# I experienced my first burnt up cell today, and got video!



## NabilAhmad (Feb 26, 2011)

Was about to take it for a test run after rebuilding the speed controller, but ran into other issues. I'm needing to test out some things to figure out what initiated the whole thing. It is actually kinda satisfying to have my own experience with a catastrophic cell failure. 

These are Headway 10ah cells that I got from Headway Headquarters in September. Jeffery commented about the headways in the Porche 911 having a 20% failure rate. It seems I haven't completely escaped that possibility. 

I'll be receiving my own West Mountain Radio CBA3 this week and will test each cell's performance and capacity. Any thoughts as to what test scenarios I should run?


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## dougingraham (Jul 26, 2011)

I have a CBA III and for testing single cells the most it thinks it can do is 29 amps on the LiFePo4 cells. What it can do reasonably is 20 amps. Mine overheats at 25A in 7-10 minutes. One problem I have had is with the Anderson power poles they used to connect the battery to the CBA III. They dont make great contact and I have to wiggle them to get decent contact at 20 amps. It doesnt have a way to remotely sense the voltage so you see the voltage drop in the wires and connectors. I used 12 gauge wire and the voltage drop across the wire and connectors is about 0.2 volts at 20A.

It makes pretty charts. You can graph the charge voltage and several options during the discharge.

I would suggest a 1C discharge and then a maximum rate discharge. Whatever you can get to work. Try the 29 amps it suggests and I would like to hear if you get it to work. That way you can compare the voltage sag and compute the actual internal resistance at different SOC.

I've considered buying the 500 watt expander which would let me test cells at up to 140 amps. Supposedly it has a way to tell the software the resistance in the wires so it compensates the voltage in the graphs.

For your headways the regular unit is probably good enough.

The video wasn't clear enough to see what happened. You may have had an internal short of that one cell which caused a thermal event and let out its magic smoke. That was a fairly tame event as these things go. But that is what is supposed to happen with LiFePo4 so you got some heat but no fire.

Several years ago when I was doing the software for the AstroFlight Inc hobby chargers I heard this story. A customer was having a problem with his charger and battery so he brought it to the factory so they could look at it. It was determined the problem was with the crummy connectors on the pack. He gave the pack to one of his guys and told him to replace the connectors. This guy picked up the wire cutters and clipped both wires at the same time shorting the pack for a fraction of a second. This was a 2AH 3S LiPo pack and that brief short of the outputs was enough to cause a thermal runaway and the pack caught fire a few seconds later. They tossed it outside the building and watched it burn.

Hope you figure out the cause!


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## DavidDymaxion (Dec 1, 2008)

Wow, thanks for posting that. I'll look forward to hearing what you figure out.

I did get another brand of 10 Ahr cell to vent like that, but that was with repeated discharges over spec, and it wasn't nearly as much steam. That cell must have been really hot. Had you been running the pack hard? I didn't see any flame, but is there any possibility whatever was venting was also burning?

I'll also ask the inevitable questions: It looks like there was no BMS, did your system do any kind of cell level monitoring? Had you top or bottomed balanced the cells? Did the charge just rely on overall voltage to determine when to terminate the charge?


NabilAhmad said:


> Was about to take it for a test run after rebuilding the speed controller, but ran into other issues. I'm needing to test out some things to figure out what initiated the whole thing. It is actually kinda satisfying to have my own experience with a catastrophic cell failure.
> 
> These are Headway 10ah cells that I got from Headway Headquarters in September. Jeffery commented about the headways in the Porche 911 having a 20% failure rate. It seems I haven't completely escaped that possibility.
> 
> I'll be receiving my own West Mountain Radio CBA3 this week and will test each cell's performance and capacity. Any thoughts as to what test scenarios I should run?


