1

I am considering purchasing a new smartphone that is several years old and could use a little electrical engineering help from the EE geniuses here.

I've performed quite a bit of research, and found so much conflicting (and just plain wrong) information that I'm hoping the voting system here on StackExchange will help yield at least one quality answer.

The device I'm considering is a Google Pixel 3XL. I don't think they make them anymore, and my understanding is that they were manufactured in 2018-2019. I think a safe rough estimate would be to say that the stock of phones is 32 months old.

The chemistry of the battery is Li-ion. I found a chart with self-discharge rate estimates for different battery chemistries here. It claims a 10%/month self-discharge rate for Li-ion batteries at room temperature. I have no idea if that chart is accurate. I also have no way to know the temperature at which the devices were stored.

The devices are still in their boxes and should have never been charged after the original factory charge. I don't know the level of the factory charge (if you do, please chime in!), so I'm thinking a reasonable estimate might be 70%.

If my estimates are close, and math hasn't recently changed, that would mean that the battery would be more-or-less discharged after ~7 months. Given the estimate that the device is 32 months old, that would mean that it has likely sat with a dead battery for a little over 2 years.

From my research, many people claim that if a Li-ion battery goes completely dead, it will never work again. But I've also read numerous stories of people buying old smartphones in the box and being able to charge the batteries and have them work fine. On the surface, it seems that only one of those claims could be true. But I'm guessing there are details of which I am not aware.

Given the above data, what is a reasonable expectation for the batteries in these phones? I realize all answers will be approximates, but about what percentage of the original capacity will one likely receive from the battery? And, approximately, how long until the battery will no longer be able to be recharged above 50%?

If you can explain the reasoning for your answer, it will help me learn more. Thank you.

RockPaperLz- Mask it or Casket
  • 315
  • 1
  • 7
  • 20
  • No battery expert, so I don't post it as an answer. I think if people talk about "fully discharging" Li-Ion batteries, they mean actively drawing large amounts of current from them. This will damage the battery, as far as my understanding goes. Having the Batt sitting there, with a little more than self discharge trickling out shouldn't be a problem. With the charge decreasing, the internal resistance increases, resulting in even lower discharge rates. The open circuit voltage will not drop into "permanent damage" region. – jwsc May 11 '21 at 15:11
  • @jwsc I'm not Li-ion battery expert either, but most of what you wrote makes good sense to me. In particular, *"With the charge decreasing, the internal resistance increases, resulting in even lower discharge rates."* If that is accurate, it's an excellent point. The big thing that I would add to what you wrote is that I keep on reading that the *length* of time that a Li-ion battery is discharged is a real problem. For example, see the "Don’t Leave the Battery At 0%" paragraph here: https://www.howtogeek.com/169669/debunking-battery-life-myths-for-mobile-phones-tablets-and-laptops/ – RockPaperLz- Mask it or Casket May 12 '21 at 00:35

1 Answers1

1

I have the Li-Ion battery from my daughter's first cell phone. It is 20 years old and is still at the 3.8V storage voltage. It is not inside a phone so it is not discharged by the clock or something.

The 32 months old battery in the phone you talk about also might still hold a storage charge if the phone circuit is sleeping.

Audioguru
  • 3,894
  • 5
  • 8
  • Thanks. Since all modern smartphones have a RTC, would it ever be able to disable the circuit for that, or would it represent a continual (small) draw? Also, I thought that the batteries themselves now have internal circuits that cause a small amount of drain. I wonder if your daughter's battery is so old that it doesn't have such circuits. Both of these would be small draws, but 32 months is a long time. Interested in your thoughts. – RockPaperLz- Mask it or Casket May 12 '21 at 00:27