# Problems with the Battery Pack



## JoeG (Jul 18, 2010)

Sorry to hear about your pack problems. To start, you need to cycle test a few individual cells for capacity. If they all test low, say at 60AH then that is why you are getting half the range. If they test at 130AH, then you might have a few bad ones. Until you start testing cells, you don't know where you are at.
Joe


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## mizlplix (May 1, 2011)

Drive your car down to where the "squealer" is going off on acceleration. Then place your cell phone on video and duct tape it to "watch" your pack to see where the Red LED is And on what cell. You might do this several times to get the varmint.

Or have an assistant watch them if possible.

Make a couple of small jumpers to go over the bad cell BMS board to isolate it. 

Drive some more to see if the squealer quit.

You can fine it with patience.

I did.

Miz


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## Old Goat (Jun 28, 2011)

Joe G, can you please share a little about testing individual cells? Do you make up a low resistance load (say with a hundred feet of 12 GA solid wire, or some such) and then purposely load the cells down one at a time, observing cell voltage drop, accounting for internal resistance? How do you go about measuring the fairly high current that will surely result? I don't have a professional load box like they use at the corner garage when you are suspecting your 12V battery in a ICE powered car. Maybe you know of a relatively inexpensive source of a device like that?


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## Old Goat (Jun 28, 2011)

Mzlplx,
I tried to follow your suggestion. I perched my wife over the open bay of batteries (not an ideal position to be in!) as I accelerated again and again. Asked her to look for green LED's extinguishing. I learned that #21 and #6 were the first to go out, and after that it was 23 , 24, 25, 26, 27, & 29. But some runs had some of those (23-29) not going out at all), It's really pretty inexact because she said the LED's got dim but wasn't sure they were all the way out in some cases. There are two battery boxes, one under the back seat (where the fuel tank used to be) and the other under the luggage area (it's a hatchback) where the spare tire used to be. In the rear battery box, the LED's are really hard to see at all, and some are obscured by the front of the battery box so I'm not all that confident that we didn't miss one or two bad performers. I'm thinking that the other suggestion, measure the AH capacity with a load tester, would certainly be in order before I just go out and willy nilly start replacing between 2 and 8 cells (as you know, they're really expensive). Does anyone have a suggestion for where I could purchase a commercially available battery load tester (measures AH) suitable for measuring only 2-4 V? I'm sure there are lots of them available for typical lead acid batteries (6 or 12 V, or even higher) but I don't even know where to go looking for one suitable for lithium. The remaining question in my mind is that if I were to replace, say, six of them, will they ever behave "close to the same" as the remaining 31 cells that have been deteriorating for 14 months?


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## dimitri (May 16, 2008)

Do you have a voltmeter showing pack voltage as you drive? Can you see how much overall voltage sag you get under 100A-120A load, which represents 1C for your cells?
If you can record pack voltage sag under 1C load on a fully charged pack and then also under same load once BMS alarms start coming in, then you will have some idea of how bad internal resistance is on your cells.

I have a feeling that IR on these cells is rising too much over time, which speaks of poor quality of these cells.

Also, you can drive around the neighborhood gently until BMS alarm is constant, even when you let go of the pedal. This way you can certainly see which LED is out, this will point out weakest cell. While this cell is still beeping, measure other cells and see if any are below 3.1V , which means they are also getting close to the low knee of the curve. 

If many or most cells are resting below 3.1V when lowest one is in alarm state ( below 2.7V ), then your pack is deteriorating more or less evenly, i.e. its not just one cell causing troubles. This is worst case, because replacing only a few cells is a waste of money.


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## Old Goat (Jun 28, 2011)

Dimitri, Yes, I am using the Android dashboard. It gives me a reasonably accurate pack voltage reading. I'll try that (testing at 1C with a full pack, and later when it's nearly depleted). I'm going on travel tomorrow for the next week, but I'll do that ASAP upon my return. I did try today running with a nearly full pack (only about 10 miles on it) with an observer (my wife, who was none too thrilled about this assignment!) watching for green LED's going out. When I was drawing around 3C, I got alarms to ring and she noted around 7 LED's going out (that means 29 cells never got to LV Alarm state). So now I am reasonably sure that more than one or two cells are troublesome. Interestingly, after stopping and taking another round of static voltage measurements, one cell (#30) which never dropped to LV alarm state, was about half a volt lower than average voltage of the pack (3.202V) which was far and away the biggest deviation from average of any of the 36. But its green LED never came off, indicating it wasn't one of my problem cells under load. The entire pack was very close to perfectly balanced when I started that little exercise.


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## GizmoEV (Nov 28, 2009)

I'm wondering if you might have been overcharging your cells, especially if you are balancing them on every charge to 3.6/3.65V. With a healthy LiFePO4 cell at room temperature at 100%SOC will rest at 3.38V. Maybe after a full charge with balancing disconnect the BMS on each cell and all loads and let it sit untouched for 12-24 hours. Then go around and measure the voltage of the individual cells. If any are resting over 3.38V then it was overcharged. It might be that the quality of the cells is such that they can't handle the overcharge as well as some other cells can.

I have over 16k miles and over 3.5 years on my 200Ah pack and it shows no signs of degredation. I only charge to 3.455vpc and when I do the charge and rest test all my cells rest between 3.356V and 3.363V. Right at the end of charge they were between 3.450V and 3.487V and have not been top balanced for 2 years. These are TS-LFP100AHA cells made in November 2009 and are in a 2p20s arrangement. (Note that I do not have BMS boards on them any more. If you have BMS boards on a LiFePO4 pack you are likely to need to balance your pack more often since it is nearly impossible to make sure the parasitic drain on each cell by the BMS boards is exactly the same.)

As for a capacity tester you might consider something like the PowerLab 8. A description is here: http://store.evtv.me/proddetail.php?prod=revolextrix


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## GerhardRP (Nov 17, 2009)

Old Goat said:


> <snip>The pack has been top balanced again and again to the point that static (after driving) voltages are all within .03V of average cell voltage and highest voltage - lowest voltage is around 0.1 V after a day of driving. The lowest cell seems to change every time I measure cell voltages. <snip>


I think there may be issues of resistance...either internally or in connection resistance--have you verified connection torque and maybe connector crimps?.
But also, after you have been signaled for low voltage, measure the resting voltage after 15 minutes or so. If not less than 3 volts, drive on for a while. Resting voltage is the only sign of actual over-discharge.
Gerhard


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## Old Goat (Jun 28, 2011)

Gerhard,

I have checked with my fingers to look for a loose or cockeyed connection but didn't find any. I haven't checked with a toque wrench; think I'll try that. What torque do you suggest? Also, since I live in Florida (95% humidity and lots of salt in the air), I used No-alox when I installed all the terminal connections. Do you think that might be a problem? No corrosion but maybe some high resistance due to the grease?


