Battery Voltage drop

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JG27

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Apr 10, 2020
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Hi guys, I'm new to this forum!
Because I know that this is a good place to ask questions and learn new things, I just wanted to ask, which Voltage drop is acceptable. For example I've some cells here that I tested a few weeks ago for capacity and resistance (some of them were 0V and recovered) and exactly the 0V cells were fully charged but most of those 0V cells dropped down to about 4.04V. Is this good/acceptable? Can anyone link to a post which has some kind of spreadsheet/explanation about Voltage droprate or so?
Thanks!
 

daromer

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Oct 8, 2016
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I would say max drop from 4.2 to 4.1 ish within 2-4 weeks.
 

SolderSoldier

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Apr 10, 2020
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Hi, my first post. Yes, another dreamer here, bit by the battery bug.

I've been struggling with the same question. How much V drop is too much ?

Background:
I have a random li-ion charger, for a video ghimbal's batteries, dont' know the specs on it exactly.
I recovered some 18650 batteries from some 970 Sony style packs.

I charged the batteries: the final voltage was 4.1V

A week or so after readings:
some <2V - obviously crap.
some 3.63V, still crap.

but some only 4.02-4.04V
Is 4.02 too much of a drop from 4.1V?
Should I bother using them in a project ?
The target is a 4s5p packs.

I don't have yet a capacity tester, or any other fancy charging station.

Thank you !
 

Korishan

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Jan 7, 2017
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6,552
Li-Ions should really drop below 4.1V after full charge. If they do, they are getting pretty old or were heavily abused.

Sub-4.1V cells could be useful in non-critical applications. Like a flash light(torch), toys, etc. I wouldn't use them in anything that requires storing them for a period of time and expect them to be functional when needed, or as a backup power supply.
 

ajw22

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Nov 16, 2018
Messages
641
SolderSoldier said:
I charged the batteries: the final voltage was 4.1V

If all the voltages right after charging were 4.1V, you have either a badly calibrated charger or badly calibrated tester. Many cheap testers are.

As daromer said: To get a better picture,best to wait at least 2 weeks and allow a greater drop (say 0.1V). You want to differenciate between initial drop in the first hours~days(not bad) and continuous self discharge all the time (bad, dying cell).
 

jonyjoe505

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Feb 28, 2018
Messages
229
any cells that were at zero, those I would not use in a pack. If they were around 2 volts then maybe you can use them as long as they don't drop too much voltage example from 4.20 it drops to 4.18, any more then that I put the cell aside.

I encounter too many packs that I build that had balancing problems, thats why I avoid any cells that drop too much voltage. It's too much trouble to take a pack apart to look for a bad cell. If you have alot of cells only choose the best, if you have doubts about a cell, set it aside. They can be used in a flashlight.

I recently bought an IR tester (yr1030) , I test every cell with. I already found some cells that had high resistance but checked good elsewhere. I don't use those cells, its not worth it. I paid 57 dollars for the tester, but for me it was worth it. Anything that can help you weed out the bad cells is useful. And this tester is quick, you can test the cells as you get them, those that have high resistance, you don't have to waste time doing a capacity check on them. The opus can check resistance but it gives you different readings all over the place on the same cell.


image_kgykqv.jpg
 

JG27

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Apr 10, 2020
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SolderSoldier said:
I don't have yet a capacity tester, or any other fancy charging station.
I am using this charger/tester all the time:
https://www.amazon.de/dp/B074GZ5MRZ/ref=cm_sw_r_apa_i_eTBKEbXRY31HF
I think it's a good one!


Korishan said:
Sub-4.1V cells could be useful in non-critical applications. Like a flash light(torch), toys, etc. I wouldn't use them in anything that requires storing them for a period of time and expect them to be functional when needed, or as a backup power supply.

Fine for me because I'm mostly going to use them in fun projects/non-critical applications!

Thanks for the help!
 

CUDAcores89

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Dec 30, 2016
Messages
83
jonyjoe505 said:
any cells that were at zero, those I would not use in a pack. If they were around 2 volts then maybe you can use them as long as they don't drop too much voltage example from 4.20 it drops to 4.18, any more then that I put the cell aside.

I encounter too many packs that I build that had balancing problems, thats why I avoid any cells that drop too much voltage. It's too much trouble to take a pack apart to look for a bad cell. If you have alot of cells only choose the best, if you have doubts about a cell, set it aside. They can be used in a flashlight.

I recently bought an IR tester (yr1030) , I test every cell with. I already found some cells that had high resistance but checked good elsewhere. I don't use those cells, its not worth it. I paid 57 dollars for the tester, but for me it was worth it. Anything that can help you weed out the bad cells is useful. And this tester is quick, you can test the cells as you get them, those that have high resistance, you don't have to waste time doing a capacity check on them. The opus can check resistance but it gives you different readings all over the place on the same cell.


image_kgykqv.jpg
Beware if you do decide to use those IR testers. The resistance between the probes and cell is going to mess with your measurements. I bought one of those awhile ago and created a custom Cell holder using magnets so I could get more reliable readings.
 

SolderSoldier

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Apr 10, 2020
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JG27 said:
I am using this charger/tester all the time:
https://www.amazon.de/dp/B074GZ5MRZ/ref=cm_sw_r_apa_i_eTBKEbXRY31HF
I think it's a good one!






Thank you JG27. It's not the first time someone recommends this charger / tester.

Thank you everyone.

