# final top balance after install for BMS-less



## dtbaker (Jan 5, 2008)

finishing up my Miata, which has a BMS-less top balance pack... I did a long slow top-balance in parallel prior to series install, just finished series install, charged first time, and had maybe a third of the cells finish a bit higher than I would like above the theoretical average vpc target....

I had hoped for less variation, and am wondering if anyone can explain why some of the cells got a little out of balance between parallel balancing and post-install series charge?

I've been thru the charge/tweak cycle several times, and its getting closer, but is pretty darn time consuming when checking each cell and knocking down the high ones a minute at a time with a 50 watt resistor... Its so hard to tell how much to take off the high ones, and I don't want to overshoot.

so..... my questions are:
- how/why did some cells unbalance between parallel balance and install
- is there any faster/better way to tweak the balance than cycle and bleed the high ones a little at a time?


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## EVfun (Mar 14, 2010)

I know there is some variation, even after tweaking. I have about a 0.1 volt scatter. 

Parallel cells still get slightly varying charges based on the connection resistance at the terminals and the differences wiring resistance between each cell and the charger. I could see slight current movement across the interconnects on a block of 6 cells in parallel. You can detect it with a meter that can read down to the ten-thousandth of a volt, across the jumpers between the cells. 

One thing that may work better is to use a single 3.6 volt cell charger that shuts off when done and charge each cell with that.


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## DIYguy (Sep 18, 2008)

dtbaker said:


> finishing up my Miata, which has a BMS-less top balance pack... I did a long slow top-balance in parallel prior to series install, just finished series install, charged first time, and had maybe a third of the cells finish a bit higher than I would like above the theoretical average vpc target....
> 
> I had hoped for less variation, and am wondering if anyone can explain why some of the cells got a little out of balance between parallel balancing and post-install series charge?
> 
> ...


at what voltage did you parallel them and for how long?


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## dougingraham (Jul 26, 2011)

dtbaker said:


> so..... my questions are:
> - how/why did some cells unbalance between parallel balance and install
> - is there any faster/better way to tweak the balance than cycle and bleed the high ones a little at a time?


My best guess is they weren't actually balanced to begin with. There are a number of reasons this could have happened. When they get close in state of charge there is not much current flowing between the cells. The flat charge/discharge curve is responsible for this. A way to work around this would be to hook the positive end of the charger to one end of the parallel string and the negative end to the far end of the parallel string. This connection will prevent strap resistance from affecting state of charge since all cells are equal distance from the charge source. Then you want to discharge for a while. This will tend to remove energy from the cells with the highest state of charge first which will promote balance across the parallel string. You don't need to take it all the way down, just enough to level the playing field. Then bring them up to what you consider full. A resting voltage after a day of 3.38 to 3.40 would be a good place to shoot for. After removing the parallel connections you could wait a couple of hours and check the resting voltage to make sure they were all the same.

I did a bottom balance but my approach would work the same for a top balance. And it is pretty simple. I used a hobby charger that could do 30 amps and discharged to 2.7V with no taper. In your case you would simply use the charger to charge each cell to whatever you consider fully charged. As long as this is done to all the cells at the same temperature they will all be fully charged when you put them in series. I have not had to do any tweaking of my pack and it is still balanced to better than a thousandth of a volt over most of the usable state of charge. In my case they start to diverge only when approaching full charge on the weakest cell but that all goes away after it rests a few hours. The whole point of balancing is so that all the cells reach the same point at the same time. The charger will do this to a very high degree of relative accuracy. I used an iCharger 3010b but any of the better RC hobby chargers would also work.

I do have a top balanced 4S pack that I use for my AUX battery. I just float it at 13.6volts (3.40V per cell) and this too has worked well. They never quite get to fully charged when treated like this which is exactly what I was shooting for.

I hope all that makes sense.


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## TheSGC (Nov 15, 2007)

When I got my CALB 180AH from Evolve Electrics they were nearly perfect. I had at most a 0.004 spread and I didn't even top balance because it would have taken months. I do, however, have the Mini-BMS and I have my charging voltage set under the shunting voltage of the Mini-BMS.

Are you batteries all from the same batch? I have 45 perfectly serialized batteries and so far with 2,000 miles on them they are still darn close. I am due to check them again this week.


