(Question) How reliable are salvaged / harvested laptop batteries for solar?

Sorry if this has been asked before.

I'm starting out and I like the idea of salvaging laptop batteries, but now I'm wondering, are they reliable?

Let's say this is my scenario:

1. I get my hands on old/dead laptop batteries.
2. I take it apart and harvest the 18650 cells.
3. Those that are below 1V (let's call them bad cells), I parallel charge them or I somehow resuscitate them.
4. The bad cells that I manage to get back up to 4V+, I include with the rest of the good cells.
5. I create 7S packs out the cells.

Question: wouldn't the bad cells that turn bad when being inside a laptop go bad again?
Wouldn't that bad cell affect the good cells in the series, for the same reason it became bad in the first place?

Is it because the use conditions inside a laptop is different than an off-grid solar system?
Will the charge controller and BMS prevent that scenario?

Thanks in advance!

Androfire said:

Question: wouldn't the bad cells that turn bad when being inside a laptop go bad again?
Wouldn't that bad cell affect the good cells in the series, for the same reason it became bad in the first place?

Click to expand...

1. Nobody can answer to your first question but it's highly possible that they would.
2. Bad cells always affect the pack because it's harder to keep the pack balanced with them. If the cell goes too bad and charging does not bring it above, say, 4V, then the other cells riskbe overcharged.

That's why people are testing and sorting cells after harvesting. Similar cells can be combined in a pack with lower risk.

thunderheart said:

Androfire said:

Question: wouldn't the bad cells that turn bad when being inside a laptop go bad again?
Wouldn't that bad cell affect the good cells in the series, for the same reason it became bad in the first place?

Click to expand...

1. Nobody can answer to your first question but it's highly possible that they would.
2. Bad cells always affect the pack because it's harder to keep the pack balanced with them. If the cell goes too bad and charging does not bring it above, say, 4V, then the other cells riskbe overcharged.

That's why people are testing and sorting cells after harvesting. Similar cells can be combined in a pack with lower risk.

Click to expand...

Thank you for your kind response, Thunderheart.

I see you've tested a whole lot of batteries in your list, but those are new, correct?
If I want to learn to test used batteries, I'll follow this guide by@canispaterchristmas?

You're right, the batteries in my tests are new. Testing used cells is a bit different because you don't need to check if they comply to declared specs or not. You need to verify are they usable or not. That's why it's better to develop your own testing rules for your own requirements and test all the cells using the same rule (like checking capacity at 1A charge/1A discharge down to 2.8V). If your scenario of usage suggests high rate discharging then you should also check them at max suggested discharge rate. After getting the results you can group the cells and use similar ones to make packs. An additional IR check would also help to detect similar cells.

We have enough users here that use the laptop batteries without issues. Usually under 1V cells are there because the entire pack is under 1V, with the exception for one bad apple that is 0V. All that means is you have a deeply discharged pack due to that someone possibly used the laptop battery until it's dead and threw it out. So the only cell that's bad is that one 0V (and maybe the other one that's paired with it in parallel). So in a 8 cell config, you might find 2 that are 0V and 6 that are 1V. The 1V you charge it up to 4.2V and let it sit for a few weeks. If it self discharges below 4.15 or more then you know you have a bad cell. Else test for capacity. If you fear a bad cell, then repeat the charge-pause-discharge cycle again. I'd say 20% of my cells are under 1V when I got them yet my system is balanced to the point that it went without a bms for 3 months without any issues.

@thunderheart
Just to confirm, as 12V battery demands a higher discharge rate than a 24V battery on the same wattage, yes?

@not2bme
Thanks for the explanation. The cells I harvested so far are either 0V or 2-4V. If I ever find <1V cells I'll try your method.

Androfire said:

@thunderheart
Just to confirm, as 12V battery demands a higher discharge rate than a 24V battery on the same wattage, yes?

Click to expand...

If by "discharge rate" you mean current (or amps) - then yes. The 12v will have to delivery twice the number of amps as the 24v for the same wattage. 12v * 100amp = 1200watts. 24v * 50amp = 1200watts. One of the key reasons people choose a higher nominal voltage (e.g. 24v or 48v rather than 12v)is that the wire size gets very thick/expensivewhen you start getting a lot of amps. So you can get the same wattage with much smaller wire if you build your battery at a higher voltage

Going back to your original question, there are some packs that go bad due to bad, rusty, CID, or worn out cells. But there are a lot of laptop packs that are recycled for other reasons: laptop died, planned obsolescence, cells out of balance making overall capacity lower, BMS failure, new old stock, etc. By testing for capacity and self-discharge, you are weeding out the former from the latter. Also, some cells have higher IR and cant hold their capacity with a higher amp draw like in a laptop in performance mode with 100% CPU for example, but will have a decent capacity at 500mA for powerwall usage.