if the power management ic gives the go ahead, the bios/firmware side refuses to go any further. The battery must be charged for some time before the bios will allow for booting.

The problem is that both power management IC and BIOS readings should be taken in same way and be of similar value. They are not. It’s not that Teclast couldn’t do this, but for whatever reason they decided to write the BIOS in that way. The 7% problem could originate in the fact that a what the BIOS considers a dead battery (0%) is actually charged to a degree and is different from 0% measured at the power management chip level. Overall the power readings are inconsistent in both measurement and reporting. It doesn’t seem to be a hardware problem.

Another problem is how „dumb” is the battery management hardware. In any modern portable computer (laptops, tablets, even phones – excluding some chinese products) you cannot simply disconect the data bus from the battery and simply feed some random 3-4 volts to power the thing. It’s like you would remove the battery from your laptop, check the label on it for the voltage rating and stick a bunch of wires on the contacts (2 of them) and expect the thing to boot. It won’t. Firstly because IT’S NOT SAFE. The battery or motherboard can’t report one to another if a fault is occuring and can’t accurately measure voltage/current consumption.

Yet another problem is that the same power circuitry does not compensate for large voltage/current swings. A simple experiment for you folks to try. Get a aa battery (a battery in general) measure it’s voltage as it’s sitting still then connect a small lightbulb/motor/led/whatever load runs on that battery and measure the voltage WHILE the battery suplies current to the load. You will find a voltage drop at the battery level. It’s normal, is how these things run. A complex electronic device must take that into account in it’s design. At idle/browsing web/viewing picture, the tablet draws about 1.1 amps from a battery that’s registering 3.87volts (at that test’s time in my case). Running a benchmark/video game produced a series of spikes to 2 – 2.1 amps and an aditional voltage drop to around 3.61 volts. Remember that some power rails require exact voltages (cpu core, main bus, 5volt usb bus etc). The power circuitry must provide those exact voltages regarding the input voltage swing. Noup... and that’s the main problem untill now. THEY DONT! I was shocked to see how the chinese engineers are pushing it right on the edge. If you desolder one battery pin and insert an ampere meter in series, that’s enough to induce the little voltage drop needed for the tablet to freeze under load or shut down alltogether. The ampere meter leads were rated to withstand 10 amps under load – and they do, yet the simple fact you inserted a piece of wire along the track is enough to disturb an already delicate balance. The thing is only barely capable of whitstanding it’s own battery voltage swing. In my opinion you can try to reduce the load by disabling turbo modes on cpu/gpu or whatever (and teclast tried with some bios/models of the x98 air) but you cannot fix this by firmware. It’s just bad hardware design. They cut costs on the power management side.

Those are my finds untill now. I’m thinking of adding some capacitance over the power rails to take the load over from the batteries when a large amount of current is drawn (spikes that occur under load). Other then that, there is not much to do about this.

Even so, i don’t know why the tablet still draws power while turned off. I wasn’t able to make it do that while measuring. Aditionally i don’t know why only one cell is reported in windows. More tests are required.

This is still an ongoing "project". Some of my conclusion could be wrong at this stage. Like i've said it's still a work in progress. It would be quite a thing if anyone with some knoledge about the BIOS code (or how it runs on this tablet) could step in and direct me to the right hardware to examine.