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## NabilAhmad (Feb 26, 2011)

DavidDymaxion said:


> Wow, thanks for posting that. I'll look forward to hearing what you figure out.
> 
> I did get another brand of 10 Ahr cell to vent like that, but that was with repeated discharges over spec, and it wasn't nearly as much steam. That cell must have been really hot. Had you been running the pack hard? I didn't see any flame, but is there any possibility whatever was venting was also burning?
> 
> I'll also ask the inevitable questions: It looks like there was no BMS, did your system do any kind of cell level monitoring? Had you top or bottomed balanced the cells? Did the charge just rely on overall voltage to determine when to terminate the charge?


I have no BMS on this pack. Frankly my intent with this whole project is to have a quick use cycle to observe the merits of the whole BMS debate for myself.

These cells have less than 5 minutes of use. I am working with original factory charge. I have not yet used the charger for them at all! My plan was to get the bike running and use the batteries up a bit to get them discharged so I could then bottom balance them. All measured voltages were between 3.3 and 3.5 before assembling the pack. 

If you check out my other video from October 8, you can see the majority of the use on these cells just before I roll of camera. The mosfets popped about 10 seconds into the run and I've been spending the time since rebuilding/reevaluating the speed controller circuitry.

The cells had each been around the 3.3v range when assembled, there was practically no use on them - just spinning up the tire a few times in air. The pack sat out in my garage for a month and voltage measured at the pack was 53.8v; no notable self-discharge, so I didn't inspect further.


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## EVfun (Mar 14, 2010)

DavidDymaxion said:


> Wow, thanks for posting that. I'll look forward to hearing what you figure out.
> 
> [snip]
> 
> I'll also ask the inevitable questions: It looks like there was no BMS, did your system do any kind of cell level monitoring?


WHAT! There was a Lithium battery fire. A BMS *had* to be involved! 

BMS = fire, no BMS = no fire -- I read it on the interwebs


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## frodus (Apr 12, 2008)

The resting voltage with these doesn't accurately indicate the SOC or that they're charged. I've had cells resting at similar voltages and one would take one or two Ah more than the other.

Did you even charge them up one by one before installing? Equalize them after installing (charge groups)? Were you monitoring voltages?


How many amps were you discharging the cell when it had the failure?


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## NabilAhmad (Feb 26, 2011)

frodus said:


> The resting voltage with these doesn't accurately indicate the SOC or that they're charged. I've had cells resting at similar voltages and one would take one or two Ah more than the other.
> 
> Did you even charge them up one by one before installing? Equalize them after installing (charge groups)? Were you monitoring voltages?
> 
> ...


I guess not all the details are typed out, and only in the audio of the video that I made and some is only inferred from them! Sorry. The most critical detail to outline is that as mentioned in the video's audio track, there was some sort of a short - so much for getting a credit; I can't ask Jim to eat the cost of something that was completely out of his control. - I only this evening was able to open things up and confirm that there was a short somewhere.

What i'm having a hard time with is understanding where the short started. The power and precharge switches were off! I am currently suspecting the digital voltmeter I have installed in the speedometer cluster - but here's the rub- it has only 18awg wires that are hardly adequate to carry a current beyond that of the cells rated power capacity. This leaves me to needing to rip out all my wiring and inspect every inch of every piece. 

I do know I had a short; I have no idea how much current it would have been pulling. I never charged any cell. They were all above the plateau of the nominal 3.2v resting voltage - meaning they were all in the upper end of the charge curve. I do subscribe to the notion that you cannot trust the voltage of a pack in active use, charging or discharging. However, a relative SOC can be deduced from a standing voltage. Or would you recommend that I put a specified load for a specified time to knock the cob webs off before measuring voltage? And I assume you suggest equalizing them based on apparent voltage - if voltage does not correspond to SOC, what would that matter? And how would you recommend segmenting a single string of non-paralleled cells?


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## dtbaker (Jan 5, 2008)

NabilAhmad said:


> ... However, a relative SOC can be deduced from a standing voltage.



no, it cannot.

you MUST either fully charge or discharge to have any real idea of balance. best to balance whole set in parallel of course, but AT LEAST take individuals cells to a specific low, or end-of-charge voltage, to balance before use. resting voltage means almost nothing w.r.t. SOC with Li cells.