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## dougingraham (Jul 26, 2011)

Old Goat said:


> I used No-alox when I installed all the terminal connections. Do you think that might be a problem? No corrosion but maybe some high resistance due to the grease?


Correctly used the greases are not an issue. And you can really glop the grease on and as long as you get the connection tight all the excess will be squeezed out. It looks terrible when you do this but is isn't an issue.


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## kennybobby (Aug 10, 2012)

*Testing*



Old Goat said:


> 14 months (6K miles) using 36 REAL FORCE 120 AH LiFEPO4 cells. ... I have never charged any cells above about 3.8V ... I could go 62 miles on a charge. Now the audible alarms start at around 19 miles. Anybody have an idea of what the fix is?


You never mentioned what pack or per-cell charging voltage you have been using, but it sounds like reduced capacity due to overcharging. 

You don't need to buy an expensive cell tester, you already have one--your car. Just buy some good heavy-duty jack stands and a clamp-on current meter if you don't have a current shunt. 

Charge your pack and let it sit 24 hours and measure pack voltage at the start, then jack the wheels up and run it on the stands at a constant current and you don't have to worry about getting stranded on the road. 

Measure pack voltage and current every 6 minutes (0.1 hr) and you can monitor pack capacity in real time until the pack voltage drops to whatever level you like to call drained, let sit 24 hours and measure voltage again to see the bounce-back. 

For example if you are drawing 50 amps on the stands, then every 6 minutes is 5 AHr--5,10,15,20 you can do the math in your head and see what sort of reduced capacity or if are close to getting 100 Ahr out of your pack.


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## Duncan (Dec 8, 2008)

*Re: Testing*

I agree with KennyBobby - except I would not use the stands,
Just drive around the block and monitor your current
You do have an Amphour meter?
If not buy a CycleAnalyst - great things mine is the sole instrument in my car
http://www.ebikes.ca/drainbrain.shtml


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## Old Goat (Jun 28, 2011)

OK, just got back from a week on the road and finally got a chance to try Dimitri's suggestion: checking for internal resistance (and/or loose, corroded, or otherwise high resistance terminal connections). I made about 18 runs from 0 Amps to 400 Amps pack current, measuring the pack voltage for every 100 Amps step). Since I have 120 AH cells (made by REAL FORCE), this roughly corresponds to 1C, 2C, 3C, and 4C. LV Alarms (mini BMS head board) started sounding at 3C and above right from the get go so I am quite sure something is wrong with the pack. (Maybe it's just one or two cells?) Total resistance ranged from 0.54 Ohms to 0.61 Ohms with an Average Thevenin Resistance of 0.57 Ohms. I calculated it on a simple spread sheet by applying the following formula [V (rest) - V (1C)]/100 or V(rest) - V (2C)]/200 . . . and so on. It's pouring rain outside (has been for about 8 days now) and I haven't run the battery charge down much yet so I haven't continued the experiment again with the pack almost depleted as Dimitri suggested. Now, my question is: Is this [R(Thevenin) = 0.57 Ohm] considered excessive resistance? In the next few days, I plan to remove all the terminal connections, wipe them clean, polish with fine grit sandpaper if necessary, check all crimp connections and reassemble. Then, I'll repeat the experiment both with a full charge (as today) and with a depleted charge. My problem is that I don't really yet know how to interpret the data. Is this considered excessive resistance? Can anyone suggest additional testing I can do before just giving up on my 14 month old battery pack?
Thanks for anyones' ideas!
Dave


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## Old Goat (Jun 28, 2011)

In reply to KennyBobby -- I charge routinely with an ELCON HF/PFC 2500W battery charger set on an algorithm to provide around 123VDC. I then use the Mini BMS cell measurements to shut down the charger when one or more cells reaches the pre-set cell voltage that turns on their RED LED (I believe that's around 3.65V). In any case, after a complete charge (starting around midnight for 3 or 4 hours) first thing in the morning, the total pack voltage always shows 120V or 121V, which is around 3.36 V per cell, after settling for a couple of hours at least. For today's testing, I had charged the pack and then let the car sit for about 8 days. Again, rest voltage started just slightly under 120V. Even when top balancing (which I've been doing a lot), I use the mini BMS to shunt cells, never driving any single cell higher than about 3.75 V (and that's pretty rare, I usually top them out at about 3.65V). I have never knowingly driven any cell much above that. I do have a clamp on ammeter and I guess I could use it while the car is on jack stands, but the Android dashboard app is pretty convenient to measure pack voltage and current and that's how I usually do it. I'll try the jack stand suggestion next week. Thanks.


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## PStechPaul (May 1, 2012)

I think your 0.57 ohm resistance is incorrect. That would be 57 volts at 100 amps, which is half your pack voltage. Your formula seems to be correct, and it is a measure of internal cell resistance as well as connections. I would expect the voltage under 1C load to drop from 120V to maybe 110V, which is 10/120 = 83 mOhms. You probably mean 0.057 ohms, which would drop your pack voltage 22.8V at 400A to just under 100V.

Sometimes it helps to use a figure of percent impedance, which in this case would be 0.057 / (120/100) = 4.75%. This type of characterization is used for power transformers, which also gives you the short circuit current as a multiple of rated current, which in this case would be 120/0.0475 = 2526 amps.


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## dimitri (May 16, 2008)

Old Goat said:


> I made about 18 runs from 0 Amps to 400 Amps pack current, measuring the pack voltage for every 100 Amps step). Since I have 120 AH cells (made by REAL FORCE), this roughly corresponds to 1C, 2C, 3C, and 4C.


Can you post here pack voltage readings you took corresponding to 1C,2C,3C,4C ?


