having trouble properly discharging, getting capacity

wattwatt

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May 21, 2018
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I'm trying to get the capacity of18650cells I harvested from laptop batteries via fully charging and then fully discharging them, but the discharger stops and theresultingmAh's are much lower than the cell's rating and I even notice the resulting/resting voltages are around or above nominal. Here's a test showing what's happening...:


Code:
Samsung ICR18650-28A

Rated specs:
2800 mAh (capacity)
1400/2800 mA (standard/max charge current)
560/5600 mA (standard/max discharge current)
3.75/4.30/2.75 V (nominal/charge max/discharge cutoff voltage)

Discharge settings:
500 mA (discharge current - 100 mA to 12000 mA are the options)
3.1 V (discharge cutoff voltage - 3.0 to 3.75 V are the options)

Test results:
1414 mAh (capacity)
3.06 V vs 3.71 V (discharger's reported voltage when it stopped vs voltage reading I got from multi-meter)
206 minutes (runtime)

Equipment:
XTAR VC4 (basic USB powered 4 slot multi-charger) to charge cells
HTRC H150AC/DC DUO (150W hobby charger) to discharge cells

Any ideas why the capacities are coming back so low or why thecells don't appear to fully discharge?

P.S. - I know the cells are good because I charged them in November 2017 (actually might of been 2016)and when I took them out today to top them offfor their capacitytest they all read 4.1 V or higher beforehand.
 

Korishan

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What are they getting discharged to?
It's possible (tho maybe not likely) that when they drop below a certain voltage under load, they rapid drop. For instance, they could discharge @ 500mA from 4.1V to 3.4V, then all of a sudden they drop to 3.1V and the discharger stops. So you could be not getting mAh ratings from 3.4 to 3.1V, which is a lot.

I would watch them and see what happens.
 

jonyjoe505

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I think your discharger is bad or not program right, it not taking it all the way down to 3 volts. I trust the voltage reading on your multimeter of 3.7 volts, still got plenty of power left.

Also 206 minute runtime on charger will give you3.5 hours of discharging at 500ma, about 1600 mah, if the cell is the full 2800 it would take about 6 hours to discharge at 500ma. Try discharging at 1 amp, it wont harm the battery I capacity test all my 18650 at1 amp.

Maybe the charger has a timer that shutsoff charger at a preset time for safety reasons. One of my triton chargers had such a timer but you can disable it or increase the the timer setting.
 

floydR

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How long after the tester stops discharging is the check with mulitmeter done?

Could the voltage difference be cell rebound?

Have read keeping cells in storage at a high state of charge is bad for them, 3.8v is a better storage voltage.
Are these cells , used laptop cells?

Later floyd
 

wattwatt

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May 21, 2018
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Korishan said:
What are they getting discharged to?
It's possible (tho maybe not likely) that when they drop below a certain voltage under load, they rapid drop. For instance, they could discharge @ 500mA from 4.1V to 3.4V, then all of a sudden they drop to 3.1V and the discharger stops. So you could be not getting mAh ratings from 3.4 to 3.1V, which is a lot.

I would watch them and see what happens.

Good point.I thinkI'll even put a camera on the discharger in case I miss it.

jonyjoe505 said:
I think your discharger is bad or not program right, it not taking it all the way down to 3 volts. I trust the voltage reading on your multimeter of 3.7 volts, still got plenty of power left.

Also 206 minute runtime on charger will give you3.5 hours of discharging at 500ma, about 1600 mah, if the cell is the full 2800 it would take about 6 hours to discharge at 500ma. Try discharging at 1 amp, it wont harm the battery I capacity test all my 18650 at1 amp.

Maybe the charger has a timer that shutsoff charger at a preset time for safety reasons. One of my triton chargers had such a timer but you can disable it or increase the the timer setting.

I disabled the safety timer shutoff feature when I first got it so that's not the culprit. I have another custom rigged electronic load/battery discharger I got off Amazon and I can set the low voltage setting to whatever I want (or disable it entirely), but it doesn't auto shutoff, it just beeps when it reaches low voltage, so that's not safe as it'll damage the cell. I'm currently running discharges at 100 mA/0.1 A and a 3.0 V cuttoff so we'll see if that gives us better results. I'll run the next tests at 1000 mA/1 A as you suggested.

floydR said:
How long after the tester stops discharging is the check with mulitmeter done?

