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update on cell drift of my pack

28K views 97 replies 21 participants last post by  EVfun 
#1 ·
I just thought I would start a thread on MY pack to record results, and see if it is what other people are seeing.

First.... my pack is 38 x 100ah Thundersky cells. I am using an Elcon pfc-1500 charger with simple CA->CV curve set to finish at 139v which gives a target average of 3.658vpc at 'finish'. I have no BMS, and leave my dc-dc 'on' all the time. I do use the vehicle almost every day, and charge at night.

Longest period of non-use was 1 week, and I left dc-dc 'on' to see what drain would be.... Kill-a-watt meter measured 1.9kWhr it took to charge back up to 'full'. Confirming that if you are not going to drive for a week, you really ought to flip the main breaker to prevent ANY drain!

I did an initial top balance in parallel to 3.90v with a bench power supply, then installed and wired in series. First series charge I monitored very closely and it didn't take more than a couple minutes since cells were charged from initial parallel balance, and was glad I did because I did have a couple cells head past 3.80 while others were below the CA->CV voltage of 3.65vpc. I then 'knocked down' the high ones with a pair of 50W resistors and repeated charge cycle until I had the whole pack +/- .02v just after end of charge.

The manual final tweaking of the balance in series is amazingly sensitive to small changes, and the voltage rise from flat to starting up the knee happens fast, and doesn't last long. I found that tweaking individual cells with no more than about 30 seconds of either draining with resistors or adding with power supply was the best way to avoid overshooting the target.

The cell voltage measurement I found most practical to do a few minutes AFTER the charger 'finished', and the pack voltage settled from the finish voltage of 139 to 136.0... because it hangs at close to 136.0 long enough for me to buzz around and record voltages from each cell.

The initial balance was +/- .02 on all cell but 4 that were -.03 ... I had to button the car up and do a show and tell and local community college, so I left it at that figuring that the 'high cells' would finish at no more than 3.68vpc, which is still fairly conservative for Thunderskies.

My first balance check was after about 1000 miles.... there was virtually no change in relative balance between cells. I decided not to change balance at all to see if there would be any drift with more use.

I just did second balance check yesterday at just over 3000 miles on the pack. There was virtually no change in relative balance between cells. I decided to spend a little time to get the 4 cells that were -.03 up a little... and after a couple charge/checks, I was able to get all cells +/- .02 at end of charge.

so.... at this point I just wanted to note that I experienced NO significant relative cell drift in the first 3000 miles.
 
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#5 ·
It took nearly 100 full cycle equivalents for my cells to be 0.1V different at the extremes. This could have been due to several different factors and likely wasn't much energy difference. I was charging to 3.485vpc and had BlackSheep BMS boards attached which draw only about 1mA. I rebalanced my pack to 4.00vpc using the BMS boards and then removed them. I take a reading each month to see what things are doing and then record and plot the results. I do have a half-pack voltage monitor on my pack but that is it besides the CycleAnalyst.
 
#6 ·
Dan,

As you know, I have a similar setup here and have about the same number of miles on the car. I did an initial top balance and did a "touch up" after about 2,000 miles. There are small differences (0.2 to 0.3) in voltage at the end of a charge cycle, but the batteries settle down nicely in a few hours. I only use 50 to 60% DOD in my commute and never get near the bottom, so the irregularities in cell capacity are not an issue for me. I think that is the key to running without a BMS.

I am considering adding one or two more cells to the string to lower the Vpc to either 3.55V (39s) or 3.47V (40s) to alleviate the differences in terminal voltages and add a mile or two of range.

Ralph
 
#8 ·
I did an initial top balance and did a "touch up" after about 2,000 miles. There are small differences (0.2 to 0.3) in voltage at the end of a charge cycle,
I hope you mean .02-.03 vpc differences?

but the batteries settle down nicely in a few hours. I only use 50 to 60% DOD in my commute and never get near the bottom, so the irregularities in cell capacity are not an issue for me. I think that is the key to running without a BMS.
agreed... but I would like to say that I think .02-.03vpc irregularity at end-of-charge is about all I want to tolerate to avoid possible overcharge issues on any one cell at end of charge. I have been surprised how a tiny difference in SOC can make a rapid and significant difference in final voltage. applying a 6amp charge or drain on a single cell for 30 seconds seems to change the final charge voltage close to .05v if the rest of the pack is 'balanced'. That gives me an idea just how steep that curve is, and how important that end-of-charge balance is.