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## drgrieve (Apr 14, 2011)

Old Goat said:


> In any case, after a complete charge (starting around midnight for 3 or 4 hours) first thing in the morning, the total pack voltage always shows 120V or 121V, which is around *3.36 V* per cell, after settling for a couple of hours at least.


Might be slow death by over charging.

Turn down your charging voltage to the range of 3.45 to 3.5 and (or) reduce the CC phase. Aim for a static resting voltage of 3.33 which is 119 to 120 V.

A BMS shouldn't be used as a trigger to terminate charge, should be used as a fail safe.

Bet way to maximize cell life is to stay away from either end. Under charge and not over discharge.

Unanswered questions are - how far from the end, and which is more pointy? 

80% DOD, 95% SOC or is 85% DOD, 90% SOC better?


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## dimitri (May 16, 2008)

drgrieve said:


> Might be slow death by over charging.
> 
> Turn down your charging voltage to the range of 3.45 to 3.5 and (or) reduce the CC phase. Aim for a static resting voltage of 3.33 which is 119 to 120 V.


I never understood the persistent notion on this forum that LiFePO4 cell is overcharged by taking it to 3.65V or even to 3.85V using basic CC/CV chargers. Is this one of those gems where one genius came up with this theory and everyone else repeating it?

Most common CC/CV chargers do not hold CV voltage for more than few minutes, in fact entire CV phase is really short, so I don't see how any damage can be done. I suppose there is some potential danger in holding CV voltage for many hours at a time, and even that idea needs some hard data to prove such effect.

I have been driving my pack for 4.5 years now, charging every night to 3.8V per cell, yet the pack is still going strong, so how can this be explained?

All commercial LiFePO4 chargers and charge controller ICs are made to 3.65V per cell, so I guess those billion dollar companies with their R&D labs are all wrong too?

So, if we call things what they really are, some people choose to undercharge their packs, and that is their choice, but it does not mean that people who chose to charge their cells more fully are overcharging them.


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## PStechPaul (May 1, 2012)

Is there a temperature coefficient for LiFePO4? The float voltage for a lead-acid cell is 2.4 Vpc at 25 C, and the temperature coefficient is (-)0.003 Vpc / DegC, or (-)0.125%/C. If it is the same for LiFePO4, then the 3.65 Vpc would change to 3.76V at 0C and 3.54V at 50C.

A discussion from some time ago says that the tempco for LiFePO4 for charging is (-)2.5 mV per DegC. 
http://www.diyelectriccar.com/forum...ure-compensation-lifepo4-chargersi-64518.html

So in that case it is (-)0.07%/C and the range would be 3.71V to 3.59V. Apparently the open circuit voltage does not change appreciably with temperature. But the usable capacity changes strongly with temperature:



> Also, the *usable* amphour capacity changes strongly with temperature.
> It delivers about half the amphours at -25 deg.C, and 10% more amphours at 45 deg.C. The OCV might say a battery is at 50% SOC, but you can't
> get more than 25% out if it's cold, for example.


Something else to consider is the effect of the resistance external to the cells (as well as their internal resistance), especially for higher charge rates. If the resistance is 0.05 ohms for 36 cells, then each cell has a resistance of 1.4 mOhm, which corresponds to 0.14 Vpc at 100A. So if the charge voltage is measured at the charger, it could be as high as 3.65 + 0.14 = 3.79 volts. This is where a distributed BMS with connections right on the battery posts may be preferable.


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## Siwastaja (Aug 1, 2012)

dimitri said:


> I never understood the persistent notion on this forum that LiFePO4 cell is overcharged by taking it to 3.65V or even to 3.85V using basic CC/CV chargers. Is this one of those gems where one genius came up with this theory and everyone else repeating it?


Hi,

This is relatively simple;

A cell is full when its OCV equals the full voltage (something a bit less than 3.40V IIRC). Then where does the 3.65V come from? It's because you need voltage differential to drive current into a cell. If you just used the "full OCV" voltage, you could float the cell endlessly, but it would also take more time to fully charge because towards the end, the voltage differential would be minimal.

Now the caveat is here; if you do use the 3.65V, you need to _terminate_ the CV phase after a specific time. It's usually given as C/20 terminating current.

So the complete procedure must be followed (CC - CV - termination at the correct point). The voltage limit of 3.65V cannot be separated from the equation as it includes the voltage differential and it is specified for a certain charging current.

You overcharge a cell if you continue the CV phase at 3.65V after C/20.

Many people find that if they only charge to about 3.4V, they can keep "charging" practically forever - no need to measure current for the stop condition. The only downside is that charging slows down a little bit more at the end, but it's only imminent in case of fast charging.

But doesn't miniBMS cut off the charger _instantly_ when the first cell hits 3.65V, without turning the charger back? Then there is no CV phase, and the cells will be (very little) undercharged, not overcharged, so there shouldn't be a problem -- unless the charging current is less than C/20 _to begin with_.


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## dimitri (May 16, 2008)

Siwastaja said:


> You overcharge a cell if you continue the CV phase at 3.65V after C/20.


Do you have any facts supporting this statement? Over past 4 years working with LiFePO4 cells from various manufacturers I have seen data sheets with wildly varying data on the topic of charge termination. Some go even over 4.0V , most mention 3.65V-3.85V range, most aren't specific at all about final charge rate.


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## Siwastaja (Aug 1, 2012)

Well, it's a simple physical fact that a li-ion cell continues absorbing current even after the 3.65V C/20 point. I hope you didn't question that. You can easily measure it if you want to verify that.

As of whether this extra charge is damaging or not, I have no personal test data. It's just general consensus among both the science and DIY scene that li-ion (incl. LiFePO4) does not have a mechanism to turn excess energy into heat without causing damage like NiCd or NiMH or lead acid, so it's best to believe that (or do some very thorough testing that shows otherwise). Maybe there isn't damage at all, or maybe the damage is minimal, but why take the risk as there's nothing to gain?

I understand that non-BMS folks may use the robustness of the chemistry as they might end up overcharging a bit -- then the gain is the saved BMS cost -- , especially in the bottom balanced application, but if you DO build a BMS, what's the point of the idea of not following the charging instructions?

