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Lenovo Yoga HD10+ (B8080) tablet repair

What

A damaged Lenovo Yoga B8080 (HD 10+) tablet was received as a gift. The display is cracked (cosmetic issue), and the battery is torn off, probably due to being dropped. The flip-off battery/stand was knocked off the assembly, the flat ribbon cable of the battery was severed and the end with the connector was lost.


Front side

Front side

Front side

Front side

Back side

Label detail

Battery

Battery end with cable

Battery end, passive

Battery end, passive

Severed cable detail

Severed cable detail

Disassembly

The tablet is held together with a snap-in clamshell with three little screws. One is hidden under the microSD/SIM cover, two more are under a label. The clamshell is also held in the middle with a pair of snap-in pieces of plastic.

The back side is merely a cover, a passive piece of plastic. Everything is mounted on the front shell.

The battery hinges are housing the speakers, the camera, and the power switch. These are placed in as removable modules attached with ribbon cables.


Inside

Inside

Right hinge with vibrator and speaker

Left hinge with camera, battery connector

Left hinge with camera, battery connector

Right hinge

Left hinge

Left hinge, look into

Battery in place

Battery in place, right hinge detail

Battery in place, left hinge detail

Battery in place, left hinge detail

Battery in place, left hinge detail

Battery in place, left hinge detail

Battery in place, left hinge detail

Battery in place, left hinge detail

Camera-speaker-switch module

Camera-speaker-switch module

Camera-speaker-switch module

Left hinge without module

Left hinge without module

Left hinge without module

Front camera connector microscope view

Front camera connector microscope view

chip under microscope

chip under microscope

chip under microscope

chip under microscope

Diagnostics and repair

Battery

The battery is enclosed in a metal tube with plastic caps on sides, attached by small screws hidden under stickers. The sticker on the cable side was removed and the now exposed screw taken out. The battery assembly then could be pulled out.


Battery end cover removed

Battery pulled out

Battery and its casing

Battery casing end detail

Battery label

Severed cable detail

The heat-shrink cover of the battery was longitudally cut and the internals exposed. The battery consists of three cells, and a battery management circuit. The connection is handled by a polyimide-foil flat cable, with six lines - a wide one on each side (the battery + and - terminals) and a pair of thin ones in the middle, for the I2C bus, the battery thermistor, and one more line. The ribbon cable is folded, behaving as a flexible double-sided circuitboard. The battery side of the cable is clearly labeled as of the signal meanings (thank you, vendor!)

The battery cable pinout is, from the outer end:

The TH and IN wires are led on one side of the cable, the SCL and SDA on the other. The P+ and P- are on both sides, connected by vias. The solder joints will hopefully make the cable replacement easy.


Battery cover cut open

Battery electronics

Battery electronics

Battery electronics

Battery electronics

Battery cable attachment

Battery cable attachment

Battery cable attachment

The connector at the end is missing. The mating connector on the board is tiny, as it goes with modern technology. A possible approach was buying a new battery, which was ruled out due to cost and being considered cheating. It was decided to attach wires - thicker ones for the power, thinner ones for the signals. This went without major problems; the pads are tiny but a stereomicroscope is a great help here.


Battery connector under microscope

Battery connector under microscope

Battery connector under microscope

Battery connector under microscope

The power terminals are the high-current ones, large and on the sides. The negative one on the right was easy to identify, as it was electrically connected to the shielding cans over circuitry and visually attached to the large ground flood fills and lots of vias, and the positive one was, well, the other one at the left.

The four remaining are I2C on one side and thermistor and the IN one on the other side. Detailed pinout to be determined.

The order should be probably

The identification should be easy-ish; monitor the wires with an oscilloscope at boot. The I2C communication has a telltale appearance - clock signal bursts on the CLK wire and corresponding digital data on the DTA wire. Alternatively, check the electrical connection with a nearby known chip (e.g. the charging one) with known I2C pins; here we assume there's one I2C bus common for the device or at least the battery/charger. Worst case, we find no connection and have to do the more laborious check.

The test was then done with feeding the circuit from the power supply set to the assumed battery voltage.

Never assume in high-stake situations. You're making an ass of u and me and can kill something.

Screwup

An assumption was made that the three-cell battery works the same way as the laptop one - three cells in series, yielding 10-12 volts. A power supply was connected to the battery input terminals and 12V DC was injected.

A suspiciously high idle power consumption occurred - almost 2 amps. The top right shielding can warmed up significantly. Increasing the current limit (stupid...) allowed powering up of the tablet, but the boot failed after showing the initial Lenovo logo. Further tests were paused.

The shielding can was removed, with considerable effort. (The higher melting point lead-impoverished eurosolders don't exactly help here.) The power management chip and its associated coil and capacitor were exposed. Further power test shown both are heating up significantly, with the chip heating up way more. The sink for the energy was localized.

The chip is a bq24196, an I2C-controlled battery charger and boost converter, with support for USB OTG power output. Its datasheet revealed it is built for a single Li-ion cell, some 3.3-4.2V on input.

OOPS!!! Here goes the initial assumption. The battery was three cells, indeed, but in PARALLEL, not in series. A single voltmeter measurement could show this.

Lesson learned.

4.5 volts were injected to the output power rail, past the choke, to the capacitor terminal. There was a high power consumption and the chip heated up. The choke was removed to interrupt the current flow, with no impact. The datasheet was consulted, the chip takes the output as its input for the USB OTG function and feedback. This connection was severed by scratching the circuitboard just next to the chip's pins.

Now the power-off current consumption dropped to zero.

The tablet was powered on successfully now. It attempted to boot, and failed. The power consumption was too high. Current limit was upped on the power supply and now it booted fully. Then it complained about excessive battery temperature and shut itself down. Which is expected, as the battery thermistor is not connected yet.

So now the chip has to be replaced. $3-5 on eBay, plus wait time. Plus the effort with tiny pins.

Also, the coil was damaged during desoldering; it is a miniature ferrite block with wire coiled inside. The wires were torn at the point where they leave the ferrite. As sourcing of a replacement coil may be difficult, the ferrite will be abraded with a diamond file and longer wire attached to the exposed copper. Butt joint can be considered too but it will be weak and unreliable.


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