I am considering adding one or two more cells to the string to lower the Vpc to either 3.55V (39s) or 3.47V (40s) to alleviate the differences in terminal voltages and add a mile or two of range.
Ralph
I would be concerned that if you target a final voltage closer to the flat, you may start giving up a significant amount/accuracy of capacity in the charge cycle because the 'final' voltage may be hit when a lot of the cells are not that close to full. I am figuring that with Thunderskies and target of 3.65 +/- .03vpc is pretty safe. They seem to only be at that final voltage of a couple minutes, and settle down rapidly after the charger turns off.
 
#9 ·
I have conducted a number of experiments with my 360ah 12v house power pack. 16 X Winston LYP90ah. The first pack I carefully balanced at 4v per cell till they accepted no further charge using solar and an adjustable PWM float voltage regulator. I took it up in 1v steps from 3.4v per cell till the amps in dropped below 5amps and moved to the next stage, just made it easier and less likely to have a run away cell. this pack has remained balanced with every day cycling for 60 cycles so far and shows no sign of changing. I charge to 3.45v per cell, gives a good safety margin just in case and I have built a simple charge cut off/reset if any cell goes over 3.85v, just in case. i use a simple Jusi cell logger and the alarm triggers a 30 min timer circuit that turns off the solar positive input before the solar regulator allowing the house load to discharge any over voltage from a rogue cell. Similar to DT Baker leaving his DC to DC on.

The next pack of 16 cells I charged by a method but no balancing. The difference with these cells was they are all older Thundersky TS cells, been it the packing case for 2 1/2 yrs, production date 03/07/2009. I didn't do the 4.0v conditioning first but rather let them charge to 4.45v per cell and put them to work. Initially there was over a 300mv difference and the 3.85v cut out operated a few times but over a few mini cycles during the day the cells started to creep closer together. Within a few days the 3.85v cut didn't operate at all and after 2 weeks the pack as within 17mv, sometimes within 4mv. I decided to do the 4v conditioning, the pck stay together all the way to 4v per cell, I returned it to 3.45v per cell float and let it run for a week, still in balance.
these packs haven't cycled below 70% DoD much yet but on the few times they have (rain for a few days) they have remained in balance.

Just a side note, when under a 1C or higher load they do go out of balance, up to 60mv and it's never the lowest cell before the test starts that drops low under load so the relevance of off load balancing becomes a bit questionable but as hat is the only real method we have it will have to do.

T1 Terry
 
#12 ·
I've noticed interesting behavior toward the end of charge and it makes me wonder how to accurately measure the balance. I have a 32 cell pack of 60 amp hour Thunder Sky cells made in February 2010.

I'm quite good at walking back into the garage just after the charger hits the constant voltage stage. The charger will be at about 10 amps at 111.6 volts. The pack will have 4 cells at 3.47, 2 cells at 3.51, and 1 at 3.52* volts. The rest of the cells will be between 3.48 and 3.50 volts. If I walk back out 30 minutes later near the end of the charge the cells will be between 3.48 and 3.50 volts and the one that was at 3.52 is one of the cells at 3.48 volts. The charger voltage will be 111.8 volts and the amps will be to low to accurately read, perhaps 2 amps.

If I check the pack 1 to 2 hours after the charge is complete the cells will all read either 3.38 or 3.39 volts except that one cell will read 3.37 volts. By the next morning the DC to DC has bled off about 1/2 amp hour and all the cells read 3.33 volts. In fact, each block of 8 cells will read the same, this morning they where all 26.68 volts (depending on how long "overnight" is they might all be 26.64 volts.)