Manufacturers indeed have varying specifications, and some manufacturers have changed their specifications over the years, but again, the most typical value is 3.65V CV with C/20 endpoint. It's unclear whether it has any built-in safety margin or not. As li-ion certainly doesn't _need_ full charging (like lead acid does), I'd add a small extra layer of safety by increasing the margin, instead of decreasing it, as you cannot gain much extra capacity anyway.

W.R.T. topic, I don't believe "overcharging" has anything to do with this case, but still, it would be nice to be ruled out.


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## ndplume (May 31, 2010)

Goat,
I've seen similar issues and tracked it back to connections with split lock washers vibrating loose creating "high" resistance. It took about 10 months in my case to see the 1st incident. And I've seen this issue on 6 of 52 cells (so far). 

When I was debugging this, I went at it in this order, retesting after each try : 

Loose Connections - Clean and re-tighten. 
Instrument Issue (Double check bad cell readings with a volt meter, then try swapping BMS boards and see if the issue follows the board or stays with the cell) 
Pack Balance - Make sure pack is balanced. Re-balance pack.
 Lost Cell Capacity - Check on PL6 
 High Internal Resistance - Check on PL6
Luckily, I've found all the issues at step 1: Loose connections.

The way I spotted the issue : Cells were getting warm (10C > average), then eventually Hot (20C > average). (My BMS monitors / reports cell terminal temperature by Cell). The temperature remains after I'm coasting, so that made it easier to identify the culprit cells. Once I found that, I looked a the cell voltage.

On the cell voltage, I'd see a drop of .3V at 125A on that cell, as it got worse, I'd see a 0.5v drop. Yet when I coasted (0 Amps), those cells would be right in line with the other cells (3.2x Volts). Since you've ID'd some suspect cells, just putting a digital volt meter on them should show you the pattern as you drive. You can compare to some of the cells running normally. 

If the cell goes low under load (100A), but doesn't recover back to normal cell voltage, then you're at the end of the cell capacity. So either cell capacity IS diminished OR pack out of balance. 

The good news is that if the issue IS the connection, its easy (and inexpensive) to fix. The (temporary) fix was to remove the strap, clean the interface, re-assemble and re-tighten the nuts. The long term answer (right answer IMHO) is to put Nord-lock washers on. I ordered some today from McMaster since I'm weary of chasing and watching for hot cells / bad connections.

If 1,2,3 doesn't find the issue, then I'd test the capacity / resistance. To test the capacity, I use a powerlab 6. They are <$200 on the net. The PL6 can use a 12V car battery for power and do charge/discharge cycles. The nice thing is that it records the voltage, current, time, AH and you can plot these to see the cell "knees". Also shows internal resistance. If you keep the data by cell number, you'll be able to retest the cell in ... a year ( or 2?) and compare to today to see how the cell capacity is changing. I think the max load is 40A, so that would take ~2.5hrs from full charge.

The PL6 will execute the tests automatically, so you don't have to worry about getting distracted and forgetting to turn the charger on or load off. I think its much easier to setup using a laptop, it will also graph the data to your laptop screen as its running. The PL6 will need the USB wire if you want to do this, it was a few extra $. When done, you can also use the PL6 to charge/discharge back to where the rest of your pack is so that you don't get it out of balance. Nice option.

IF the resistance is inside the cells or capacity is severely diminished, then your call on what to do. (Do these cells have a warranty?)
If you don't replace them, then you're down to remove or leave in place. If you simply remove the cells, don't forget to adjust your charge voltage!


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## dimitri (May 16, 2008)

Siwastaja said:


> Maybe there isn't damage at all, or maybe the damage is minimal, but why take the risk as there's nothing to gain?


This is exactly my point, something unknown has become true on this forum by repeating it over and over. Perhaps first person provided as much data as you did above, taking charge rate into account and maybe even admitting that actual damage is unknown, but over time those details got stripped and now average response is, if you charge to 3.65V you are damaging your cells.

So, again, how is that several of my own packs routinely charged to 3.85V don't show any signs of damage? I have a 22kWH pack in my Mazda, 10kWh pack in my wife's Prius, 2kWh pack in my lawn mower and they are all happy after years of regular service.

As for OP's issues, most important point IMO is that he has RealForce cells, which aren't often mentioned on this forum. I have not used these personally, but I read a few horror stories in the past, so I personally don't trust their quality. 

I really hope the issue is with terminal connections, and I still want to see voltage sag numbers under load.


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## Old Goat (Jun 28, 2011)

Wow, looks like I stirred up some good conversation. Mea culpa. I did a typo. Yes, you are absolutely correct in that I slipped a decimal. Here is my data:

Avg Voltage: resting, no load: 119.3V
AVG Voltage (100A): 113.1V
AVG Voltage (200A): 108.0V
AVG Voltage (300A): 103.2V
AVG Voltage (400A): 96.6V

Resistance (100A): .061 Ohm
Resistance (200A): .057 Ohm
Resistance (300A): .054 Ohm
Resistance (400A): .057

All figures shown are averages of approximately 18 runs starting with a nearly full pack.

I am heartened to hear that perhaps the problem is a split washer or a slightly mis-torqued nut. Can anyone provide the appropriate torque to use? I must confess that I did not use a torque wrench when I assembled the pack. But I did turn them all down pretty tight with a ratchet. Also, can anyone help me realize what a "good" resistance figure should be? Is 0.57 Ohm excessively high for a 14 month old battery pack?

I am really disappointed not to be able to drive my car much more than about 20 miles these days in fear that I'll have to call for a tow home. I REALLY need to get this problem nipped inthe bud and if that means another whole new pack, I'm going to have learn a lot more about this stuff before I drop another $5K on something that won't last any better than mine have. I swear, I have NOT overcharged or undercharged any cells that I know of. 

I realize there is some disagreement over whether I should use the BMS signal to shut off the charger or whether I should set the charger algorithm to do it first. But gee whiz, if I have damaged these puppies, it sure hasn't been because I haven't been careful.
Dave


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## GizmoEV (Nov 28, 2009)

dimitri said:


> This is exactly my point, something unknown has become true on this forum by repeating it over and over. Perhaps first person provided as much data as you did above, taking charge rate into account and maybe even admitting that actual damage is unknown, but over time those details got stripped and now average response is, if you charge to 3.65V you are damaging your cells.
> 
> So, again, how is that several of my own packs routinely charged to 3.85V don't show any signs of damage? I have a 22kWH pack in my Mazda, 10kWh pack in my wife's Prius, 2kWh pack in my lawn mower and they are all happy after years of regular service.


