Ways to re-charge a lipo battery after it drops below 2.7v

Skip to the bottom if you want the TL;DR version.

I have 3 of these batteries: http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=18631

Took them out for their first test ride yesterday, and everything was working great. I stopped after about a mile and grabbed my voltmeter to check the cells. Things were looking good. The first pack was at 3.73 for all 4 cells, the second pack was at 3.72 for all 4 cells, and the third pack was at 3.74 for all 4 cells. This was after 1 mile of riding. I take it about another mile and a half, and stop to check the cells again. Suddenly, there was a MUCH higher drop in voltage than there was after the first mile. Before I set out, the cells were all at 3.79v, across all 3 packs. After 1 mile, they had dropped down to 3.73v, 3.72v, and 3.64v, so a loss of about .04v to .07v over 1 mile.

Now, that second mile and a half, for some reason, drained the batteries much, much more. Whereas before, 1 mile took an average of .05 volts across all cells, that second 1.5 miles took the first pack from 3.74 to 3.29, the second pack down from 3.72v to 2.66v, and a couple cells at 2.8v, and the third pack down from 3.74 to 3.34v! Why would the second leg of that ride have taken so much more out of the pack? If 1 mile takes approximately .05 from each cell, then 1.5 miles should take approximately .075v from each cell. Of course there will be differences between uphill/downhill, speed, wind resistance, etc, but THAT much of a difference doesnt make sense. The second leg of the ride was actually abour 40% downhill, and there was no more wind than the first part, although I did go slightly faster for the non-downhill part.
But, even with all that take into consideration, it still doesnt make sense that it would be that drastically different. I mean, i can understand if the first mile took out .05v, and the second mile and a half took out, say, .15v (which is triple the first mile). But, going from .05v loss to over .5v loss, and even 1.02v loss in the middle pack?! WTF?

Another thing I dont understand, is why the middle pack would have been drained so much more. I realize not all packs will be drained exactly evenly, but for two of them to be drained by .5v, and another to be drained by 1.01v is a very big difference.

So, all that being said, my biggest concern is this: When I got home, I plugged all 3 batteries in to charge, each with one of these: http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=21044
being powered by a 12V 5.4A SLA car battery. The two packs that werent drained as much (the ones that ended up at 3.29v and 3.34v) charged up just fine, no issues. But, the middle pack, the one that dropped down below 2.7v per cell, wont charge at all. When I plugged it in to charge, the pack was at 10.8v total. A few hours of charging later, it had actually DROPPED down to 10.5v total! Whats even more confusing is that after leaving them all unplugged and not charging overnight, it had gone UP to 11.3v WITHOUT being charged! I understand that not going below 2.7 is one of the 'Golden rules of Lipo" but this still doesnt make sense. Everything Ive read just says that going below 2.7v will result in an overall shorter lifespan for the batteries. As in, go below 2.7v and you might only get 3000 cycles instead of 10000. Ive never read anything that said a lipo battery wont charge AT ALL after dropping below 2.7v.

None of this seems to make sense, so any help is greatly appreciated.

So, to recap, the main issues I need help with are:

• Why would the battery pack(s) be drained by only .05v on the first mile of the ride, but then drained by over 1.01v over the next mile and a half?
• Why would the middle pack be drained by so much more than the other 2 packs?
• Why wont the middle pack charge at all any more?
• Why/how would the middle pack not charge when plugged in, but go from 10.5v to 11.3v overnight when NOT plugged in at all?
• Is there any way to 'fix' the middle pack so that it will charge again and get back up above 2.7v?

TL;DR version:
First lipo setup, running 3 of this battery http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=18631
Took it for a ride, first mile took each cell down by about .05v, second mile and a half took each cell down by over 1.01v, and that doesnt make sense.
The pack in the middle was drained far more than the other 2, going from 3.72 all the way down to 2.6, whereas the other two packs went from 3.73 down to 3.3-ish. Why would the middle pack be drained so much more than the packs on the left and right?
Now, the middle pack wont charge at all. Is there any way to make it charge again?


Any and all help is greatly appreciated.

You need to test capacity of all of your packs individually to make sure you're matching things well, and be extra careful with any that over-discharged.

When the charger won't accept a pack to charge due to low voltage simple go to the next lower cell count. eg For one of your 4s packs that the charger rejects as voltage too low, simply set the charger on 3s and start the charge. Once you've raised the voltage enough you can switch it back to 4s and balance charge it.