I'm paranoid enough that I keep checking near the end of most charge cycles. These current numbers have held for about 1 week, prior to that I was still doing small balance adjustments at the end of most cycles. These adjustments where measured in amp seconds and I doubt any cell was adjusted by 3 amp minutes (0.05 amp hours.) I have been comfortable enough to monitor the finish charge only every other cycle. I've never seen a cell get above the others by 0.1 volts, even before balance adjustments. Most of the adjustments where knocking down that cell that pops up to 3.52 volts. I want to leave it alone for awhile and see if it creeps up.

* The one at 3.52 volts is the largest cell in the pack. After a deep test cycle it was at 3.18, the low one was at 3.06 and the rest where between 3.08 and 3.16 volts.
 
#14 ·
I'm quite good at walking back into the garage just after the charger hits the constant voltage stage. The charger will be at about 10 amps at 111.6 volts. The pack will have 4 cells at 3.47, 2 cells at 3.51, and 1 at 3.52* volts. The rest of the cells will be between 3.48 and 3.50 volts. If I walk back out 30 minutes later near the end of the charge the cells will be between 3.48 and 3.50 volts and the one that was at 3.52 is one of the cells at 3.48 volts. The charger voltage will be 111.8 volts and the amps will be to low to accurately read, perhaps 2 amps.
I've noticed a similar behavior with my pack. What I think might be the cause is the cell which goes high at first has a higher internal resistance so its terminal voltage will be higher when the current is higher. As the current drops the IR doesn't factor as much in the terminal resistance so the voltage drops. I think that this points to a reason to only top shunt balance when the charge is basically finished and then only at very low current meaning less than 0.5A. That is assuming you are going to top balance. The behavior you see is the reason I take cell measurements at the end of charge immediately after the charger quits pumping in current. If I can't wait for that and need to get the readings I only make them when my charger is pulling 20-30W out of the wall.

Like Pete said, I wouldn't worry about it especially since you are charging to a low vpc like I am.
 
G
#13 ·
What are you worried about. That is just fine. There will always been some variance near the end of charge and discharge. None are even in the 3.6 volt range. I have one that goes into the 3.8 volt range and it is consistent and always rests at 3.34 or 3.35. What more can you ask for. My pack is also bottom balanced as you know.

I am in Cape Girardeau, Mo right now. I will be posting during the evening after each day of the conference.

EV, you have nothing to worry about. Keep watching until your just plain sick of doing so and sure in your own head that all is OK. Don't get too anal about it but do what you must do. We all check. Except on my Leaf.

Pete :)
 
#18 ·
8-2-2012.... finally made time to check balance on my pack in the Swift again. It went a little further than I had intended, but all is well. I now have 8431 miles on my pack (38 x 100aH Thunderskies). my usual daily use is about 20 miles (50% DOD or less), so this represents about 425 partial charge cycles. I have taken pack down to 75%-85% DOD about twice I think.... and the wimpy Curtis 1221 can only maintain 2C, and max 3C, so pretty gentle use.

I am pleased to report that all cells are still +/- .03 vpc from theoretical average vpc target at end of charge after letting pack settle from 139. to 136. and starting measurements just after surface charge dissipates. I re-charged and double checked the highest ones right at end of charge and found that the very highest was at 3.80v for about a minute at tail end of charge until pack settles and voltages drop closer together.

There were 7 cells on the high side by .03, so I applied a 50 watt resistor for 10 seconds to each of the highest ones as long as I had battery covers off. Probably didn't need to tweak balance, but why not!

tests are bearing out hypothesis that top-balance without BMS stays in balance just fine. at this point I plan to do just annual checks/tweaks.... about every 5000-7500 miles.
 
#19 ·
Kind of a boring thing to do, isn't it? After a while you just don't worry about it. I just checked my pack today and the voltage spread is 0.033V. The low was 0.020V, high of 0.047V. It is interesting to look at a graph of the spread. The cells change places frequently. The attached graph is for my 40 cell TS-LFP100AHA pack in a 2p20s arrangement.
 

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#20 · (Edited)
Kind of a boring thing to do, isn't it?
I am loving this particular boredom! I thought it was time to check terminal nuts and figured I might as well go thru and check balance. Thrilled that balance is basically unchanged without having any BMS.