It has been a while but IIRC I read in this scientific report, "Nonaqueous Liquid Electrolytes for Lithium-Based Rechargeable Batteries"
by Kang Xu, Electrochemistry Branch, Sensor and Electron Devices Directorate, U.S. Army Research Laboratory, Adelphi, Maryland 20783-1197, that lithium is plated out when a cell reaches full and continues to charge. I do know that when trying to charge when the cell is well below 0°C will cause lithium plating so that is why it is not recommended that charging take place when the cell is below 0°C.

As for your pack doing fine after several years, mine is doing fine too after being initially charged and balanced daily to 4.00V for the first few months. Unfortunately I have no capacity data to say what "fine" really means. It is possible that the damage is minimal and won't show up for over 1000 cycles. My pack was put in service in January 2010 but it has only delivered about 38,000 Ah which is less than 200 full cycle equivalents for my 200Ah pack. For most of that time I have been only charging to 3.455Vpc. With out cycle data the number of years may be meaningless. Furthermore, as Siwastaja said, there is very little if anything to gain by charging above 3.4V. Yes, I have data to back that up. It is what convinced me that I wasn't gaining anything by charging to 4.00V and risking shortening the life of the cell. A NASA study I read showed that even LiFePO4 cells had a shorter calendar life when kept at 100%SOC than at a lower SOC. It wasn't clear how much shorter so it might actually be minimal or lost in the noise.


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## dimitri (May 16, 2008)

I am not arguing that charging above 3.5V gains any useful capacity, but there is a big difference between "not useful" and "damaging", that is all I am trying to point out. As for usefulness, its not just about capacity, its also about ease of top balance. So, obviously for those who don't top balance it makes no sense to go over 3.5V, but I find it a lot easier to top balance my packs if I take them higher, so that is the only reason I do it.

Back to OP's issue. Your voltage sag is not bad at all, IMO, pretty close to what I get on my own pack. This tells me your BMS alerts might be coming from just one cell, or a couple of cells, but not majority of cells.

So, my recommendation would be to follow ndplume's advice to clean and tighten your terminal connections, then drive the car until BMS alarm is constant, so you can see which cell lacks green LED after you stop and replace that cell if your range is still too short. However, if you are buying a spare cell, I would get 2 or 3 of them, so you can repeat this process if another weak cell shows up in near future.


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## Old Goat (Jun 28, 2011)

OK, Dimitri, that sounds like a plan! Haven't had time [yet] to start disassembling the pack, removing and torquing all terminals after a thorough visual inspection of the terminals, washers, bolts, crimped connectors, etc. Maybe that will tell us something. Whenever I look at the whole pack, it seems like everything's fine, but the bms LV alarm tells me different and I cannot find a consistent poor performer amongst the 36 in the pack. I have done the "drive it until the alarm is consistently on and then look for which green LED's are dark" test several times before, and sometimes it's one cell, sometimes another. No consistent bad behavior there. But I'll try it again [and again and again, if necessary]. It's just a real pain when I have to change horses in mid stream (so to speak) after driving only about 15=20 miles, and then shifting to fossil fuel. I sure don't want to purchase a whole new pack, so I'll try anything here. Once again, does anybody have a torque setting to use when I'm tightening terminal bolts? Everybody tells me to use a torque wrench, but nobody (including the vender) tells me how many foot pounds (newton meters).\\
Dave


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## Old Goat (Jun 28, 2011)

ndplume, not sure what "check on pl6" means. Can you please elucidate?


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## EVfun (Mar 14, 2010)

I would like to add that if you have aluminum cell terminals (pretty common for Lithium cells, particularly on the positive terminals) you should remove the cell strap and abrade the top of the terminal. Aluminum oxide is an insulator and quite stubborn. You should use Noalox, or similar aluminum terminal grease, on all connections. A light film on each aluminum surface is all that is needed. Clean connections are very important!


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## TEV (Nov 25, 2011)

Power lab 6

http://www.google.com/search?q=powe...AQ&biw=369&bih=615&sei=0hQUUvH2BOXwyAHuqYCoBw


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## GizmoEV (Nov 28, 2009)

Old Goat said:


> Once again, does anybody have a torque setting to use when I'm tightening terminal bolts? Everybody tells me to use a torque wrench, but nobody (including the vender) tells me how many foot pounds (newton meters).\\
> Dave


I just found this: http://www.evsource.com/tls_lithium_calb.php. I have no idea where the info came from. 


```
Cell Terminals Specifications:
[FONT=Courier New]        Recommended Bolt  Recommended Lug  Torque (ft-lbs, Nm)
40AH     M6x16 - M6x20         1/4"             7, 9
70AH     M6x16 - M6x20         1/4"             7, 9
100AH    M8x16 - M8x20        5/16"             15, 20
130AH    M8x16 - M8x20        5/16"             15, 20
180AH    M8x16 - M8x20        5/16"             15, 20
400AH    M14x16 - M14x20      9/16"             45, 60[/FONT]
```
I just go by feel. I use brass bolts with bronze lock washers and they don't loosen. At least they haven't over the past 3 years. I have TS cells.


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## ndplume (May 31, 2010)

Old Goat said:


> ndplume, not sure what "check on pl6" means. Can you please elucidate?


PL6 = Power Lab 6 Battery Tester/Charger. I hope you won't need to go that far to solve your issue. However, if you get one of those units, its nice to chart a few cells to so you can compare specs of the same cells down the road. Also, can use it to determine the correct torquing for a cell connection.

Did you use nord lock washers on your setup? 

On screw torque :
Doug Ingraham explained a scientific way to determine the torque required for a cell connection that really makes sense and isn't too difficult to perform. I use my PL6 to do it, but it could be done with independent instruments.
The idea is simple. Connect a cell to a load using one of your battery straps in the circuit with 1 side connected to a cell in your pack. Once you get a load current (I use 25A from PL6) and are monitoring the Cres, tighten the screw that goes to the battery strap and watch for the Cres to bottom out. Record the torque value and add some for margin

Once you see the voltage drop vs the current, you should be able to estimate your total pack "sag" based on the amps out. The voltage sag should be linear with the amps out. The contact service between the cell and strap should be clean and free of dust for best electrical and thermal conductivity. So watch for that as you re-assemble.

