Lifepo4, never below voltage?

dogman dan

dogman dan

1 PW
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We know what Luke told us about lico (rc lipo), never let a cell get below 2.7v. Below that voltage, the cell gets damaged and might be dangerous to recharge.

We also know that some have revived lifepo4 cells that went lower, but not to 0v.

I know my pingbattery bms will shut the pack down if a cell gets below about 2.7v.

But what exactly is the lowest voltage you could expect to revive a lifepo4 cell from and succeed without a severe loss of capacity? Is it 2v, 2.3v? I can't imagine it being much below 2v?

What's your opinon, or experience with it?
 
my direct experience is that the ping v1 and v2.5 signalab will shut off at almost exactly 2.1V. the headway will shut off at exactly 2V.

i have accidentally discharged my headway pack down to .75V under a .5C load while testing a BMS and it bounced back, but i put it on the charger in seconds.

i have had some headways that self discharged from sitting to low voltages, .25, .41, ,55, .75V and they charged right back up and seemed to be unaffected.

i am finally getting a CBA so i will test them if i can to see if they show any after affects, but all the headways seem to head right to 3.41V after they come off the charger so these were no different. the screw tops anyway, the tabbed spot welded cells don't drop off as fast.

i have a ping pack i am working on now that had the first 4 cells discharge to low levels under the drain of the circuit current and they ended up being over discharged under load. they now will not charge up and hold that charge, and drop back to the 3.35-3.37 level. they will not balance with the other cells in the pack which all hold their voltage at 3.75-3.8V.
 
dogman said:
We also know that some have revived lifepo4 cells that went lower, but not to 0v.
<snip>
But what exactly is the lowest voltage you could expect to revive a lifepo4 cell from and succeed without a severe loss of capacity?
Click to expand...
I have attempted recharge of cells at 0V (in the Vpower packs) but they definitely have capacity loss and greater IR (not sure what the IR is, but it's obviously increased, watching voltages during charge/discharge).

Sometiems the cells are totally dead, and wont' even recharge. Others will recharge but leak quickly down to nothing. Others will recharge and hold it, but have severely reduced capacity. So far, none of the ones from less than a volt have recovered close to their orignal capacity, AFAICT, though I haven't done a truly scientific study on it, and it's been a while since I worked on the pack.
 
I came from the RC world before here, and I never took lipo's down below 3.0V per cell. Even the discharge option in my RC chargers are 3.0V..... 2.7V may be on the wild side...

I have had a A123 Cylindrical cell (lifePO4) that I bought used and the voltage was down to 1.2V or 1.4V I can't remember for sure. I managed to revive the cell, but the capacity loss was severe. It will only hold 1200mAh or so. Normally they are 2300mAh. I also don't know how long the cell was sitting at that voltage. 2.0V per cell LVC is normal. I have my ebike controller set for about 2.3-2.4V per cell though. really once you get down in that voltage range there is not much capacity left. Around 50mAH from the 2.3V to 2.0V, and thats 3 in parrallel.
 
The lifepo4 seems to have a flatter discharge curve. I have a chart from A123 showing discharges in different temps, and seems that 2.6 would be about the beginning of the cliff on the cells. I'm planning on running mine from 3.3-3.4 down to 2.6-2.7.

I can get the URL if you are interested, dogman.
 
Thanks for the info. I confirms what I was thinking, but by now you well know I get things backwards or just plain wrong at times. Like typing 50 amps, the othe day when I meant to type 50watts.

Sounds like never letting lifepo4 get below 2v is definitely the aim. I just wondered what the effect on the resistance and capacity was if you let em get to 1v for example. Clearly 0v is definitely too low, and I'd be feeling less optomistic about a cell that got as low as .5v. But it sounds like reviving a cell from 1.5v is definitely possible.

Only time it ever happened to me, my first ping was at .5v, for the whole stack of 12s! It was puffed the size of a beach ball, and clearly past revival. So I haven't had a chance to play with lifepo4 revival.
 
I find different cells in series come to have a personality that shows up over time. Yes even with the pre balance and always bms balance cells develop a little different over time as what's the rule. No true rule. Lighting bolts in a box sometime 16 sometime 24. Pick out your best lighting bolt. There are all a little different. It's when we demand to see thier personality. hvc-lvc ect.
 
Yeah, every battery I own now is at least 2 years old. Both my ping and my lipo's are getting personalities. What's odd is I mark down which cell is quirky, and a few months later that cell is fine and another one is the cell that gets slightly out of balance. No telling what causes that.
 
I watch my 24s A123 as I don't run a bms and the bottom cell on the rear rack seem to be the weak one. Took the a16ah ride and a different cell in that pack was a little lower. I thought I found a weak pouch and put a alarm on that one but the weak cell moved ? The other 12s is perfect. I don't like it sitting flat on the rear rack. Just watch and see.
 
Its always weird to see the bottom end balance of packs they can be so far out of balance sometimes. Less than 1 minute on my charger they all sitting back at the same voltage again. Certain Lifepo4 batteries you have to allow for a deeper DOD or LVC to account for the high IR. Thundersky is one. The have a large voltage sag and the rebound is pretty big after the load is taken away. I used to use 2.0v a cell on those and still never quite got full capacity out of them cause the way it used to trip the LVC but it wasnt truely dead. This was before I had an Rc charger so I never quite knew the capacity of the pack. I just had a meanwell and some medics.
 