Could the voltage difference be cell rebound?

Have read keeping cells in storage at a high state of charge is bad for them, 3.8v is a better storage voltage.
Are these cells , used laptop cells?

Later floyd


Good point about the rebound - sometimes I don't check voltage until an hour or two after discharging stops.I'll payclose attention after the nexttest and check immediately.

The cells were charged sometimes years ago when I didn't knowabout storage voltage. Rookie mistake.

Yes these are laptop cells. The main use of the harvestedcells will be an e-bike battery and other tinkering projects (like powerwall).
 

thunderheart

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crashintoty said:
Any ideas why the capacities are coming back so low or why thecells don't appear to fully discharge?

1. Your cell is 4.3V but your charger charges it at 4.2v. Here you lose about 15% of capacity.

2. Your cell's discharge cut-off voltage is 2.75V but your discharger is set to 3.1V. I don't know how much capacity you lose here, but i'm pretty sure it's minimum 15%.

3. To calculate the capacity you should discharge it at 0.2C (0.56A in this case) so your 500mA setting is pretty good here and should not affect the result.

Conclusion: try to follow the end voltages mentioned in the datasheet :)
 

wattwatt

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So I used the electronic load/custom discharger with current at 1000 mA/1 A and cutoff voltage lowered to 2.5 V and I got 2000 mAh discharged in about 2 hours then the 2.5 V warning sounds off. The discharger actually stops discharging for a second when hitting that 2.5 V, but while it's stopped for a second or two the voltage reads 3.11 V on the display and then the discharger kicks back up immediately. Unfortunately the discharger starts looping off and on soon after jumping from 2.5 V and 3.11 V. I turned the current down to 250 mA/0.25 A and it discharges for about 10 to 15 minutes but then starts looping off and on again when it hits 2.5 V again.
 

rev0

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Under load the cell will drop voltage also, but at only 500mA I wouldn't expect that much drop. Check your cell connections, you can probe directly on the cell terminals with a multimeter to check the real voltage vs. what the discharger is saying.
 

Korishan

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Just curious, why are discharging to 2.5V? Is it because that's the what datasheet says or because you plan on draining the cells to that point? If it's the latter, keep in mind that charging to full 4.2V and discharging to full DoD of 2.5 will yield the shortest cycle life of the cells. With a range of at least 4.1 to 3.2V will give you 1000's more cycles.
Also note that there is not much capacity between 3.0V and 2.5V. It's like driving on fumes in a vehicle at that point. Barely any throttle just to go an extra 1000 meters or so.
 

thunderheart

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Korishan said:
Just curious, why are discharging to 2.5V? Is it because that's the what datasheet says or because you plan on draining the cells to that point? If it's the latter, keep in mind that charging to full 4.2V and discharging to full DoD of 2.5 will yield the shortest cycle life of the cells. With a range of at least 4.1 to 3.2V will give you 1000's more cycles.
Also note that there is not much capacity between 3.0V and 2.5V. It's like driving on fumes in a vehicle at that point. Barely any throttle just to go an extra 1000 meters or so.

4.2V is not its charge end voltage. It's a 4.3V cell with 2.75V discharge cut-off. So charging it at 4.2V will increase the cycle life by 100s but not 1000s (in case of not discharging below 2.75V of course).

I'm going to publish a series oftests of a 4.35V cell charged at both 4.35V and 4.20V with capacity difference comparison. Hope it will be helpful in such cases:)
 

wattwatt

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May 21, 2018
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Yes, it appears both dischargers drop sizable voltage under just about any load (100 mA to 1000 mA) towards the end of discharging and then rebounds after a couple of minutes to at least 3.6 V or higher. From all of your input I'm assuming this is not suppose to happen (sizable voltage drop towards after discharging for a while) even though these cells are used/harvested and only have a 2C/5600 mA/5.6 A rated discharge current? If this is normal, then what can I expect if I stubbornly used 140 of these anyway for a 14s10p battery pack for a 52 V/30 A (1560 W) e-bike motor?
 

Korishan

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wow, if they are swinging that much even under 100mA load, I don't think it'd be worth it. No cell should swing that hard when under light load. Especially at those voltages. Maybe if they were around 2.2V it would, but not jump from 3.6V
 
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