I'd be curious to hear out how this goes, so let us know,
Thanks


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## PStechPaul (May 1, 2012)

Aluminum does pose a problem because of its rapid oxidation on its surface, and Aluminum Oxide is a very good insulator as well as very hard. One possibility to consider is plating the surface with a conductive metal coating such as copper or nickel, and possibly a final coat of silver or gold. Here is a discussion of a method to accomplish this:
http://dalmar.net/aluminum_plating.htm


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## drgrieve (Apr 14, 2011)

dimitri said:


> Do you have any facts supporting this statement? Over past 4 years working with LiFePO4 cells from various manufacturers I have seen data sheets with wildly varying data on the topic of charge termination. Some go even over 4.0V , most mention 3.65V-3.85V range, most aren't specific at all about final charge rate.


Ok some facts about charging.

1. OCV of LiFeP04 as determined by the voltage potential of the chemistry involved is 3.38V
2. Charging at a higher voltage than 3.38V such that current is achieved will at some point in the future cause the cell to be fully charged and then over charged
3. Charging at a random voltage say 3.8V doesn't indicate what the finial SOC will be - it is dependent on several other factors including but not limited to: charge rate, cell diffusion rate, temperature, and apparent internal resistance. So when you say you charge to 3.8V every night, you say nothing meaningful at all.
4. The only way to measure to SOC is to a count AH in and out, or try to divine SOC from an OCV lookup table. Since an OCV lookup table doesn't exist and for accuracy you need to wait 3 to 4 days for the cells to rest, only AH can tell you the SOC. So when you say you charge to 3.8V, how much AH are left? 
5. Now for as assumption. Repetitive charging recipes - for an individual pack - either CC or CC/CV, will charge to the same SOC given similar temperatures.
6. Given 5, then by determining AH left for a recipe, a pack owner can then use that recipe to charge to a known SOC by using a combination of voltage and current.
7. 6 is the only practical way to charge a pack, given only counting AH introduces errors over time due to inaccuracy of measuring equipment.

Ok now for some facts about over charging or full charging.

1. Deliberate or abusive overcharging can cause fire, venting, and swelling of the cell. Causing the cell to be at least inoperable, and at worst smouldering ashes and destruction of car, garage and home.
2. Assuming each car uses a dependable charger, which stops charging predictably and if any errors detected, and the car is built with a secondary failsafe device (BMS or other method) then 1 is a very low risk.
3. Each lithium ion moved during charging causes an intercalation event. Each intercalation event has a low risk of causing crystalline structure damage which locks the lithium ion away and may block other free spots in the structure from intercalation. This is a natural decline in capacity with usage.
4. When the cell reaches higher SOC, each lithium ion moved, may need to bump already intercalated ions, to fit. This is called diffusion. The more full the cell the higher the diffusion rate, and a higher voltage is needed to maintain current flow. But each individual cell has a different diffusion rate so when one cell exhibits a higher voltage doesn't mean it is at a higher SOC relative to another cell.
5. As the cell becomes fuller, each ion moved causes an exponential number of intercalation events. Assumption - Wether this count as a full event or partial, or not really counted as they as just bumps I'm not sure of. But if they are full or partial events, then charging after the knee causes accelerated capacity loss relative to middle SOC charging due to increased intercalation events per ion moved.
6. When an ion cannot be intercalated it plates instead on the SEI layer causing capacity loss. Plating can occur before the cell is fully charged and plating begets more localised plating.


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## EVfun (Mar 14, 2010)

I have issues with a few of those:



drgrieve said:


> Ok some facts about charging.
> 
> 1. OCV of LiFeP04 as determined by the voltage potential of the chemistry involved is 3.38V


Why is that particular resting voltage considered full? They accept and release at least a fraction of an additional amp hour if the finish and rested voltage is 3.40 volts (as an example.) Perhaps 3.36 volts should be considered full and 3.38 volts be considered slight overcharging. 

At what temperature is the full charge voltage 3.38 volts? Does this finish voltage vary, based on differences in composition of the electrolyte? Does this vary based on the total Lithium made available?



> 4. The only way to measure to SOC is to a count AH in and out, or try to divine SOC from an OCV lookup table. Since an OCV lookup table doesn't exist and for accuracy you need to wait 3 to 4 days for the cells to rest, only AH can tell you the SOC. So when you say you charge to 3.8V, how much AH are left?


Why do we need to leave them for 3 or 4 days to reach resting voltage? That is quite a bit longer than most other reports.



> 6. When an ion cannot be intercalated it plates instead on the SEI layer causing capacity loss. Plating can occur before the cell is fully charged and plating begets more localised plating.


The SEI layer is not metallic (plated) Lithium. It is much more complex than that. Does overcharging have paths besides Lithium plating? Perhaps it thickens the SEI layer (that is reported to increase internal resistance.) Perhaps it cracks the electrolyte.


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## Siwastaja (Aug 1, 2012)

drgrieve said:


> Ok some facts about charging.
> 
> 1. OCV of LiFeP04 as determined by the voltage potential of the chemistry involved is 3.38V
> 2. Charging at a higher voltage than 3.38V such that current is achieved will at some point in the future cause the cell to be fully charged and then over charged
> ...


Unbelievable! Someone posts 7 detailed points of li-ion charging theory on DiyElectricCar and gets everything right! Thanks.


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## Old Goat (Jun 28, 2011)

Finally got around to completing the data collection that Dimitri suggested. Today, I drove 30 miles and managed to get one cell (#30) to continue to register as low voltage when I shut down (it was all the way down to 2.1V) when most of the other cells were still up around 3.1V. There were 4 other cells (#20, 21, 22, and 6) that also usually (but not always) had their green LED extinguished (indicating low voltage on each cell detected by the mini bms) when the LV alarm started to sound. So I am reasonably convinced that I will need to replace five cells to get a little more service out of my 36 cell pack. Of course this is a gamble since the pack will obviously not be from the same production lot and will not ever be as well matched as a new set of cells. But it would be a whole lot cheaper than a whole new pack.

For the first time, I'm getting some cells to consistently underperform the others.

