Laptop motherboard repair

[SentinelAeon]

My friend's laptop has a wierd problem. When only using a battery, it is working fine. As soon as you plug in the power suply, it will not turn on (with or without battery). Using killawatt i measured 6W power draw when pc is off and battery removed. I circled the part of mb thats heating. After that i measured the DC input on mb and the terminals are shorted. I cleaned the motherboard with isopropynol to remove possible dirt short, sadly didnt help. I couldnt find any visible damage on the board.
This is an old laptop but my friend is used to it. So i was wondering if you have any easy solution of what could be done ?

1) identify and fix the problem
2) my friend would even be ok with soldering appropriate power source to battery pins on mb and just use that instead of battery instead. The problem is that by that you lose the protection offered by battery BMS and people online adviced against it

 

[Diver300]

There appears to be a self-resetting fuse under the ICs that are getting hot, so it could be that. A self-resetting fuse will stay hot if it has tripped and the power is left on.

The protection that the BMS offers is to protect the battery. If you connected an appropriate power source in place of the battery there is nothing to protect. The biggest issue could be that the laptop would need to communicate with the battery pack before it would work.

 

[SentinelAeon]

There appears to be a self-resetting fuse under the ICs that are getting hot, so it could be that. A self-resetting fuse will stay hot if it has tripped and the power is left on.

The protection that the BMS offers is to protect the battery. If you connected an appropriate power source in place of the battery there is nothing to protect. The biggest issue could be that the laptop would need to communicate with the battery pack before it would work.

But replacing the fuse wont solve the problem or am i misunderstanding ? The DC pins (where you plug in power suply) are shorted, power and ground. So the protection on board is preventing the startup obviously. But i dont know enough about the board to figure out which element is shorting the pins.

The battery has many pins. How would i figure out what to suply to control pins for board to startup ? Some people said to just plug in battery and solder the DC input to 2 pins, but that sounds like a very bad idea. You are basicaly bypassing board and applying voltage directly to battery input pins.

 

[Diver300]

I don't know what is shorting the power input.

I was suggesting supplying power to the laptop via the battery connector. The battery would not be used at all.

It isn't a good idea at all to charge the battery without a BMS.

 

[SentinelAeon]

I understand that, but if i just connect the DC power to battery connector, the pc will not turn on. Probably because there are no battery control pins present and i dont know how to suply that signal to the motherboard

 

[Pommie]

Have you considered buying the exact same model secondhand and transferring the hard drive? There are many places that refurb and sell older models.

Mike.
Edit, you also end up with two batteries, a spare hard drive and (maybe) more memory.

 

[SentinelAeon]

Well thats always an option, right now i am looking at what can be done with this laptop.

 

[SentinelAeon]

Maybe someone could help me out. Im trying to connect 12V directly to battery input pins on motherboard. The motherboard is JE70-CP, from laptop Acer Aspire 7741G. See the attachment for schematic. I connected the 12V to 2 pins and Ground to 2 pins, like in the schematic. The pc will not turn on, clearly i need to connect something else, like a sense pin. Any idea what?

 

[sagor1]

That diagram shows a SCL and SDA data pins for the battery. One can assume the battery communicates via some protocol to the device off the left side. It may be that the laptop won't turn on unless it detects that signal.
You can't just hook up 12V to battery terminals in a laptop, the laptop has lots of sensing circuits to detect the battery and determine its condition.

 

[SentinelAeon]

I figured it out and learned some interesting things. It turns out all u need to connect is 12V, ground and temperature pin. I measured it and temperature pin in battery is connected to ground via 10K resistor. So thats what i did, connected 12V and ground from my DC source, and then bridged the ground and temperature sense pins on motherboard.

 

[SentinelAeon]

Hmm, interesting. Even though im plugged through battery input pins, the windows OS thinks im running a wire (which i am, but not through the dedicated plug). And since i only have 12V 5A DC handy, i will need to limit the power draw of the system because as soon as i stress the discrete gpu, the laptop hits 60W usage and powers off. Thats no problem, the system can use igpu of I5 460M, its more than enough for youtube and such.

But this brings me to another question. This system uses a 10.8V (12.6V) 48Wh 3S battery pack. Now, im only assuming that when running a battery the power draw is less than what i am seeing now. Because otherwise ... on a fully charged battery, when stressed voltage will drop from 12.6V to 10V ... and that means in order to get 60W or more, u need 6A or more from those batteries. Now i am pretty sure i remember that those 18650 batteries found in laptops are not high amp ones, meaning they are designed to run at like 3A, not 6A and more. Am i missing something here ?

Bonus question: how to trick the motherboard into thinking that i am indeed running a battery and not a DC connected to it. Maybe that would force the system to use less power ?

 

[Dr_Doggy]

Maybe that would force the system to use less power ?

You can adjust energy effiency in "Power settings" to run the same as plugged in as when on battery.

 

[SentinelAeon]

Yeah, i did that. Though it still makes me wonder how computer runs on battery. Lets say you set it all to max. Surely it cannot draw 70 or 75W from 3S .. thats like 7 amps .. i clearly remember when i wanted to use those laptop cells for some high amperage usage, people warned me that its a big no-no, as i will have a fire if i run them at like 5A. I just asked chatgpt about that and he (she, it ?) says that usualy max amp draw can be calculating by dividing the Wh rating with voltage. In my case that would be 48Wh / 10.8V = 4.4 Amps. Though im not satisfied with this equation because its basicaly the same whether u have normal or high amp cells. But yeah, that would mean 48W limit. I saw jumps to 66W when loading windows. But like i said, system thinks im connected with cable and probably assumes its the default 90W one connected through DC input, while in reality i have 60W power suply connected through battery tab input.

Anyway, its working now and wont go over 55W. If anyone knows the answer to any of my questions would be great to know for future projects.

 

[Diver300]

Batteries can usually withstand peak currents that are well above the 1 hour rate.

In your example, dividing the Wh rating by the voltage gives the Ah (Amp hour) rating of 4.4 Ah, which means that if you take 4.4 A, it would be discharged in 1 hour.

That does not mean that 4.4 A is the maximum current. The maximum current allowed isn't clear from the Ah rating. It depends on a lot of things, but heating is one of the factors. The battery might well overheat if you took a high current from it for a long time, but the laptop could well take peaks of 10 A or without problems. As long as the current peaks are short enough that the battery doesn't have time to overheat, that would be fine.

The 1 hour rate is a very crude guide, and I would say that it is usually for continuous use, and there are lots of applications that discharge a battery far faster than 1 hour. Drones and cordless power tools are two examples of common devices that don't last anywhere near 1 hour if used continually. On the other hand, some battery chemistries, especially on non-rechargeable batteries, are not suited to short discharge times. Lithium Thionyl Chloride batteries are an example, as they have a huge capacity but can't be discharged efficiently in less than about 100 hours.

As for peak current, many applications have peak discharge rates that are far more than the 1 hour rate. 2G mobile phones take large current peaks, as do vehicle starter batteries. Electric vehicle maximum power ratings are often around 3 - 4 times the kWh rating of the battery.