John in CR said:

You need to test capacity of all of your packs individually to make sure you're matching things well, and be extra careful with any that over-discharged.

When the charger won't accept a pack to charge due to low voltage simple go to the next lower cell count. eg For one of your 4s packs that the charger rejects as voltage too low, simply set the charger on 3s and start the charge. Once you've raised the voltage enough you can switch it back to 4s and balance charge it.

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Im not sure what you mean by 'set' the charger. There's nothing to set... Theres a slot for 2s, 3s, and 4s balance leads, and a plug for the power supply, and thats it. Thers no controls or switches or anything like that, so I dont think its possible to set anything.

Also, its not that the charger wont accept it; it operates like normal and doesnt kick up any error message, it just doesnt charge for some reason.

how do you know it is not charging? did you measure the current going into the terminal of the battery pack?

Does your charger have a nimh mode? That should get those packs started.

dnmun said:

how do you know it is not charging? did you measure the current going into the terminal of the battery pack?

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I know its not charging because the voltage isnt going up, even after being plugged into the charger for several hours. (As I said in my original post.) The other two charger just fine, all the way to full in a few hours. The middle one isnt charging at all even though it was plugged in for the same amount of time.

bee said:

Does your charger have a nimh mode? That should get those packs started.

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As I already said, there IS no 'mode' or 'set' or anything. There is NOTHING on this charger except for the slots for the balance lead, and the hole for the power supply. No buttons, no switches, no dials, no nobs, no levers, no NOTHING. There is no setting ANYTHING on this charger.
Take a look at the link. You can clearly see there is NOTHING on this charger that allowed you to charge or switch or set anything.

Did I read this right? batteries in to charge, each with one of these: http://www.hobbyking.com/hobbyking/stor ... duct=21044
being powered by a 12V 5.4A SLA car battery.
Once again 12v 5.4 Ah SLA? YGTBK!
It's pretty hard to put more Ah in, than what's powering the charger.

What are you running that kills a battery in 2 miles? I know you are only running 5Ah so 4-5 mile limit at 20mph on flat laod.

Dan

It's best to charge the cells at a very low rate until they get over 3.0v. It sounds like your charger is in some kind of safety mode that won't let it start. You may need some other kind of charger to bring them up.

fechter said:

It's best to charge the cells at a very low rate until they get over 3.0v. It sounds like your charger is in some kind of safety mode that won't let it start. You may need some other kind of charger to bring them up.

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Hmmm... Ok, humor me for a moment.
I have an adjustable-voltage power supply, very similar to this: http://s3.amazonaws.com/kpsurplus_images/0ba45ae81d2ed61f8f60893be603baaec36fe160.jpg
Except that the one I have is 6-12V, and 2.5A.
Could I, in theory, simply connect the positive and negative wires of this power supply directly to the appropriate contacts on the balance lead, and charge the pack one cell at a time? Yes, yes, very risky, super dangerous, I know. I would be monitoring the pack very closely the whole time. Aside from the potential risks, could this work?

I mean, the balance lead on a 4S pack has 4 contacts. Ive been checking the voltage of each cell with my volt meter by putting the prongs from the volt meter on adjacent contacts, which gets me the reading I need for one cell. So, could I basically do the same thing, but with a straight power supply? Im not sure if Im doing a good job of explaining this, but let me know what you think.

DAND214 said:

What are you running that kills a battery in 2 miles? I know you are only running 5Ah so 4-5 mile limit at 20mph on flat laod.
Dan

Click to expand...

This is what this battery pack is running: http://www.ebay.com/itm/Rear-Wheel-Electric-Bicycle-Conversion-Kit-24V-36V-48V-250W-500W-700W-800W-1000W-/290754592384?pt=LH_DefaultDomain_0&var=&hash=item43b252fe80 (36v, 800w version of that)

the problem with that is the limit on the current you can push through the sense wire and the fact the pins are so close you would short out anything you tried to stick into the plug. plus it will damage the contacts too and that will affect the future connections to the charger during balancing. but it would be interesting to know if your charger is pushing current all this time and the voltage is not going up. if it is pushing current and the voltage does not go up that is a serious problem. imo

omegagamer89 said:

fechter said:

It's best to charge the cells at a very low rate until they get over 3.0v. It sounds like your charger is in some kind of safety mode that won't let it start. You may need some other kind of charger to bring them up.

Click to expand...

Except that the one I have is 6-12V, and 2.5A.

