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History

My laptop battery has shown less and less capacity. So I got a new battery (LA04) in Jun 2018, with a warranty of 6 months. After about 8 months, the new battery also showed problems. It went to zero suddenly from 10%. Over the next few months, the battery suddenly went to zero on higher and higher percentage. Somewhere about 10 months after the purchase of the battery, it had become completely useless. The laptop turned off right as I pulled the plug. I had the battery setting to show a warning at 20% and hibernate at 10%, but it didn't show any warning or anything; just instant death. When I turned it back on, the Windows behaved like an unexpected shutdown. The battery icon showed "0%, Charging" from then onward, but the percentage never changed. (With later analysis, I think at this stage the laptop thought the battery was charging, but it was actually not charging because the low voltage on one of the cells had hurt the ego of the battery management circuit and it needed someone to listen to its problems before it started working again.)

Revival of the battery

In Oct 2020, I opened it up. It contained an IC (SH79F329AX) that seemed to the be boss of the charge management system and two transistors (MT8103) connected in series with opposite polarity (typical of battery management systems). The picture of one of the cells is also attached. There was no visible sign of physical damage anywhere on the battery's constituents. The voltage of three of the cells were about 3.6 V and fairly close together. The cell #2 from the negative side was at about 2.4 V. The series string of the battery showed about 13.3 V but the output terminals of the battery management circuit (that connected to the laptop) showed 0 V. I guess the battery management controller had gone to some fault state due to low voltage on one of the cells. I charged up cell #2 using an external Li-ion charger to the voltage approximately equal to the rest of the cells. The output of the charge management circuit still showed zero. So I looked up on internet and shorted the positive output of the circuit with the positive end of the cell string, in accordance to one of the suggestions. The circuit came up live. I used the battery in the laptop. It gave a good back up of 30-40 minutes with full CPU usage and up to 2.5 hours with light document editing. The battery now charged up at about 1% per minute with the stock charger, which is a sign of a healthy battery in my understanding. I usually used the laptop while connected to the charger except for occasional moving around for a few minutes and discharged it to 15 % 2-3 times a week. The setting was warning at 20%, hibernate at 10%.



The problem is coming back

On 19 Dec 2020, the laptop showed a battery warning at 20% and suddenly hibernated. When I connected the charger and turned it on, the battery icon showed the old "0%, Charging". The percentage never actually changed. I guess the charging circuit had gone to some fault state again and needed someone to come and end its misery. I opened up the battery. The output of the charging circuit was 0V, and the series string was at about 13 V. The cell #2 was at 1.5 V and the other three cells were at about 3.8 V each. I charged up that cell with the same external charge controller. Then I shorted the positive terminal of the circuit with the positive terminal of the cell string to reset the circuit and closed up the battery. The battery seems to be good now.



My assessments

It seems to me that there is some problem with the cell #2 or its charging segment. It took about 90 minutes to bring the cell from 1.5 V to 4.0 V using a Oppo 2A mobile phone charger in combination with the external charge controller, which indicates a healthy cell in my opinion. The cell may not be charging up correctly with the BMS. Or there may be some problem with the balancing circuit in the battery management circuit. If the BMS measures the SoC with the battery's voltage, then the cell #2 may be having low voltage and the other cells might be getting over charged. But I don't sense any heat from the battery even when it is connected to the charger for hours a time.



The questions

  1. Is there a mode of battery failure mode for Li-ion batteries in which the cell has good charge capacity and low internal resistance but suddenly goes to low voltage? This might be the case with the cell #2.

  2. If anyone has better knowledge of the BMS systems in laptop batteries, is there any way that one of the cells doesn't get charged up correctly? There maybe something wrong in the balancing circuit if the cells are usually charged in series.

  3. Can anyone suggest any more measurements I need to get to have better understanding of the situation? I guess opening up the battery at a higher SoC (60-80%) and check the voltages of the cells would help.



The request

I am a little confused with the way Stack-exchange works and I don't have the energy to find and go though the constitution, by-laws and best practices spanning dozens of webpages. But I have tried to be generally polite and as helpful as possible. I would be welcome to any constructive feedback to improve my question to help people read and answer it in a better way. The objective of this question is the same as the title of this post (What is wrong with one of the cells in my 4-cell laptop battery? How can I make it better?). I want my laptop battery to work better and I want to learn some theory behind its problems and possible solutions. If it seems to anyone with admin rights that this post is disruptive or off the topic, please give me a chance and some feedback to improve it.

BMS controller transistors in series with opposite polarity one of the cells

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First if one of the cells of this battery has been down to 1.5V the battery should be replaced.

You bought a battery with a warranty of six months so I expect the cells and BMS not to be of the best quality. Quality battery warranties are in the 1-3 year ranges.

The cell seems to charge ok and retains its good state for some time so I suspect the BMS is the problem.

The BMS could be causing this problem. As the battery comes up to full charge the voltage across each cell increases. If this increase is uneven the BMS connects a small load across the higher voltage cells in order to attempt to equalize the cell state of charge and hence voltage. This is referred to as top balancing.

The load is connected using a MOSFET. A common failure mode of MOSFETs is to go short circuit. This would leave that load connected across the cell resulting in it constantly discharging, which seems to fit with the symptoms of your problem.

The way to confirm this is to measure the current in the balance wires while the battery is charging at a low state of charge, below the balancing threshold. Any differences in the cell 2 wires compared to the others would indicate that this is the problem.

One of the problems with your question is that this is not a discussion forum. This is a complex problem which can have many answers and trying to pin it down to one cause is very difficult, hence this is just my best guess. In any case the battery should be replaced. Running a cell down to a low voltage can cause the cell to short out at some time in the future. See Is draining a Li-Ion to 2.5v harmful to the battery?

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  • \$\begingroup\$ I tried to confirm the mode of failure you mentioned, i.e. the cell #2's balancing MOSFET shorted. I left the battery disconnected for about 16 hours. When I checked the cell voltages after this time, they were 3.91, 4.00, 3.90, 3.90. The cell #2 voltage is still 0.1 V higher than the others probably from the last manual charging a few days ago. With these results, I suspect the balancing MOSFET of cell #2 is not permanently shorted unlike as you suggested. If the cell #2 has been discharging for 16 hours, its voltage should have been somewhat lower than the other. \$\endgroup\$
    – paki eng
    Dec 24, 2020 at 10:06
  • \$\begingroup\$ I have a bit confusion with your point about a discussion forum. The question may seem to be a prompt for discussion because I have provided too many details and I am ready to perform any more experiments needed and provide the results as a help to someone trying to answer the question, "What is wrong with one of the cells in my laptop battery." \$\endgroup\$
    – paki eng
    Dec 24, 2020 at 10:17
  • \$\begingroup\$ Thanks a lot for your answer. \$\endgroup\$
    – paki eng
    Dec 24, 2020 at 10:18
  • \$\begingroup\$ You are the second person to suggest me to get a new battery. Thanks for your suggestion. But what to with the battery was not the original question. I will consider replacing it if it causes too much problem and I can't find a workable solution. \$\endgroup\$
    – paki eng
    Dec 24, 2020 at 10:22
  • \$\begingroup\$ You obviously don't understand the risks of buying a poor quality product , pushing it past safe operating limits and then expecting us to provide you with information to "fix" it. Voting to close. \$\endgroup\$
    – RoyC
    Dec 24, 2020 at 14:54

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