Asus X751S not working on battery power, AC is ok

[Trofazni]

Hello, i got a few days ago x751s, the client said that it does not work on battery anymore. Tried it myself, while on charger i unplugged dc connector and it shuts off immediatly.
I oredered and replaced battery, it is internal battery on this model, powered it on, it charged to a 100%, plugged in, charging, everything seems to be working. BUT! When I unplugged DC connector it shuts off immediatly. Also, even though it is full battery, i can not power the laptop on only on battery.
It comes on only on ac adapter, but it says that bat is full.
I measured voltage directly across main batt leads, sanwa reads ~0,4V dc... yet it's full. The same voltage is with or without charger.
Can someone suggest steps for solving a problem, i can not find service manual for this motherboard...

[BlueMidnight]

Re: Asus X751S not working on battery power, AC is ok

I cannot find a schematic for this board. I also don't have the boardview but it's available for a price. However, it looks like it has a pretty normal charging circuit.

The first step is to measure all the pins on the charging IC and FETs related to feeding power to/from the battery. Do this while the battery and AC adapter are connected. It might be helpful to do the same thing again with just the battery connected. And it's better I ask for that now since forum replies take time.

I did find a blurry picture to show you which components to check. The bottom chip is the charging IC. The 2 above that are the buck regulator FETs for charging the battery. The one at the top is the FET that lets the battery power through to the board when the AC adapter is disconnected.

It would be very helpful if you took a picture of both the front and back of the board where the component labels and chip markings are clearly visible (especially the areas near the battery connector).

[Trofazni]

Re: Asus X751S not working on battery power, AC is ok

This is charging ic https://cdn.badcaps-static.com/pdfs/...efe8ea3bff.pdf
I measured voltages, they are on pics.

When I measure battery itself, not connected to laptop, should i read about 14V on battery terminals, red and black? Because i have nothing on them, either on old or new battery?
The only difference is new battery is able to charge to 100%, but old is stuck at 9%.

All fets around charging circuits are ok, on last fet (pj8809) that connects directly to batt terminal on one side i have 19V, but no output, and ~0,8V on gate.

When adapter is disconected, i have no voltages whatsoever anywhere...

[BlueMidnight]

Re: Asus X751S not working on battery power, AC is ok

The pinout of the battery connector on the board from pin 1 to 8 is the following:

Ground
Ground
Overcurrent signal (active low)
Data
Clock
Power
Power
Power

If you measure pins 3-5 while the battery is connected to the board (and if the board is receiving power), they should all be 3.3V. Without the AC adapter connected, those should all be at 0V.

Unlike a few batteries out there, this battery has no presence detection pin that would let it know when it is connected to something. And the data bus can't be powered unless either the AC adapter or battery is providing power to the board. So logically, since the battery can't tell when it should or should not be outputting voltage, it should be outputting voltage all the time. The fact that you are not seeing the 14V does indeed seem wrong.

If we assume it's not a faulty battery, then that can only mean the battery is not actually getting charged. In that case, the charging IC (BQ24735) and/or the charging (switching/buck regulator) FETs may be bad. Or another component in that area. Or maybe even the firmware (as the EC controls some aspects of the charging circuit via the SMBus).

Just to be thorough, you can do the following:

- Make sure there isn't a low resistance to ground on the battery rail with both the battery and AC disconnected (the coil near the label PJ8810 is a good spot). Ideally, use diode mode on your meter with the red lead on the ground and the black lead on the rail. If it doesn't read as no continuity, then it should at least read one diode drop (~0.3-0.6V).

- Make sure pins 12 and 13 of the charging IC have continuity (through resisters) to the 10 milliohm sense resistor near the label PJ8810. And that sense resistor itself should be 10 milliohms.

- Use something like the battery utility in Linux (Mint or Ubuntu). It will let you read the info from the battery to see what the actual charge level is compared to the design capacity. There must be some Windows utilities that do the same thing, but I haven't tried any, so I have no recommendations.

- Make sure that if this battery or another battery is connected and the system claims that it is being charged, you actually measure the charging voltage on the battery rail.

- If nothing else helps, you can try replacing the charging IC and/or FETs and see what happens.

[Trofazni]

Re: Asus X751S not working on battery power, AC is ok

I got new charging controller, replaced it, now it wont turn on at all... This simple change of battery became a totaln nightmare...

[BlueMidnight]

Re: Asus X751S not working on battery power, AC is ok

Okay you can get through this step by step. When a board won't turn on at all, it usually doesn't take too much work to narrow down the problem. We'll figure it out.

First, make absolutely sure the new chip is soldered correctly and is rotated correctly.

Next, you will want to do another round of measurements. You still want to concentrate on the same area of the board. Measure the two MOSFETs on the bottom of the board next to where the DC jack cable plugs in. Also, measure the charging IC and FETs again just like the last picture I posted.