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You can voltage wise (being ever watchful as you said) , but as fechter suggested you should charge it up at a very low rate - ideally you'd have something limiting the current (resistor, light bulb, lower amp power supply).
Something like 100 to 500ma would be my idea of slowly.

With a 5Ah pack you can only really plan on using 4.5Ah, so don't expect too much range at full throttle even when fully charged.

EDIT: just gonna go ahead and reverse the order of my advice for safety's sake:

Be really careful if you try to charge that pack back up (as in, do it outside)...technically you shouldn't even try to recharge a cell after it's dropped below 2.7V as it has been compromised. You might be just fine, but you might be risking a fire.

If you are set on reviving the pack, I'd suggest ordering a new and better charger with actual settings on it. Charging equipment is not the place to save bucks when using Lipo. Once you get your new charger, as John in CR said, you can just select the next setting down, then once they are charged up enough, go back to the proper setting. Also know that even if you can bring it back to life one time, the damage has already been done and you are taking a risk every time you charge it thereafter.

FYI, I once accidentally drained a fairly expensive 10s pack below 2.7v/cell and I cut my losses...just not worth the risk IMO

No offense, but someone who doesn't know a lipo at 3.7v/cell no-load is dead shouldn't use them.

You started at 3.8v/cell.. thats the storage charge they are shipped with! You didn't even charge them before the ride.

Those batteries may be trashed. Please educate yourself before you burn down your house.

Edit: i just noticed most repsonses here are telling you how to revive your ruined batteries without explaining how you destroyed them. Yikes.

Why would the battery pack(s) be drained by only .05v on the first mile of the ride, but then drained by over 1.01v over the next mile and a half?
because voltage drop isn't linear, or a reliable way to measure capacity/state of charge
Why would the middle pack be drained by so much more than the other 2 packs?
because totally empty batteries are unpredictable
Why wont the middle pack charge at all any more?
because lipos discharged below 80% even once are damaged.
Why/how would the middle pack not charge when plugged in, but go from 10.5v to 11.3v overnight when NOT plugged in at all?
Damaged lipos are unstable. Internal shorts, chemical decomposition,
Is there any way to 'fix' the middle pack so that it will charge again and get back up above 2.7v?
You can't undo damage. Sometimes packs are usable, but die soon after. not worth the risk.

Here's a cool picture you could learn from:
http://neptronix.org/forumpics/lipotutorial/24_dischargingmechanics.gif

+1 to what RedLine19K said...
you started with near empty packs and simply sucked those packs dry and ran them over the cliff on that discharge chart.
I suspect your "test ride" was also drawing Amps at a higher rate than the 8A used for Neps test chart data, so your discharge curve would be much steeper,,,not to mention the accuracy of a voltmeter needed to accurately check the cell state.
Sorry buddy, but thats an expensive way to learn LiPo basics.

Do you have access to a 12V auto battery? If so parallel your low 4s pack with a fully charged 12V battery, which should be 13-13.6V. That should bring the cell voltages up enough to use a regular charger on them. If you have a regular auto battery charger, you could also use that to charge a 4s pack. 12V chargers put out ~14.5v so should be safe to use and bring the pack up in voltage. After that use your regular charger to fully charge.
Now, I don't know where you got the idea to to start riding your bike with the battery at only about a 30% charge. That's about what a pack capacity is with cells ~3.8V. So that's about 1.5ah of battery you started with. Assuming you rode pretty much wot, that's about a 1.5-2 mile range. Next, I don't like your chargers. They should work, but you'd be much better off with a single 6s charger and parallel charging, or a 12s charger to charge them all at once in series. Those chargers you bought output 1-1.5A max, so it's going to take up to 5 hours to charge a completely discharged pack. Good luck.

After you get your pack going again, get one of these and stop riding when voltage hits 44V.
http://www.ebay.com/itm/Waterproof-Digital-Voltmeter-DC-15V-To-120V-Red-Led-Voltage-Digital-Panel-Meter-/170846310527

Also, if you are using a 48V controller, it should have hit LVC before you ran your pack down that low. I can only assume you don't have a 48V controller and it's lvc is lower than 42V.

You may have a dud pack.
I made a detailed tutorial for charging and identifying dud packs here:

http://endless-sphere.com/forums/viewtopic.php?f=14&t=52240

2.7V is not destroyed, just empty of charge. but if you are pumping coulombs in and no voltage comes out then it has to be going somewhere. sorry you missed that part.

omegagamer89 said:

I have an adjustable-voltage power supply, very similar to this: http://s3.amazonaws.com/kpsurplus_images/0ba45ae81d2ed61f8f60893be603baaec36fe160.jpg
Except that the one I have is 6-12V, and 2.5A.
Could I, in theory, simply connect the positive and negative wires of this power supply directly to the appropriate contacts on the balance lead, and charge the pack one cell at a time?

