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Dead Cell Boost Circuit
#1
I'm just getting into cell recovery and this is my first post to the forum. I work at a small computer shop and found that our used system supplier has a lot of dead laptop batteries available. Due to our working relationship, I negotiated a deal to buy 40+ pounds each week at $1/pound.

The recovery rate has been really good, but I am coming across a good number of dead cells (0v). I'm pretty sure these dead cells have only dropped below the magical 2.5v triggering the protection circuit. They just need to be boosted to get them back where my chargers can see them. I've been using the good / bad battery pairing with a resistor to bring up the dead cells. It's been working, but it's not really a good long term solution.

I've been toying with the idea of using my ATX power supply that I modified into a DC bench power supply to boost the dead cells. It provides a well regulated 12v, 5v and 3.3v. Using the 3.3v power supply instead of a good battery will allow me to boost multiple cells at the same time and does not pose a danger of overcharging the cell since it's well below the 4.2v limit. The charging time will be greater, but I think I can live with that if I can boost 4-8 at a time with fire and forget functionallity.

At this point, I just need somebody to check my math. And make sure I'm not going to burn the shop to the ground.



As the battery comes up from 0v the supplied current will drop. With a 4Ohm resistor the current will start at 82.5mA and drop to 0mA at 3.3v.

0v 82.5mA
1v 57.5mA
2v 32.5mA
3v  7.5mA

The resistor rating should be the Max Voltage * Max Current giving us this:

3.3v * .0825A = .27225W

This is just a bit too much for a 1/4W, so a 1/2W is warranted.

Let me know if I got this right.  It's been a long time since I've done descrete electronics in college. I appreciate any comments or suggestions. Thanks.
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#2
Excellent idea. However, my trick is to use a AAA NiMh in parallel with my 18650s. Then I hook up to an RC charger. The RC charger will not over charge. That and I only charge at 50ma per cell. 0v cells are quite often almost 100% recoverable, but you have to be 'gentle'. Often the parasitic draw of the BMS drags the cells down to 0v or close to it.

So I would opt for a slightly lower current. The lower the better. I believe your math to be correct. However, you would have to factor in the internal resistance of the battery. So that may make your idea - ideal.
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#3
Actually, the maths is out by a factor of 10, careful!
3.3V/4 ohms = 0.825Amps
I think you'll need to use a 40 ohms resistor instead.
Since 40 ohms resistors are not a standard value & a little less current might be better, the nearest standard value up is 47 ohms
So for 0V battery, 3.3V/47 ohms ~70mA
Next up again: for 0V battery, 3.3V/56 ohms ~59mA
rodagaster likes this post
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#4
(08-11-2018, 05:43 AM)Redpacket Wrote: Actually, the maths is out by a factor of 10, careful!
3.3V/4 ohms = 0.825Amps
I think you'll need to use a 40 ohms resistor instead.
Since 40 ohms resistors are not a standard value & a little less current might be better, the nearest standard value up is 47 ohms
So for 0V battery, 3.3V/47 ohms ~70mA
Next up again: for 0V battery, 3.3V/56 ohms ~59mA

Good catch. That's the reason I posted here. I found the error in my spread sheet, missing a 0 on one of my divisors.

I like the current curve for the 56 ohms resistor. May just need to add a second 4 battery holder to make up for the extra charging time.

This also means I can drop to a 1/4W resistor as well. ( 3.3v * 0.059A = 0.1947W )
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#5
The cells which read zero and refuse to accept charge will have the cut out in the positive terminal triggered . You can reset this with a very small screwdriver ...see Youtube

I had about 20 of these , I reset, charged and tested them and put them in a pack of their own , they seem to perform well.
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#6
(08-11-2018, 01:41 PM)ozz93666 Wrote: The cells which read zero and refuse to accept charge will have the cut out in the positive terminal triggered . You can reset this with a very small screwdriver

That's fine and good, but if they popped, they popped for a good reason. They don't normally fail on a fluke. Many users here will warn against this practice as it is not worth the possibility of the cell failing in a really bad way. Reseting the CID
Would be like putting your vehicle airbag back in the steering wheel. Would you trust it to protect you as intended?
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#7
(08-11-2018, 03:13 PM)Korishan Wrote: That's fine and good, but if they popped, they popped for a good reason.  They don't normally fail on a fluke.

I agree that any battery that tripped the CID should be discarded. Luckily, my supplier will take back truly dead cells so they can be properly recycled.

That's also why I added the Voltage Check Switch, so I can check the standing charge of the battery after 5 minutes. In that time the battery should read something above 0V. If not, it's headed for the discard bin.

Automatic thermal protection might be next up on my list to add to this simple circuit.
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#8
Well probably the CID triggered because it got too hot and pressure built up , those laptops can get pretty hot if not well ventilated ...

In a normal cool environment this should not recur ...

Anyway , I have put 20 of these in a separate pack and am watching them very closely for any issues as I cycle them ...

You will be the first to know of any problems .
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#9
I have been using a string of 8-10 cells in parallel to bring 0V cells back to life. To start, I add one good cell to the string to get the charger going, once it's running, I swap the good cell for a bad cell quickly before the charger notices! If the cell will take a charge, the voltage will increase. If the voltage starts to increase, I start adding more and more 0V cells to about 10 cells in parallel.

Even if I set my charger to 20A, it will not increase the amperage until the voltage is around 3V. So with 10 cells, the charger is only putting out 500mA to 1000mA, during this time I check for heaters. If I find a heater, I swap it out for another 0V cell, and keep watching until there are no more heaters and voltage starts to rise. When there are bad cells, the voltage never really increases, once the string is all of "fair" cells, the voltage starts climbing pretty steadily, right along with the amperage.

I do like the idea of a simple lot voltage jumping station! and that's kinda what I am doing with a smart fully adjustable charger.
Korishan likes this post
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#10
Have You checked how much capacity You get from these resurrected cells?

Reviving 0 volt cells may be successful, but my experience is that they hold only a fraction of the rated capacity, so they ended in my bin.
Have You tested how long they keep working?

Best of Luck Wink
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