The terminal studs/nuts were all good too. I did not use regular bolts; I switched out to stainless set screws (with loctite), toothed washers and nuts. I did this primarily to be SURE I was getting full thread engagement in those soft AL and Cu terminals.

I did look at your graph, and am a little surprised to see the spread opening up a little, and the specific cells showing relative drift/change with respect to each other. It also appears that you wait until after the charge cycle has settled almost completely to check (by the voltage shown)... what is your measurement procedure, and how did you do your initial balance?
 
#24 ·
I thought it was time to check on my top balanced pack without a BMS. Since my previous post in this thread I added 6 cells to my pack, taking it from 32 cells to 38 cells. All my 60 amp hour Thunder Sky cells are from the same Feb. 2010 batch. I purchased 42 cells. In 2010 40 of them where in a Datsun EV. In 2011 32 of those where in the EV Buggy. This year 38 of them are in the EV Buggy (it runs much stronger with the voltage increase.) I have 4 cells on the shelf in the garage. No cells have showed signs of damage or failure.

It was a good thing I checked...

I found the 6 cells I added have creeped up a bit. I charge the pack to 3.5 vpc (133 volts for 38 cells.) The 32 cells that have been used for 3 years where all between 3.43 and 3.50 volts. Most of them where at 3.44 volts. The 6 added cells ranged from 3.61 and 3.99 volts at the peak during charge. These cells matched the pack in the spring. I top balanced the 32 in the car and added the 6 cells that had been top balanced by charging in parallel. I installed these cells in the car as 2 blocks of 3 cells. Each block of 3 has only been treated as a unit (no individual balancing within each block of 3.) I had removed a fraction of an amp hour from each block of 3 to make them match the 32 in the car.

The right hand block ranged from 3.74 to 3.99 volts. I removed 0.15 amp hour for now which lowered the high one to 3.93 volts. The left hand block ranged from 3.61 to 3.82 volts. I removed 0.1 amp hour from that block of cells which lowered the high cell to 3.80 volts. I will pull more out of both banks before recharging the pack again. I may do a capacity test to see if the bottom is getting more ragged or if the smallest cell is different. I will certainly make sure the top is balanced again, or is at least not more than 0.1 volt high at the end of the next charge.

I'm not so sure adding cells is a good idea, even when they come from the same batch. I may end up installing shut regulators to nudge the top in line. It wouldn't take more than 20 milliamp hours each cycle. Even though all 38 cells have seen the same number of amp hours go into and out of them the less used cells are ending up at a higher state of charge over time.
 
#25 ·
I thought it was time to check on my top balanced pack without a BMS. Since my previous post in this thread I added 6 cells to my pack, taking it from 32 cells to 38 cells.
so.... did you ever put the WHOLE pack in parallel and top balance to something like 3.8 or 3.9? then put back in series and check end of charge to knock down any high cells at very end of charge? if not, then it sounds like initial re-balance was not quite 'complete'.
 
#26 ·
I had the 6 cells at a slightly higher SOC than the 32 cells. After I installed them I put a resistor around each block of 3 added cells to pull it down while charging for several cycles. After working the SOC down on the added cells I verified they where matched to the rest of the pack by monitoring the voltage during the end of charge for 2 cycles. The cells finished in the same voltage range (3.47-3.54 volts in the last 10 minutes of the charge) without intervention for 2 charge cycles. I think I had the added cells matched to the rest of the pack. I checked several more times this spring and everything looked normal.
 
#27 ·
The cells finished in the same voltage range (3.47-3.54 volts in the last 10 minutes of the charge) without intervention for 2 charge cycles.
So what were the voltages at the moment the charger shut off or just a few seconds before? As you know, even 10 minutes at a low charge rate can have a drastic effect on the ending voltage.

Edit: Also, if you look at the graph of my cell voltages in the .zip file I posted earlier you will see that the voltages do move around a bit. Maybe after the "new" cells get cycled a little more they will match up.
 