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## frodus (Apr 12, 2008)

I bet that negative wire and fuse that you run right next to that cell shorted through that heatshrink somehow. It's routed right next to that cell, and could carry full pack current. If it was low, it could have gone into overdischarge.

Is that cell shorted? or is it open circuit?

What is the voltages on all of the other cells?


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## Jozzer (Mar 29, 2009)

Another possibility is that the cell shorted through it's casing, which are live (under the plastic wrap). This would explain why damage was limited to one part of the pack (if that is the case).

Steve


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## NabilAhmad (Feb 26, 2011)

frodus said:


> I bet that negative wire and fuse that you run right next to that cell shorted through that heatshrink somehow. It's routed right next to that cell, and could carry full pack current. If it was low, it could have gone into overdischarge.
> 
> Is that cell shorted? or is it open circuit?
> 
> What is the voltages on all of the other cells?


Haven't openned the pack up yet. Im letting it air out, but I plan to dig into it more tonight. The vented cell is physically adjacent to the fuse, but it is not adjacent in the circuit. So if it did short through the can to the fuse, I expect the cells on either side of the circuit to be dissimilar.


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## DavidDymaxion (Dec 1, 2008)

I bought a pack of 10 Ahr HiPower cells to test. From the factory all the cells were very close in Voltage (3.27V to 3.3V). On the first discharge, after only a 33% discharge one cell dropped 0.4 V below the rest. So even though the delivered voltages were close, the state of charge was not!

After bottom balancing all the cells delivered spec capacity and stayed closely matched, until I vented some by running over spec.

Anyway, an educated guess is one cell was lower in capacity, and it was greatly overheated by being over discharged. I'd suggest doing a bottom balance first next time. Despite what Jack Rickard says, do not assume your cells are delivered with equal states of charge!

If you'd like my test numbers:

http://explodingdinosaurs.com/9electric/celltest


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## Guest (Nov 26, 2011)

Nabil,

Have you figured out any thing? Got more photos of the damage? We are waiting for results of any kind. 

Pete


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## NabilAhmad (Feb 26, 2011)

Jozzer said:


> Another possibility is that the cell shorted through it's casing, which are live (under the plastic wrap). This would explain why damage was limited to one part of the pack (if that is the case).
> 
> Steve


Congratulations Steve, your hypothesis has been the most correct so far.

I have learned a couple things: do what you intend to do, and optimism has no place in design.

Indeed the reason that that specific cell vented was that the mounting strap for the battery box made contact with it. I had intended to put another plastic liner between them, but didn't get around to it yet...

It still seems that it all began in the super expensive hong kong special voltmeter. I had contemplated putting it on its own DC - DC isolator, but again got lazy and didn't.

I am still ripping the wiring out of the frame to discover any more burnt wires, but it seems the primary short path was through the vicor 48-12v isolated regulator. The high side is still isolated, but the 12v side is 0ohms across the outputs.

In short, I'm rewiring the whole bike to be cleaner and better isolated.

Also, I am using the WMR DBA3 to see what each cell looks like still and bottom balance them while i'm at it!

Oh, also - the electrolyte destroys acrylic. Fractures started where the fluid had pooled and spread out from there. The battery box is a complete loss - I'm actually glad for the opportunity to make a better one anyway!


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## GerhardRP (Nov 17, 2009)

dougingraham said:


> They tossed it outside the building and watched it burn.
> QUOTE]
> I'd think that step one would have been to wheel the damn thing out of the garage and away from anything flammable.
> Gerhard


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## NabilAhmad (Feb 26, 2011)

I've been having a most exciting time with my new CBAIII.

I have been doing a two stage approach - pulling each cell to 2.0v at 28a, which is right at 3.5c. Then pulling them down to 1.8 at 1c to have an end resting voltage of 2.8 (idealy). The actual ending rest voltage has been slightly under that at 1.7 and some change... one being 1.69, I think.

Attached is a pdf with 10 of the cells analyzed. I started from the end furthest away from the burnt cell. 