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## kennybobby (Aug 10, 2012)

dtbaker said:


> so..... my questions are:
> - how/why did some cells unbalance between parallel balance and install
> - is there any faster/better way to tweak the balance than cycle and bleed the high ones a little at a time?


 It seems to me that if you want to top balance a pack, it would first be necessary to know which cell is the weakest (i.e. has the lowest capacity) and then only charge the pack to the point at which the weak one is at it's full capacity. 
In your pack the weak ones are those that you keep bleeding off the higher voltage...why is that?--because they are already 100% full and heading up that steep (and rapidly changing) voltage slope of the charging curve. 
The stronger (higher capacity) cells act like a current sink--they will happily take more charging up until they too reach the point of being full. But that extra current flowing thru the weaker cells will overcharge them and results in a higher OCV and the variations between your parallel balance and installation. 
Don't know if it would help understanding, but here is my experience with a bottom balance pack: In a pack of 44 prismatic 100Ahr cells i measured the capacity of each cell by discharging them completely and monitoring the current and voltage over time. The capacities varied from 101 to 105 Ahr. Then did a parallel balance at the bottom using a 5 amp load to get current flowing and speed up the charge diffusion in the cells. After 3 days disconnected all cells were within 1 mvolt of each other. The OCV tended to slowly drift up over time, but all cells drifted up together. Connect in series and monitor current over time--charge them up putting 100 Ahr into the pack, resulting in full pack and no overcharged cells.


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## EVfun (Mar 14, 2010)

You are describing bottom balance Kenny. Top balance is making them all full at the same time so the smallest one would be empty first. Since there is no (measurable) self discharge there would be no ongoing need for bleeding charge off the smaller cells. Top balancing is used to flatten the voltage spread at the end of charge so if the charger timer fails or the charger output voltage drifts up none of the cells would exceed 4 volts. 

I've had a charger timer fail and the next morning none of the cells where up to 3.70 volts.


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## kennybobby (Aug 10, 2012)

EVfun said:


> ... Top balance is making them all full at the same time so the smallest one would be empty first.


So would it even possible to do this in parallel at all, or must it be done individually? In parallel the smallest cell would fill up soonest, then you would have to raise the input voltage sufficiently in order to continue to charge and fill the larger capacity cells. But in doing that it would tend to overvoltage the smaller cells. In either top or bottom balancing it would seem that a knowledge of each cell's capacity would be necessary in addition to a time-history of the charging current.


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## palmer_md (Jul 22, 2011)

I think you are missing one piece of the puzzle. If they are all at the same voltage, then they are all at the same SOC. If all cells are at 3.3000v then all the cells are at the same SOC. The difference is that as you discharge, the smaller capacity cells will lose voltage quicker and when the smallest one is at 0% SOC then you have to stop.


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## EVfun (Mar 14, 2010)

In parallel they will all have the same terminal voltage. You can do it with a 3.6 volt power supply, but they won't completely balance until the current is down to a fraction of an amp. By that point they are really overcharged, though people have done it this way without obvious damage. 

If you leave them in parallel after charging only to full (3.65 volts at a current equal to 5% of the total amp hours in parallel) they will continue to balance, but the voltage across jumpers will be a few tenths of a millivolt. The current is correspondingly very low so it would take a very long time to equalize that way, if it is even possible (the closer they get the slower the rate.)

Once the cells are very close to full you can take a single cell charger with auto shutoff and use it to charge each cell individually. When it turns off you move to the next cell. If the cells are very close to full this should take less than an hour per cell. Since there is no measurable self discharge for good LiFePO4 cells it should work fine. 

I bulk charged the series string by hand and then finish charged a few cells slower to come up. For the next 5 or 8 cycles I manually watched the end of charge and evened out the finish voltage by manually clamping a resistor across cell that where higher. After a few cycles of this all the cells came up together without my intervention. I still check every so often. When I walk into the garage and see the charger blinking (that indicates it is in the last 15 minutes of the charge) sometimes I grab a volt meter and quickly check each cell before the charger turns off. If I check the next morning, after the charger has turned off the night before, all the cells will be within a few thousandths of a volt. 

On the other hand, I've never had the resting voltage of any cell below 3 volts. I don't go down there.


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## GerhardRP (Nov 17, 2009)

DIYguy said:


> at what voltage did you parallel them and for how long?