Here are my pack resistance measurements:

Full Pack:
Rest (0A): 119V
100A: 113V -- .06 Ohm
200A: 108V -- .055 Ohm
300A: 103V -- .053 Ohm
400A: 97V -- .055 Ohm

Near Empty Pack:
Rest (0A): 115V
100A: 108V -- .07 Ohm
200A: 101V -- .07 Ohm
300A: 94V -- .07 Ohm
400A: 87V -- .07 Ohm

[Of course, I'm showing too many significant figures in my resistance measurements above, but suffice to say my pack Thevenin Resistance is somewhere between 50 and 60 milliohms on the full pack and about 70 milliohms on the near empty pack. I still don't know whether this number is meaningful or within normal range.

I am considering another experiment and would appreciate comments from the crowd here. I looked up the resistivity of AWG 14 copper house wiring. I figure if I make a coil of 100 feet of the stuff, I will create a 0.25 Ohm resistor. If I find the half way point of the wire and fold it back from the middle, I could wind a pair of parallel wires around a home-made wooden spool (actually I would nail together 2X2's into about a 1 foot diameter by around 18 inches or maybe 2 feet long) and one wire would wind on clockwise while the other would be counterclockwise thus essentially eliminating inductance and the whole length would be almost perfectly resistive with almost no reactance. [It would be like parallel transmission lines, something like 300 Ohm twinlead TV transmission line]. With a full cell connected to this resistor (3.3V), I would draw about 13 Amps and dissipate about 40 Watts (13^2 * .25). This would be about 0.1C. My guess is if the wire gets really hot, I could blow a fan across it to help dissipate heat. In any case, if my cell really is 120AH, it would take around 9 hours to go from full (3.3V) to near empty (approx 2.6V). If I measured the time and multiplied by the current, I would have a measure of one cell's capacity.

If it's way below 120 AH (say, half), I would know if I had to replace the whole pack. Of course the whole assumption here is that I'm choosing one cell at random and it's pretty close to the average of all the cells. I wouldn't have the patience to do this 36 times.

Comments, please?
Dave


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## PStechPaul (May 1, 2012)

You can get two 50W 0.5 ohm resistors for about $6 and you could put them in parallel to get the 0.25 ohms. They are made for a heatsink but you could also submerge them in a bucket of water which will heat up and possibly be useful for something.
http://www.ebay.com/itm/2x-0-5-OHM-50W-Wirewound-Aluminum-Housed-Resistor-50-Watts-/400310239040
http://www.ebay.com/itm/0-25-Ohm-0-...l-Shell-Case-Wirewound-Resistor-/370550984105

If you want to make your own resistors, baling wire or coat hanger wire (steel) has more resistance and is cheaper. And in any case it's better to heat water than air.

You can also get a battery tester for under $50:
http://www.ebay.com/itm/300720475540

If this is a properly designed product, it will read actual ampere-hours even if current fluctuates. That's another reason to submerge the resistance in water, especially if you use copper or steel, which have large temperature coefficients. The water will keep the temperature more constant.


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## kennybobby (Aug 10, 2012)

Good catch on finding the weak cells. 

That one cell is definitely damaged and will show reduced capacity if you test it, but that doesn't necessarily mean the rest of the pack is bad also. Bypass those 5 cells and run the test again to determine how well the pack holds up without the slackers. 

i've have used burner eyes off a stove (second-hand thrift store) as loads when testing some high dc voltage and needed some high currents--really cheap and will handle 100's of amps. 

i think the reactance of your coil is zero due to the frequency of the source, not the winding configuration.


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## ndplume (May 31, 2010)

kennybobby said:


> Good catch on finding the weak cells.
> That one cell is definitely damaged and will show reduced capacity if you test it, but that doesn't necessarily mean the rest of the pack is bad also.


Before tossing the cells, I would make sure they were in balance with the rest of the pack. If those cells were less charged, then they would run out of juice first and hence the low voltage. I realize, if they have lower capacity, same could happen. The question is how to figure out which is the issue. (Steps 3 & 4)



kennybobby said:


> i've have used burner eyes off a stove (second-hand thrift store) as loads when testing some high dc voltage and needed some high currents--really cheap and will handle 100's of amps.
> <snip>.


Using stove coils as loads, thats sounds like a great idea! I'll have to remember that. Have you stuck them in water during testing? With some crab boil and shrimp, could be the best tasting aspect of the test.


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## dougingraham (Jul 26, 2011)

ndplume said:


> On screw torque :
> Doug Ingraham explained a scientific way to determine the torque required for a cell connection that really makes sense and isn't too difficult to perform. I use my PL6 to do it, but it could be done with independent instruments.
> The idea is simple. Connect a cell to a load using one of your battery straps in the circuit with 1 side connected to a cell in your pack. Once you get a load current (I use 25A from PL6) and are monitoring the Cres, tighten the screw that goes to the battery strap and watch for the Cres to bottom out. Record the torque value and add some for margin


The minimum torque to get good conductivity may not be enough to keep the connection tight long term. On my batteries I found that the head would pop off the bolt at between 50 and 55 in-lbs for the stainless M4 bolts used on the GBS terminals. So that is the upper limit for my setup. I don't recommend you try to find this upper limit. Mine was found by accident (twice). The first time it happened I just thought it was a bad bolt. The second time I realized I had reached the physical limits of the materials. I removed the terminal strap and had no trouble removing the thread portion of the bolt. You might not be quite so lucky and I would hate for someone to ruin a cell.


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## ndplume (May 31, 2010)

dougingraham said:


> The minimum torque to get good conductivity may not be enough to keep the connection tight long term. On my batteries I found that the head would pop off the bolt at between 50 and 55 in-lbs for the stainless M4 bolts used on the GBS terminals. So that is the upper limit for my setup. I don't recommend you try to find this upper limit. Mine was found by accident (twice). The first time it happened I just thought it was a bad bolt. The second time I realized I had reached the physical limits of the materials. I removed the terminal strap and had no trouble removing the thread portion of the bolt. You might not be quite so lucky and I would hate for someone to ruin a cell.


Good point on risking cells. I still like this method for bracketing the torque.

One could twist a few heads off on some screws in a scrap piece to find the upper limit. That way we wouldn't risk the cells. 

Use the Resistance drop to establish the minimum torque. Then use judgement to go somewhere between the 2.