There are some dips in the voltage that it then recovers from - I measured with my muliti-meter at the screws on the terminal and the voltage stayed steady throughout the dip and recoveries. I'm going to revisit the solder joints on my test lead in the morning.

Also, I'm doing this in my garage with an ambient temp of 13.5c or around 55F, i think - rather chilly and I can occasionally see my breathe. There is a fairly consistent increase in voltage after about a minute or two by a few hundredths of a volt. Another trend that I noticed is that around the 4.5Ah mark on them all, the voltage declined at a gentle curve then a very linear grade nearly to the end. 

Enjoy!


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## frodus (Apr 12, 2008)

That's good to know that the cell didn't fail on it's own and that it shorted to the case. 

why on earth are you overdischarging your cells to 1.7-1.8V? That's not good for the cells. Overdischarging will damage the cell.


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## NabilAhmad (Feb 26, 2011)

https://docs.google.com/spreadsheet/ccc?key=0AiuiE3dlZnv7dDd2MFhFZTBWbDltVGJrZTBEaDhmV3c


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## GizmoEV (Nov 28, 2009)

Wow, looks like those cells were way out of balance for being only 8Ah cells. You might do a proper charge on them and then do another capacity test to see how closely matched they are.


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## NabilAhmad (Feb 26, 2011)

GizmoEV said:


> Wow, looks like those cells were way out of balance for being only 8Ah cells. You might do a proper charge on them and then do another capacity test to see how closely matched they are.


That's the plan. I have them all bottomed out now and will charge tonight. My resting bottom voltage target is 2.8. I find that those at 2.9 - 3.1 resting voltage had around .09ah draw at 1c to get them +/-.05v of the 2.8v target. 

My bottoming process was a dual stage. 3.5c to 2.0v (that's where I got the spreadsheet linked above), then a second stage of 1c to 1.8v. The first stage ah counts can be read from the spreadsheet. I have not compiled the second stage data yet, but it seems an average ah drawn out was .8ah! So yeah, I'd say I'm most certainly on the face of the cliff. (I'd be interested in anyone's data on these cells in this region.) This process left most at 2.8 - 2.88v. Three were above that and were treated to the process explained in the preceding paragraph. 

Since the initial state of charge was unknown, I'm not sure what conclusions can be yet deduced. The second charge/discharge cycle will be more informative, Im sure.


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## frodus (Apr 12, 2008)

frodus said:


> why on earth are you overdischarging your cells to 1.7-1.8V? That's not good for the cells. Overdischarging will damage the cell.


Any answer on this?

Not sure why you'd ever have a reason to overdischarge a cell unless you were doing a destructive test on the cell.


I wouldn't overcharge or overdischarge any of those cells, especially if you want to keep them.


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## MN Driver (Sep 29, 2009)

That's some rough treatment, it would kill a TS, WB, SE/CALB, HP, etc prismatic through extreme heat and venting and even though Headway cells generally perform better in terms of voltage sag, I wouldn't think of doing this to them since you are essentially running them down to where they are sagging very low anyway, where they generate copious amounts of heat to continue performing. Good luck.

Can you let me know how hot the Headway cells get at your procedure of 3.5C to 2.5v and then 1C to 1.8v when measured at the terminals as soon as you are done with that discharge?


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## NabilAhmad (Feb 26, 2011)

frodus said:


> Any answer on this?
> 
> Not sure why you'd ever have a reason to overdischarge a cell unless you were doing a destructive test on the cell.
> 
> ...


My post immediately prior to your request for an answer is an outline of my explanation... but not the full story, I suppose. My goal is to get all the cells to 2.8v resting. While using my 17th cell as a guinea pig on the CBA3, I figured out that taking it down to 2.0v only got me to 3.1v after resting for about an hour. However, taking it down to 1.8v @ 1c got me to 2.8v resting. This is of course, a procedure I intend to do once and not habitually


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## NabilAhmad (Feb 26, 2011)

MN Driver said:


> That's some rough treatment, it would kill a TS, WB, SE/CALB, HP, etc prismatic through extreme heat and venting and even though Headway cells generally perform better in terms of voltage sag, I wouldn't think of doing this to them since you are essentially running them down to where they are sagging very low anyway, where they generate copious amounts of heat to continue performing. Good luck.
> 
> Can you let me know how hot the Headway cells get at your procedure of 3.5C to 2.5v and then 1C to 1.8v when measured at the terminals as soon as you are done with that discharge?