#28 ·
They are not getting closer. They where all between 3.47 and 3.54 in the early spring right after I worked the 6 additional cells into the pack and got them agreeing at the end of charge. I would just go around in circles checking the cells continuously after fitting the new cells into the pack. Must have been 4 or 5 cycles where I would burn off a little charge with resistors to knock the added cells down to match the other 32. Then I watched all 38 cells stay in the same range for 2 charges without intervention. Today my total pack range was 3.43-3.99 volts! 32 of them where between 3.43-3.50 volts. The added 6 cells ranged from 3.61 to 3.99 volts. I only knocked them down down a little and will tighten up the added cells to the rest of the pack when I charge tomorrow (at least get close, balancing may take several cycles.)
 
#31 ·
They are not getting closer. They where all between 3.47 and 3.54 ....The added 6 cells ranged from 3.61 to 3.99 volts. I only knocked them down down a little
if you are top balancing, my belief is that for at least the balancing you need to be checking and adjusting right at the tail end of charge... thats what you are trying to control after all! If they are balanced close to 3.65vpc at end of charge, they'll be even closer if you set your regular cycles to end lower.

I took my initial balance in parallel ABOVE intended pack voltage (to 3.9), the checked and tweaked a little after putting them in series until all were 'finishing' very close to 3.65 vpc.

trying to balance at lower voltages is a waste of time as you can't tell where you are if the curve is flat.
 
#33 ·
I'm wondering if it has anything to do with the how the cells degrade. I remember seeing a 500 cycle test put up over at evtv that he managed to get out of thundersky. While the decline in capacity was slow and straight for the most part, other than the gain within the first 50 cycles, there was also the occasional spike or valley in the chart for one charge then a continuation of the slope. Is it the same cells showing variation or does it move around?
 
#35 · (Edited)
It is consistently the same cells that are high at the end of charge. It is the 6 I added to the pack. I'm going to discharge the pack today, constantly scanning with the volt meter to find out how the bottom of the pack numbers have changed.

After a drive to take the cells down about 1/2 way I let them rest a couple hours then checked the voltage. All 38 where 3.297 +/- 0.001 volts. I'm doing a discharge test now to see what I see at the bottom. I'll post back whatever I find.
 
#37 ·
I pulled my pack down until a cell hits 3.000 volts. My main 32 cells where between 2.996 and 3.104 volts. The added 6 where between 3.107 and 3.125 volts. I pulled 0.2 amp hour from them. I added 0.06 amp hour to the smallest cell because I know it isn't one of the highest voltage at the end of charge. The pack has remained consistent, the smallest cells are still the smallest and the largest cells are still the largest. Once I get the 6 added cells in line I will figure out where they are compared to the rest of the pack.
 
#56 ·
I added 0.06 amp hour to the smallest cell because I know it isn't one of the highest voltage at the end of charge.
Just in case you don't know or if someone reading this equates charging voltage to SOC.

Charging voltage doesn't equal SOC. The only way to measure SOC is using static voltage or counting amp hours from a known point.

Using charging voltage as a point to balance your pack is wrong and is a failing of using an active BMS to balance your pack.
 
#39 ·
It was easy to get the pack back in balance and it is surprising just how little it takes to run a cell from 3.5 volts to 3.9 volts. The 6 cells had 0.2 amp hour removed and now they finish with the rest of the pack. I had a total range from 3.46 to 3.53 volts. The 2 cells at 3.46 where both the added ones that had been finishing at around 3.8 volts. The 2 cells finishing at 3.53 where from the older 32 cells and have always been the highest ones at finish.

Just 0.2 amp hours was all the difference between one cell going to 3.99 volts and now only going to 3.51 volts at the end of charge. It makes me wonder what really happens in the last 30 minutes of a charge with a bottom balanced pack.
 
#40 ·
Now you can find out if those two creep up in voltage at the end of charge after several cycles. If they do there might be some merit to the efficiency hypothesis.

For a bottom balanced pack the ending voltage has to be lowered. As long as the charger shuts off when the current drops to about 0.05C then a cell ending up at 3.65V is likely ok and not overcharged. I'm sure that if the cells' capacities were spread a long ways out then it could be a real problem.
 
#41 ·
I have chosen a different cell strategy, one I'm sure that will ignite many critics and cause a lot of "You're a dummy" posts. I don't care. It works for me. I only present it here as an abstract idea and mental exercise.