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## pdove (Jan 9, 2012)

The fix would be to take off the BMS and bottom balance your cells. Charge to 3.6 volts and turn off charger.

You will get all the range back if you haven't damaged a cell. If you have it shoulkd be obvious by diving the pack voltage by the number of cells if the voltage is less \than 3.3 volts then you have a damaged cells and need to test the voltage of each cell to find the culprit.


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## dimitri (May 16, 2008)

If only people actually read and comprehended original post and OPs' follow up responses, then number of conflicting advises could be reduced to a minimum


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## Old Goat (Jun 28, 2011)

PSTech Paul: Good call. I really appreciate those leads. Incredible what great buys one can find on EBay for items like this. I shopped all over the place and didn't find either of these; thanks for passing along the info.
Dave


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## Old Goat (Jun 28, 2011)

If anyone is interested in this continuing saga, after yesterday's continuous LV alarms after about 30 miles of driving, I simply charged the one offending cell (#30 until the alarm stopped sounding -- it went down to about 2.1V when nearly all the rest were around 3.1V up so when I charged it alone to about 2.8V the alarm to shut off. Then, I simply ran the pack charger in a normal fashion. By the next morning, the entire battery pack had settled out at 120V (which is normal). I then measured all 36 cells in a static fashion and the average per cell voltage was 3.332 -- the highest was 3.336V and the lowest was 3.328. Highest minus lowest was thus 10 millivolt and biggest deviation from average was 6 millivolt. I conclude that the pack is pretty darn close to "well balanced." Interestingly, the offending cell (#30) measured in at 3.331 which is only 1 millivolt from average cell voltage. To date, all the balancing I have been doing is top balancing as per Dimitri's suggested procedure. I really appreciate all the comments, opinions and suggestions. I knew you guys wouldn't all agree on this stuff but I can easily catch some really helpful hints here and I want to say THANKS.


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## kennybobby (Aug 10, 2012)

So did you get back to your 62 miles of range...?


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## dimitri (May 16, 2008)

Old Goat said:


> I simply charged the one offending cell...


By adding some charge to low cell you likely caused this cell to be the first one to hit HVC at the end of charge, which disturbs the top balance you previously confirmed. You did not notice it because you did not observe end of charge, since you only checked resting voltage after some time. Resting voltages around 3.3V don't tell you much about the state of balance. Only voltages close to either end of the curve can tell you something useful.

If you want a balanced pack, you need to pick either top or bottom balance and stick to it, you can't jump back/forth all the time.

Whatever your lowest capacity cell , it will always limit usable pack capacity, regardless of which balance scheme you pick. Since you already had a top balanced pack and your range is too low, the only way to address it is to replace weakest cells. Assuming your issues are not related to dirty/oxidized terminals of course, but since you confirmed a cell at low voltage after 30 miles, this pretty much means a cell is weak and it limits your range.


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## Lopezjm2001 (Nov 22, 2011)

dimitri said:


> I never understood the persistent notion on this forum that LiFePO4 cell is overcharged by taking it to 3.65V or even to 3.85V using basic CC/CV chargers. Is this one of those gems where one genius came up with this theory and everyone else repeating it?
> 
> Most common CC/CV chargers do not hold CV voltage for more than few minutes, in fact entire CV phase is really short, so I don't see how any damage can be done. I suppose there is some potential danger in holding CV voltage for many hours at a time, and even that idea needs some hard data to prove such effect.
> 
> ...


I do not disagree with most of what you are saying but it is not so much the high CV value of 3.8 volts but the low C rate during the CC stage that can lead a battery pack to being overcharged resuting in lithium plating. It has more to do with not correctly matching the battery pack configuration with the charger.

http://priuschat.com/threads/discus...battery-university.119569/page-3#post-1713759


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## pdove (Jan 9, 2012)

My cells are all 3.339 volts after charging the pack. No BMS whatsoever. I just cut the charge off at 3.6 volts per cell contant current up to that point. Cells are exactly the same in voltage every time. You guys are living in a fictional world. My pack is 146.9 volts after resting over night since my charger cuts off before I go to bed. that is 3.339 volts per cell (44 cells). Been the same every time for 6 months. They don't vary from cell to cell more than two millivolts.


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## pdove (Jan 9, 2012)

Dimitri is correct. That is just a procedure. What is important is the entire amount of energy (watts) one puts into the cell. I have read a lot of papers on the subject but more importantly I took them in the lab and played with them.

What I found was that I could charge and discharge them at 3C with not rise in temperature of the cell. It doesn't care, however, I took a full cell and put it on a c/20 charge and within 5 minutes the cell temperature went up 50 degrees F. 

They were 100AH Bestgo cells. so I charge at 300 amps for 20 minutes with no rise in temperature but on a full cell 5 amps the temperature climbed rapidly.


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## ndplume (May 31, 2010)

pdove said:


> <snip>
> 
> but on a* full* cell {charging at} 5 amps, the temperature climbed rapidly.


And what if you charged a *discharged* cell at C/20? (Say 50% SOC) 
Other than it taking about 20 hrs?


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## dtbaker (Jan 5, 2008)

pdove said:


> My cells are all 3.339 volts after charging the pack. No BMS whatsoever. I just cut the charge off at 3.6 volts per cell contant current up to that point. Cells are exactly the same in voltage every time. You guys are living in a fictional world. My pack is 146.9 volts after resting over night since my charger cuts off before I go to bed. that is 3.339 volts per cell (44 cells). Been the same every time for 6 months. They don't vary from cell to cell more than two millivolts.



what do you use to monitor, and cut charge when cell(s) hit 3.60v ? I'll go back and catch up on this thread, but wanted to ask that as I am looking for alternatives to control/end charge gracefully...

I am a big believer in no BMS, top balance, and just cranking in the watts until you hit a specific end voltage.


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## GizmoEV (Nov 28, 2009)

pdove said:


> I have read a lot of papers on the subject but more importantly I took them in the lab and played with them.


More people need to do that. Get a bench power supply and a DMM and play around with the cells to get a sense of how they behave.

Like you, I don't see the voltages at the end of charge move around very much. It has been over 2 years on my pack and still no need to balance. Since my charger doesn't cut off at a particular voltage but merely tapers to nearly 0A then the timer shuts it off I charge to 3.455V/cell. After an over night rest the cells are all at 3.36V.


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