Granted - I was doing this in my garage with a temp of 10-14c according to my temp probe (I measured air and the cells as sitting untouched all day). The temperatures measured at the negative terminal varied quite a bit - I'm not sure if that's due to inexperience with the probe or if the cells really did vary that much.. They were as follows (all in Celsius): 32.4, 24.5, 26.8, 40.7, 27.6, 35.5, 35.4, 33.7, 29.8, 35.8, 33, 31.8, 36.9, 34.5, 34.3, 34.3. Maybe I mistyped, but the 3.5c was to 2v. I then rested the batteries about 3 minutes before moving to the bottoming phase. I didn't measure the temps after the bottoming phase since it was a lesser current draw.


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## NabilAhmad (Feb 26, 2011)

I have now cleaned up the spreadsheet for public consumption. I am cataloguing the actions taken on the cells each step of the way. I have not yet included the capacity measured from the 2.0v to 1.8v bottoming discharge. I'll see how far I get tomorrow...

In the notes I cited that the charge took a total of 70 minutes to complete and switched from CC to CV at 50 minutes in. This is a 10a charger, so that suggests that there was just over 8ah put into the cells during the CC stage - ignoring any losses.

As the chart makes painfully clear - the second barrage of bottoming took two cells much further down than I would have wished.. They were my learning curve. However, if the Ah taken out at that time is any indicator, I think that amounts to close to 1% of the rated 8ah capacity.


Cell History Spreadsheet


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## frodus (Apr 12, 2008)

2V is cutoff, meaning DO NOT GO BELOW THAT. I don't care if you're top, bottom, left or right balancing your battery, that's LVC. You will damage a cell if you go below that. Same reason you don't charge a headway to 4V, it'l overcharge it.

3.65V down to 2V, that's it. 



Where did you get 2.8V from anyway, trying to figure out why you chose that number as a bottom balance voltage?


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## Guest (Dec 1, 2011)

Once the LiFePO4 cell reaches the 2.8 volt mark of the discharge curve measured at a steady state, you ARE on the down slope of the end of charge. From 2.8 volts to 2 volts will take but moments to reach. The same reason you charge to like 3.6 or so volts at the top. From there to like 3.8 volts your only moments away. It is usually a faster drop on the bottom end if your out for a drive as your driving high current through the cells. Much less on the charge end of things. 2.8 is an arbitrary number but one that is in the down slope of the curve and since its not at the absolute bottom it is in the save zone. He could have chosen 2.5 and been fine as well. Or even 2.4 or 2.3. But the further down you go the faster you are going to reach the absolute end of 2. You can balance your cells there but never go below the 2 volt range while discharging on a heavy current. EVER. Now if the discharge is a trickle discharge below that then all bets are off because I have cells that are just fine that have been sitting below 1 volt for over 2 years now that are still fine. But during active discharging or active charging you MUST remain in the safe zone. 2.8 is safe. Because it is a Headway cell I'd be inclined to stay at 2.5 to 2.8 as my LVC point. But not during SAG. A full cell sagging into those zones will be fine. An empty cell will not be fine in those zones. 

Pete


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## Guest (Dec 1, 2011)

Even at 3.1 volts your in the beginning of the down slope. 2.8 is a safe middle of the down slope point to balance. I used like 2.5 for my Hi-Power. I am going to use a higher point from now on. I do loose some usable AH but not enough to warrant the damage that can be done. I'd rather remain in the power band of the cell then stop and recharge. So if I have a 100 AH cell I will use 80 AH. That is still OK by me. If I had an 8 AH cell I guess i'd have to use only 7 AH. Stay safe with your cells. 