I named it -Middle balanced-:

I paralleded my pack in their as-is delivered state. I do not much care as to their real charge state just as their not full 3.9 or 3.0

I left them for a week. After disconnected another week, they were all 3.310

They were installed, series, and brought up to full charge. (recorded)

I had one wild cell, #12. I put a cell voltmeter on it. It was 3.90

Driven, recorded, charged, driven, charged....4 complete 35-40 mile cycles.

The figures all repeat, every-time accurately.

The same cells charge and drain in an accurate, predictable manner.

The basis of my "middle charging" idea is that when it charges it only gains 50% of the deviation of a bottom balanced pack. It also drains at 50% of the deviation of a top balanced pack.

In short, It uses a 50% median line to limit the deviation to 50% above or 50% below instead of the full 100% as of top or battom balanced.

The top charge is repeatable and "watched by the charger. The problem cell that will drain first has a cell monitor on it and will give me a really obvious red light on the dash.

Yes, it has been only 4 cycles so far, but it has repeated accurately.

Miz
 
#46 ·
I did exactly this when I got my pack new over a year and half ago. I wanted to get driving and didn't want to take the time to bottom balance accurately. I never even go below 50 -60% so, it worked fine and never drifted or had any issues until I got around to bottom balancing.
After my rebuild, I re-did everything, so I decided to redo the bottom balance a bit lower. I used the Power Lab 8 and discharged each cell to 2.7 volts. It took a bit of time, but I did it while I worked on other things.
After seeing that the finish charge in CV at 3.45 volts per cell did not cause any shooters at the top above 3.6, I upped the charger to 3.5 vpc. I have 3 cells that I watch with slightly lower capacity . . about 1% is all. They climb to about 3.65 and interestingly enough, they start to go back down before the charge clicks off. It's perfect.
So, ya, it can work and perhaps if you have a pack that is not so perfectly matched, maybe is a good compromise.
 
#42 ·
So what is your end of charge target? That one cell could have been 5 AH out of whack from the others and could very well be exactly the same AH in the cell as the others and would be sitting happy with the others if you had lower balanced your pack. If you even take your pack to 3 volts per cell you know your not on the bottom but pretty much on the down swing. All should easy be within an AH or so from each other. Then on the upper charge your swing won't be so far as long as you stay off the very bottom which is pretty easy to do with a smart controller and AH counter. I would not just do a middle because it is hard to KNOW what the MIDDLE is. It is easier to do the balance on the upswing or downswing. Downswing being the safer but either works if you so choose. My last balance is more in the range of 2.7 volts rather than 2.4 volts like I did before. Still no cell goes over 3.8 by the time the charger cuts off. All cells just before termination are 3.7 and lower. I now use 3.55 volts as a cutoff voltage since I can choose any termination voltage with my controller when its used as a charger.

Pete :)

I might suggest you bring your one cell down a touch to better match the others so your not charging it to such hight voltages. 3.9 is in the range where you may not want your cell to live and if it has the capacity you surly don't want to leave it there.

The purpose of balancing is to also find which cell is the lower capacity one or ones and to find out by how much. If your termination voltage is like 3.7 and lower for your highest voltages your doing well.
 
#43 ·
No matter how you set up your pack your still charging in series single cells so each will charge accurately and repeatably every time. That is not in question. The question is how well balanced is your pack of cells to each other cell in the pack. The closer together they are the better. Top, Bottom, or Middle. Just harder to balance accurately in the middle vs the top or bottom swing.
 
#44 ·
For the last 4 cycles, my pack has done the exact same thing. They all come back to .014/volt of each other when in that mid-balanced area. Even that high one.

I do not intend to further alter any even if I lose that strange one. That is the premise of this method.

I am aware I am leaving some capacity on the table with the way my charger is set, but that is another story.

When I get my transmission put back in, I will get some lower discharged cycles on them. If that follows predictions, it will bear me out. If not then I will just top balance like everyone else and keep them that way.

In my world, when a cell tops out first AND bottoms out first, it is lower in capacity than the rest. I have 14 that deviate from the rest of the pack when on the top end. I still need to check them on the bottom end to say for sure.

