Best method to discharge cells to storage voltage

[mickyd]

I have to bring down about 400 individual 18650 harvested cells from full charge to ~3.6V storage voltage. What's the best method to use to discharge this number of cells in a reasonable time?

I started today using a 4-port Liitokala Li-500 NOR mode at 1000mA. This process is VERY time consuming, taking several hours for each group of cells AND having to watch the display closely pulling them off discharge when they shows about 3.3V . The cells measured directly at this point are about 3.6V when removed.

Looking for a faster method. Suggestions?

 

[docw009]

Atorch load tester, about $36 on aliexpress and 15 days. Twice that amount on amazon and get it sooner. It will discharge at a constant current 1-20 amps, limited by power dissipation. It can only take about 150W. I use it mainly to test batteries at 3 amps. It can take a single cell down to any voltage you set, but it suffers from voltage bounce. The cells will usually rebound .10-.20 volt after you remove the load.

Or you get various power resistors and connect them in place of the tester. The danger is you forget and drain them flat. The load tester has a minimum voltage that I set.

If you get one, only change settings with no battery attached to prevent power glitches which blow up the tester.

 

[mickyd]

Atorch load tester, about $36 on aliexpress and 15 days. Twice that amount on amazon and get it sooner. It will discharge at a constant current 1-20 ....

Thanks for the post. That looks like a good option. Found the Atorch DL24 with bluetooth on the Ali website. Approximately how long does it take to discharge at say 3A?

You mentioned also using power resistors. I once used a 1ohm 20W ceramic resistors to bring a single cell voltage down about 0.15V and seem to recall it only took a minute or so but the resistor got very hot to the touch after 20 seconds or so so I'd switch to another resistor (I have 10 total). I = V/R so the current was 4.2V / 1 ohm = 4.2A, right? Would that also be a good method to bring the cells down from 4.2V to 3.6V? If so, how hot can those babies get?

 

[eMark]

Found the Atorch DL24 with bluetooth on the Ali website. Approximately how long does it take to discharge at say 3A?

3A discharge rate with 400 cells from 4.2V to 3.65V will take forever. If you decide to buy get the DL24P discharging at 15-20amps rate with no more than 150W drain. It's the ATorch DL24P (180W rating) shown in above photo, but best not to discharge above 150W for extending usable life as long as possible.

Here's one example with six used (good condition) 18650 30Q cells connected in parallel. They were at 4.11V. Then discharged at 15amp rate for 45 minutes with bounce back resting voltage at approximately 3.70V. Figured cell capacity of 2700mAh x 6 = 16.2Ah capacity.

Wattage draw as high as 58W at beginning and 46W at end of 45 minutes. 150W divided by 50W drain = 3 x 6 cells equals 18 cells in parallel figuring 150W drain. So, 400 cells ÷ 18 = 22.2 times. So, 23 x 45 minutes = 1035 minutes to discharge 400 18650 cells (2700mAh) from say 4.20V to 3.80V (bounce back resting voltage after 2 hours).

That's a rough estmiate of discharge time if actual mAh capacity of each cell is about 2700mAh. If only storing for a few months then 3.80V or even 3.85V is ok. Best to at least balance all 400 within 100 mV (0.10V) of each other before discharging to storage voltage. You can always balance them later within 25 to 30mV before fabricating your battery pack.

Another quicker way with 30 cells at a time (6s5p) using a Vruzend V.2 kit. Then with two used 12V headlights (connected in series) discharging to desired resting storage voltage. Nice if you could use a Watt Meter inline to track discharge wattage, voltage, amps. There may be an ES member that would give you his V.2 kit that he no longer uses (just pay shipping/handling). Don't use the older V.1 kit (even if free) as it doesn't come with shoulder bolts so end caps aren't always compressed against cell ends for good contact.

Hope this isn't confusing, but maybe helpful.

 

[mickyd]

Here's one example with six used (good condition) 18650 30Q cells connected in parallel. They were at 4.11V. Then discharged at 15amp rate for 45 minutes with bounce back resting voltage at approximately 3.70V. Figured cell capacity of 2700mAh x 6 = 16.2Ah capacity.

Wattage draw as high as 58W at beginning and 46W at end of 45 minutes. 150W divided by 50W drain = 3 x 6 cells equals 18 cells in parallel figuring 150W drain. So, 400 cells ÷ 18 = 22.2 times. So, 23 x 45 minutes = 1035 minutes to discharge 400 18650 cells (2700mAh) from say 4.20V to 3.80V (bounce back resting voltage after 2 hours).

Appreciate the math and process of coming to the 1035 min. [17 hr.] estimate using 18P. Makes sense and help me and others see the thought process. Ohms law in action with power and time added.

Another quicker way with 30 cells at a time (6s5p) using a Vruzend V.2 kit. Then with two used 12V headlights (connected in series) discharging to desired resting storage voltage. Nice if you could use a Watt Meter inline to track discharge wattage, voltage, amps. There may be an ES member that would give you his V.2 kit that he no longer uses (just pay shipping/handling).

Rather than draining using headlights as suggested, I'll configure a 48V (13S) E-bike pack and also get some sun and fresh air while discharging. Monitoring the pack during the ride and stopping at 48V should get most if not all parallel groups close to 3.7V. Then, I could measure the individual parallel groups and if needed, charge / discharge any groups outside a range.

The loaner Vruzend V2.1 kit is also a great idea. Getting 2 would be even better. Could do 52 cells (13S4P) at a time. I'll post separate on that loaner request or who knows, maybe buy instead as I'll probably continue to harvest cells. $50 per kit with shipping isn't a bad price. It will compliment the ATorch DL24P that I think I'll buy too.