Frodus what is your cut off voltage set at? I am quite sure it's above 2 volts. The idea is to balance your cells below the voltage you'd normally consider empty. That way you won't damage any cell or force any below the safety zone. It works. It's so simple it really does work. At the top that is the reason to stay at like 3.6 volts. That helps keep the ragged top from going over the max limits. Safer to have the ragged edge at the top than than bottom. It works.


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## Guest (Dec 1, 2011)

NabilAhmad said:


> I have now cleaned up the spreadsheet for public consumption. I am cataloguing the actions taken on the cells each step of the way. I have not yet included the capacity measured from the 2.0v to 1.8v bottoming discharge. I'll see how far I get tomorrow...
> 
> In the notes I cited that the charge took a total of 70 minutes to complete and switched from CC to CV at 50 minutes in. This is a 10a charger, so that suggests that there was just over 8ah put into the cells during the CC stage - ignoring any losses.
> 
> ...



Those are not bad numbers. Charging at 10A seems pretty high for such small cells. In other words your doing fast charging. The upper numbers look fine and all within the safe zone so your not overcharging. All the low numbers from balancing look OK. You could do some tweaking. Any that go below you just need to bump them back up to match. And yes close enough is usually close enough. They don't all have to be perfect, just close. What will tell is after you discharge to like 2.8 volts then check your numbers. At rest if your still above the 2.8 mark you still have usable voltage left but if you decide to stop there the numbers should be pretty much dead on the number but some may only be very close. You know, like within a hundredth or so of a volt.


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## frodus (Apr 12, 2008)

gottdi said:


> Frodus what is your cut off voltage set at? I am quite sure it's above 2 volts. The idea is to balance your cells below the voltage you'd normally consider empty. That way you won't damage any cell or force any below the safety zone. It works. It's so simple it really does work. At the top that is the reason to stay at like 3.6 volts. That helps keep the ragged top from going over the max limits. Safer to have the ragged edge at the top than than bottom. It works.




I have my BMS set so that it starts reducing throttle below 2.75V and gives a warning, and when it hits 2.3-2.4V (trying some diff settings) it will fault (This is my Low voltage cutout). I do try to stay above 2V for these, although they can go to 2V...... it happens VERY quickly after the knee voltage. I limit my current via the Curtis 1238 controller. 

These cells say they can charge at up to 6C, but I suggest only charging at 1C otherwise you will effect the life of the cells. The lower current you charge and discharge a cell, the better off you are. 10A should be fine, but 5A would be even better.

Long and short, going below the manufacturer LVC will permanently damage cells, even if you only do it once. This is not refersable. How much that effects the life of the cell depents on the cell.... some are more robust, some aren't..... but they're both effected. I agree with Gottdi, this should never be done under a large load, even at 1C.


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## NabilAhmad (Feb 26, 2011)

frodus said:


> These cells say they can charge at up to 6C, but I suggest only charging at 1C otherwise you will effect the life of the cells. The lower current you charge and discharge a cell, the better off you are. 10A should be fine, but 5A would be even better.


Right, I understand that. My charger is preset, I can't throttle it back, unfortunately. I chose to have a 1c charge rate so that I could comfortably ride the moped anywhere in town.



frodus said:


> Long and short, going below the manufacturer LVC will permanently damage cells, even if you only do it once. This is not refersable. How much that effects the life of the cell depents on the cell.... some are more robust, some aren't..... but they're both effected. I agree with Gottdi, this should never be done under a large load, even at 1C.


Right. All the admonitions I have received made me realize that I had misunderstood the process. I of course, thought that the 2.8 was resting voltage (knowing how these cells bounce back) and that the voltage measured during discharge is sagged down. My intent was to emulate the discharge routine featured on EVTV last year - on the large format cells. I naturally assumed that these cells would behave differently from those. Frankly I felt comfortable continuing to discharge because I was expecting a much more vertical drop in voltage than was exhibited from my cells.


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