The problem with this forum is that there are too many different opinions. All state they are right, making it very confusing for anyone seeking knowledge.

Miz
 
#50 ·
The problem with this forum is that there are too many different opinions. All state they are right, making it very confusing for anyone seeking knowledge.
There is the right way, the wrong way, and the military way.

Here is a helpful (or not) guide for grading opinions.

1) If the opinion comes from a manufacturer or vendor it is suspect. They want you to spend your money and will tell you what they have to to get you to do so. There are exceptions to this rule.

2) If the opinion comes from a product user their opinion will be against if they have had problems with a product but will be in favor if they have had no problems even if the product does nothing of value for them. So their opinion cannot be trusted. There are exceptions to this rule.

3) If the opinion comes from an "Expert" it is suspect. You don't know what they do and don't know and what is just their opinion. Experts are good at "typing themselves smart". There are exceptions to this rule.

4) If the opinion comes from a noob then who cares. But it doesn't mean their opinion is wrong. It just doesn't mean much.

5) If the opinion comes from me then it isn't an opinion, it is a fact! There are exceptions to this rule. (I usually, but not always, will tell you I don't know when I don't know.)

So whats the answer? There is no correct answer. There is only what works for you.
 
#45 ·
The problem with this forum is that there are too many different opinions.
The information given is factual and I see no opinions. Information is already out there that supports what is said, including mine. Believe it or not some still do things based on what has been done and not what is done on a typewriter.

If you don't know where the top or bottom truly lies then how do you propose to properly balance the pack in the MIDDLE.
 
#54 ·
If you don't know where the top or bottom truly lies then how do you propose to properly balance the pack in the MIDDLE./QUOTE]

Middle balance is what Jack R was originally promoting. Take the batteries out of the box as they were shipped and use them. As he points out the problem is replacing or adding cells later. If you think bottom balancing is difficult it is a lot more difficult to middle balance for the purpose of adding or replacing a cell because you need to balance to better than 0.001 volt or the ends will be off significantly.

So if you have to add cells it is quite a bit easier to bottom balance and add in the cell or easier still to charge up the new cell and charge up the top balanced pack and add in the cell.
 
#48 ·
Are you sure they vary by "about 1%" DIYguy? I ask because the 6 cells I had there where getting a little high near the end of charge (3.61-3.99 volts) only needed 0.2 amp hours removed to knock them back in line, now finishing between 3.46 and 3.51 volts. I have put a second cycle on them since pulling the newer 6 down and got the exact same finishing numbers.

It is a shame nobody makes a simple 100 milliamp 3.5 volt shunt reg (no BMS function, simple shunt reg.) That would be more than enough for slight variation, even for added cells of the same type. A red LED would make inspection easy.
 
#57 · (Edited)
I didn't start this thread as an opinion rantplace.... it is intended as a thread to post my actual results over time of running a top-balanced BMS-less configuration for those wanting to follow how that is working out.

Net is that so far ... with initial balance in parallel to 3.9 vpc, and 'tuned' after installing in series at end-of-charge until within .02v... cell drift has been un-noticable after more than 10k miles without ANY bms. Average day is about 25 miles (50% DOD), so thats about 400 cycles.
 
#60 ·
CALB has a different charge/discharge curve and a different voltage profile. CALB max charge is 3.6V and most people use 3.45 max. They also seem to have better QC and more consistently matched cells than TS/Winston.
Voltage is of course a good indicator of SOC, especially near the ends, but you also have to consider the influence of current and temperature.
 
#61 ·
CALB has a different charge/discharge curve and a different voltage profile. CALB max charge is 3.6V and most people use 3.45 max.
I know they list that max, but perhaps they are just picking a different part of the curve. I'm not so sure there is a definite "full charge" for LiFePO4 so much as a choice between just how many Li ions to drive across vs. lifespan. Has anybody been taking CALB cells to 3.8-4.0 volts routinely often enough to know it has a disproportionate impact of cell life? I would think that at least Jack Rickard would have found out if that is a problem -- he runs more or less bottom balanced packs without any battery management hardware. I would think some must be seeing over 3.65 volts at the end of charge.
 
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