I have had my a123 26650 LifePO4 packs down to .8v and .6v per cell respectively. Ran the batts down to LVC, parked the bike, then left the controller on all weekend. I lost a couple cells the first time and five the second. But, after the replacement of bad cells I noticed no dropoff in the miles I could travel even after a year plus. I seldom ballance charge these either run the 66V packs down to 40V LVC then bulk charge to 73V. These are the old paperstock wrapped cells. Hard to ask for more from a cell in todays market.
 
Yesterday I plugged the ca in on my pack (a stand alone) it wouldn't even light up. A 23s 12 p konion V cells. Pack read 5 v
put meanwells on and charging real slow. Seems to be coming back, at 92v now.
Killed it cause I left controller on , 6w draw.
 
Katou sent me a bunch of A123 cells that were at 0V to test my welder. On a whim I charged them up. Half were perfectly good and are still going strong. I did manage to kill one by leaving a 4S pack connected to a load for a month.
 
none of this has presented any factual information. i have a ping pack i am working on that has three cells 2,3,4 that were over discharged badly and they will self discharge back to 3.35-3.37V within a few hours of being charged up. they will not balance with the other cells in the pack and they seem to just eat any charge that goes into them above 3.35V.

so i let them sit for a few days to settle at 3.35V and now i have started a discharge to test their capacity at this point, to see how much capacity is lost from this kind of abuse.

but it takes time, i am discharging into 3 cement power resistors, 14 ohms equivalent, so about .71A current to drain 20Ah is gonna take a day and a half if they are close to full.

after 7 hours the voltage is still 9.94 down from 10V at the start, almost 25% of expected normal capacity already but i gotta wait until tomorrow to see how much they deliver.
 
ok, those ping pouches which would drop to 3.35V have now finally been discharged. after a day and a half.

at .03C, the three cells of 4 pouches each, 20Ah, discharged into 14 ohms of power resistors, .71A initial current, 10V for the three, discharged to .47A, 6.78V when two of the cells reached 2.1V. took a total of 31 1/2 hours at an average of about .67A so these 'dead' cells which had been over discharged severely still produced the full 20Ah of juice, and then some.

i think this is the result of the super low discharge rate so i am currently repeating the test with some more load in parallel initial discharge is 1.5A so this should be done in 14 hours or so.

i had planned to remove these cells from the pack because they discharged so fast and would not balance but now i think i will leave them in the pack for now. unless this current test indicates otherwise.
 
Any idea how badly overdischarged those cells were? I'm assuming below 2v of course.

One thing I've noticed, is that my ping packs can get "stale" if not used regularly. After 2-3 cycles, they perk back up, much like the initial break in process, which really is just getting a good balanced charge on the pack more than anything else. The first few cycles tend to leave a few cells taking a full charge, but not developing a full capacity.
 
Good Morning Dogman!

I've posted before the near total discharge of my Headway 38120HP Red Cells. 40S plugged into the low draw controller for 35 days.
Apparently if you take them down slowly with little load, they can come back. 36 out of 40 came back for me. I've since built a
12s2p using the best 24 cells. This is now in my Catrike which see's 30amps peak (around 2c). I've put voltage alarms on all the cells
set to 3.0v (as this is the cliff I noticed for these cells). So under the 2C loads, no alarm has gone off yet.
I should go for a long ride and see if Capacity is down.

Tommy L sends... \\m//
 
dogman said:
Any idea how badly overdischarged those cells were? I'm assuming below 2v of course.

One thing I've noticed, is that my ping packs can get "stale" if not used regularly. After 2-3 cycles, they perk back up, much like the initial break in process, which really is just getting a good balanced charge on the pack more than anything else. The first few cycles tend to leave a few cells taking a full charge, but not developing a full capacity.
Click to expand...

i don't know how badly over discharged they were before he brought me the battery. when he arrived, they all read 1V on the first four cells. i put the single cell charger on them for an hour or so while he was here and they never got above 3.35 then and when i rebuilt the pack they never got above 3.35 until i charged them up to 3.7V with a power supply, but they fell right back to 3.35V.

i finished the second discharge test at 1.5A last night and got more than 20Ah out of it again so i decided to leave them in the pack.

his BMS had the ouput mosfets shorted so there is no way to know how low it went. he said he had to stop and reset the BMS several times when it went to LVC on the road and the leds would never ignite on 1-4 after that. so it never got above 3.6V after that event.

i was totally surprised to find they still held that much charge. really changed my opinion about how i view cells that will not charge up and balance with the others. i used to think they were totally dead, not anymore. just lower voltage at rest, and i am still assuming the internal resistance is much worse too.
 
having the same internal resistance during charging allows all the cells to fill at the same rate and balance together at the same time.

on discharge, higher internal resistance can cause that cell to hit LVC long before the others and restrict total available capacity. which is what was happening to this pack, imo.
 
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