Click to expand...

No, because you need voltage between 3 to 4.2 to charge individual cells.

omegagamer89 said:

I mean, the balance lead on a 4S pack has 4 contacts.

Click to expand...

You sure it isn't 5? precision is kind of important if you are considering something "super risky"

wesnewell said:

Do you have access to a 12V auto battery? If so parallel your low 4s pack with a fully charged 12V battery, which should be 13-13.6V.

Click to expand...

STOP giving this guy bad advice! Do you know what the amps will be connecting an 11v battery to a 13.6v source?! No, you don't because you have no way of knowing the resistance of these damaged cells. Could be 2A.. or 20A and the pack bursts into flames before he can disconnect it. Do you even know how to calculate the current on a 4s pack with 2.6v differential and .015 ohm/cell?

The OP is a total beginner, totally lacking the basic knowledge of why 2.7v/cell is bad, or 4.3v is even worse. The packs are ticking time bombs and yes I am exaggerating because a little extra caution right now is a good thing. Instead, we have a series of helpful but dangerous suggestions on how to force the packs back up to normal voltage - a practice usually discouraged except for the most cautious and experienced EVers.


Be careful while you sit in a comfortable chair miles away from these unstable batteries, their desperate owner and the possible consequences of your advice.

a battery drained to 2.7V is not an unstable firebomb. it is just discharged.

Ok, not gonna reply to each post individually as is almost 5AM here and I need to get to bed,\.

I tried the idea I had above about using the radioshack power supply and charging one cell at a time, and it seems to have worked. All the cells are up over 3v now. Tomorrow Im going to balance and fully charge it.

As for the people saying it was dumb to ride with only 3.8v per cell, I fail to see how. 3.8 is not "almost empty." In fact, 3.8 is much closer to full than it is to the minimum discharge.
If the max charge per cell is 4.3v, then 3.8v is only .5v away from full. Whereas its 1.1v away from 'empty' (AKA 2.7) I mean, sure, 3.8v isnt full, but its also nowhere near the 2.7v cutoff. I mean, if I had started riding with, like, 3.0v per cell, then yeah, thats pretty dopey. But if 2.7 is 'empty' (AKA 0%) and 4.3 is full (AKA 100%, then that means 3.8 would be close to approximately 70% full.


To those saying the chargers Im using are lousy, yes, I figured that out already. Theyre not 'bad' per se, they just charge really slowly. Ive already order better ones.

Also, to Wesnewell, why on earth would I stop riding at 44v? If the minimum voltage you ever wanna hit is 2.7v for any individual cell, that would mean I could keep riding until 32.4v total. (2.7v per cell x 4 cells per pack x3 packs = 32.4v)
Now, obviously I wouldnt wanna ride RIGHT up until they ran out. I would want to head in and stop riding when they started getting CLOSE to empty. But even then, even if I turned in once they hit 3v each, thats still 36v, waaaaaay lower than the 44 you suggested. Im just curious as to why you would say stop once it hits 44v, when thats not even 20% drained.

P.S. I already have some low-voltage alarms. I just didnt hear them in this instance. Ive repositioned them to be easier to hear next time.

This is a great example of how lipo becomes lipoo.

You took off and went riding with a pack that was at least half discharged, and were completely noobish to expect another mile from that pack when it was down to 3.7 ish volts. Now you know, there aint much below 3.7v in a pack that small. You are the kind of guy that needs to learn a lot in a hurry, before you burn your house down.

Technically you broke the lipo rules with that one cell.

Never discharge below 2.7v per cell. One cell went to 2.66.

So we must ethically advise you to dump that pack from your bike's battery assembly. It does have cells that remained above 2.7v, and those cells should be safe to recharge and use.

Your charger is trying to tell you something!!!!

But just between you and me, I'd just mark that pack with tape, and try to recharge it if I owned it. Then treat it as hazardous by always charging it and storing it outside. Soon as possible replace it, but I'd use it cautiously for now till another pack arrived.

I'm a dumbshit that always played with fire. I see a lipo fire and think, "that's all you got?" And yeah, in 1976 I did set myself on fire. Even that couldn't teach me. But dumb as I am, I still don't put known to be damaged lipo in my house or garage and go to bed.