As a newbie I am lacking the confidence, but I also may be grasping at straws with the few cells I have played around.
I can't wait for this pandemic to be over so I can put some money in this new hobby.
Right now, I am counting my pennies. Selling the chairs from under me to the highest bidder lol.

All the best.
 

Generic

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Oct 23, 2018
Messages
217
If you want a chart, ask Wolf, he's probably got some good data on that!

Here's my opinion, if you are interested:
1) Any cell that is 0.1V or less when liberated from its pack is a waste of time. It's permanently damaged. I don't have good data on it, but it's just not worth it.
2) Cells between 0.1V and 1V are almost always permanently damaged. If a cell is at like 0.85V, I might give it a chance, sometimes I get surprised. I do try to avoid them, though.
3) Now as far as voltage drop goes, you have to be precise. If you are using more than 1 charger/discharger/tester or even different slots in the same charger/discharger/tester, not all cells will be "full" at the same voltage. And your cutoff current will make a big difference, too, whether it is 100mA, 50mA, 20mA, or whatever. I usually charge to full on 1 charger, and mark the voltage of the cell 12-24 hours after the cell is charged to get a baseline voltage. I then go back 30 days later and test the voltage with the same meter that I used 30 days earlier. For me, if it's more than 0.05V lower, I put it in a second 30 day waiting period and test voltage again. The second time, if the voltage dropped more than 0.02V, I discard it as a self-discharger. Self-dischargers are bad, regardless if you are going 2P or 1000P.
 

Lux_Gamer

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Oct 11, 2016
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Speaking about voltage drop, what is an acceptable voltage drop while discharging,
I discharge my individual 18650 cells with 1A or 0,5A.
I also have a battery pack that I am testing right now that had four 18650 cells in parallel, the pack drops from 4,0V to 3,7V when discharging with 1A.
Seems to me that the cells are dead (the pack also didn't charge above 4,0V with a TP4056 module), but what are your opinions about voltage drop while discharging?


(My first post since a couple of years, good to be back!)
 

Redpacket

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Lux_Gamer said:
... the pack drops from 4,0V to 3,7V when discharging with 1A....

It should not drop instantly. Of course it's normal to gradually drop under load from full to empty per the usual voltage curves.
Some of your cells in that pack are likely poor capacity &/or have bad IR (internal resistance).
 

Lux_Gamer

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Redpacket said:
Lux_Gamer said:
... the pack drops from 4,0V to 3,7V when discharging with 1A....

It should not drop instantly. Of course it's normal to gradually drop under load from full to empty per the usual voltage curves.
Some of your cells in that pack are likely poor capacity &/or have bad IR (internal resistance).

It drops instantly when i connect the load. Of-course the voltage will drop overtime, but the voltage of the pack drops instantly when I connect the load. The pack is a 7s4p prebuild pack from a machine, I don't have fancy equipment to test the pack in one go so I discharge each parallel block on its own trough the balance connector.
 

OffGridInTheCity

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Dec 15, 2018
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Lux_Gamer said:
Redpacket said:
Lux_Gamer said:
... the pack drops from 4,0V to 3,7V when discharging with 1A....

It should not drop instantly. Of course it's normal to gradually drop under load from full to empty per the usual voltage curves.
Some of your cells in that pack are likely poor capacity &/or have bad IR (internal resistance).

It drops instantly when i connect the load. Of-course the voltage will drop overtime, but the voltage of the pack drops instantly when I connect the load. The pack is a 7s4p prebuild pack from a machine, I don't have fancy equipment to test the pack in one go so I discharge each parallel block on its own trough the balance connector.

When I've seenthis 'instant drop' in my 18650 battery experiments it was its because the cells cannot (easily) support thepower drain being placed on them. The max discharge specs of a particular cell don't always mean that they support that max discharge (or even 1/2 of the max discharge) 'easily' (e.g.without significant voltage drop).
 

Lux_Gamer

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OffGridInTheCity said:
Lux_Gamer said:
Redpacket said:
Lux_Gamer said:
... the pack drops from 4,0V to 3,7V when discharging with 1A....

It should not drop instantly. Of course it's normal to gradually drop under load from full to empty per the usual voltage curves.
Some of your cells in that pack are likely poor capacity &/or have bad IR (internal resistance).

It drops instantly when i connect the load. Of-course the voltage will drop overtime, but the voltage of the pack drops instantly when I connect the load. The pack is a 7s4p prebuild pack from a machine, I don't have fancy equipment to test the pack in one go so I discharge each parallel block on its own trough the balance connector.

When I've seenthis 'instant drop' in my 18650 battery experiments it was its because the cells cannot (easily) support thepower drain being placed on them. The max discharge specs of a particular cell don't always mean that they support that max discharge (or even 1/2 of the max discharge) 'easily' (e.g.without significant voltage drop).


I don't know what the discharge specs are for the cells that I'm using but the battery pack is used in a electric lift with a motor, and I only discharge them with 1A per pack of four 18650 cells in parallel so about 250mA per cell in theory if the cells where equal.
 

not2bme

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Oct 16, 2017
Messages
493
Voltage drop is normal during discharge. Look at this curve for a panasonic. At 650mA during the start it drops to 4V quickly from what I assume is the full charge of 4.2V. At 3250mA it is almost .25V less than 650mA.


image_dceblp.jpg
 

Redpacket

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4.0V to 3.7V drop with only a load of 250mA/cell sounds like a steep drop to me?
 
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