Stopping at somewhere between 3.7v and 3.5v will help you keep the pack better balanced, and help you to not drive that ruined cell even lower every ride. If you try to ride to 2.7v each ride, you will quickly find that about one block of riding will take a pack from 3.5v to 3.0v. There just isn't shit below 3.5v.

What I've figured out is when the cell voltage drops to around 2.7v, the internal resistance starts to increase rapidly. If the discharge current is high when this happens, parts of the cell can get overheated, resulting in breakdown of active material, gassing, separator damage, etc. If the cell was discharged at a very low rate, you could take it all the way to zero without damaging it (not recommended).

During charge the internal resistance does the same thing. If you charge at a high rate while the cell is below ~2.7v, you will risk rapid heating and possible cell failure. Once the cell is brought above the critical level, you can charge at a higher rate. A cell that was overdischarged might be OK, only to be destroyed when it gets charged at too high a rate next time.

In a series pack, any overdischarge that results in cell reversal is going to be fatal for the reversed cell. This is true for almost all battery chemistries.

Cell puffing is a sure sign that some damage has occurred. A cell can be damaged without puffing though, so it's not the only sign.

Cells that have not been overdischarged or abused can still spontaneously fail and explode (though this is rare). Always charge with an arrangement that will prevent catastrophic damage if a pack lights up.

I have dozens of lithium batteries that were stored with a BMS attached and drained every cell to zero and they sat there for over a year. Every one that I tried reviving has recovered to their previous capacity. Granted these are good quality cylindrical cells, not Hobby King lipos, but they still use the same cobalt chemistry.

omegagamer89 said:

Also, to Wesnewell, why on earth would I stop riding at 44v? If the minimum voltage you ever wanna hit is 2.7v for any individual cell, that would mean I could keep riding until 32.4v total. (2.7v per cell x 4 cells per pack x3 packs = 32.4v)
Now, obviously I wouldnt wanna ride RIGHT up until they ran out. I would want to head in and stop riding when they started getting CLOSE to empty. But even then, even if I turned in once they hit 3v each, thats still 36v, waaaaaay lower than the 44 you suggested. Im just curious as to why you would say stop once it hits 44v, when thats not even 20% drained.

P.S. I already have some low-voltage alarms. I just didnt hear them in this instance. Ive repositioned them to be easier to hear next time.

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You have a misconception of when a lipo cell is fully discharged. Probably because of listening to a some on this forum that put out bogus info. A lipo cell is completely drained at ~3.3V. Discharging it below that will do nothing but damage the cell and shorten its lifespan. Manufacturers recommend not going below 3.5V, at which point you have drained the battery to about a 4% soc. IOW's over 95% of its capacity. This is easily proven. http://endless-sphere.com/forums/viewtopic.php?f=14&t=47294
Low voltage alarms are great I guess in the RC world where there's much heavier loads on the pack than there is in the EV bike world. Personally, I've never even considered using them on any of the packs I've put together. I think they're just a waste of time and money, and if left connected will drain and ruin your battery pack in time. So be sure and remove them when you park your bike for any length of time.
When a 12s pack reaches 44V (3.67V per cell), it's over 80% drained, not 20%. Not knowing what you are doing is what got you into trouble. Personally, I couldn't care less what you do. It's your money, not mine. Run them down to 2.7V if you want to. Just don't come back here bad mouthing rc lipo when you've ruined your pack in a few months or less. I've got over 5,000 miles and over a year on my current 24s2p lipo pack and won't use anything else. I expect at least another 5000 miles from it if I don't build another bike before that.

omegagamer89 said:

....
As for the people saying it was dumb to ride with only 3.8v per cell, I fail to see how...

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I'm glad you used the right language in this assertation. With the proper attitude, you'd be amazed at the number of people on this site willing to help you.

What you need to know about LiPo and what someone else already tried to say in different words is that there isn't a 1:1 relationship between voltage and SOC (state of charge)

That's why you noticed that the pack was draining faster (in what you considered) the "middle of the pack". At around 4.0v give or take some, there is a large amount of energy in the batteries. And those batteries will continue to give that energy without dropping too much voltage. But once you get closer to nominal voltage (3.7v for LiPo), the voltage drops much faster. That's why it is VERY difficult to tell pack SOC from voltage on LiPo.



BTW, did someone on this site recommend LiPo or did you choose it based on your needs? I'm asking honestly so I can gauge where people are coming from when considering LiPo.