Hey, Dan, Can you answer that question?
Seems to me, the parallel balancing should be done by taking the voltage up to just where the end of charge knee starts. Than drain, say, 5% of the capacity an let them sit parallel connected for a few days.


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## corbin (Apr 6, 2010)

palmer_md said:


> I think you are missing one piece of the puzzle. If they are all at the same voltage, then they are all at the same SOC. If all cells are at 3.3000v then all the cells are at the same SOC. The difference is that as you discharge, the smaller capacity cells will lose voltage quicker and when the smallest one is at 0% SOC then you have to stop.


What? This is not at all correct. If all the cells are at 3.300v, they may not all be at the same SOC. Voltage is a bad indication of state of charge. However, it is a good indication of when the cell is fully discharged or fully charged.

corbin


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## Joey (Oct 12, 2007)

I have 60 CALB CA 180 cells. I paralleled them with 12 AWG wire and crimp ring terminals. I charged at 16 amps to 3.42 Volts. I used the JLD 5740 and a relay to terminate charge. It took 20 days to charge and another 30 days in parallel to settle. Cells were within 4 mV when they arrived, 40 mV under charge, 0.3 mV 1 day after charge termination (still connected in parallel) and 3 mV several days after disconnecting the parrallel connections. There was no correlation in relative cell voltage (that I could see) before and after the charge.

I first tried the method of connecting the positiove lead and one end of the string and the negative lead at the other end. Two reasons I didn't continue with it: There was 50 mV of cell to cell variation (and climbing) after a couple days with cells closer to either lead reading higher voltage, and I had to set the power supply to 15 V in order to push 16 amps of current. No cell was reading higher than 3.38 volts at the time, but if my voltage controller failed to terminate charge properly, I didn't want to risk having my cells go that high.

I ended up stringing 7 runs from the charger to various points spaced across the pack. Then the charger was pushing 16 amps with about 0.2 volts higher on the charger than the cells. The cell variation under charge stabilized until the cells got over 3.38 V, when the voltage would climber faster. That's why I don't think the knee in the curve is a voltage knee, but rather a state of charge knee, that depends on current and and voltage. This is why Calb specifies a termination voltage and current rate (3.6 volts and C/20).

I had plenty of time to do the charge and balance process because my project is still under construction. To speed up the process (I mean if the 1,800 amp, 3 volt charger is out for repair) is to charge in series to get the cells closer to full, then switch to parallel for the last portion of the charge.


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## palmer_md (Jul 22, 2011)

it is not a useful indicator when the battery is in use, but it is the only reliable indicator there is for SOC, and is very useful after the battery has had a chance to rest for 24 hours.


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## dtbaker (Jan 5, 2008)

kennybobby said:


> It seems to me that if you want to top balance a pack, it would first be necessary to know which cell is the weakest


....uuummmm, no.

in a top balanced pack, especially one without BMS, this initial balancing process is the key not the capacity of the cells. The whole trick is to get a nice close top balance and just let the charger do its default job cutting off at a specific top series voltage after that.

my question is really to others who have done an initial top balance in parallel first, installed into series, and whether or not they noticed significant variation between the cells to the point they decided to do an initial tweak before calling it done.

My first charge was not *bad* in that no cells went dangerously high before charger hits its EOC and shut down, but there was enough spread where I have decided to tweak, knock down the high cells, and get everything a little closer balanced before buttoning the lids on...


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## dtbaker (Jan 5, 2008)

DIYguy said:


> at what voltage did you parallel them and for how long?


I had them in parallel at 3.8v in one big string with pos and neg coming into string from opposite ends for almost a month w a Mastech power supply.... actually I had it cranked up for the first couple weeks to get the most out of the mastek I could, and then cut back to 3.8v untill current dropped to about 2 amps, then cut back to 3.6 and held for a couple days and the amps dropped to less than 1, indicating they were full and just dissapating surface charge. Then unhooked power and let them sit in parallel for a couple more days thinking they would complete voltage balancing as they fell to rest.

More I think about it, maybe thats where the imbalance crept in, and I should have disconnected them from parallel as soon as I pulled the parallel charge.

my target series end of charge is 169v, which is at an average of 3.52vpc if all cells are perfectly in top balance.

I would LIKE to see an initial balance with all cells plus/minus .02v of that value at end of charge....(3.50-3.54 at end of charge.)


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## Joey (Oct 12, 2007)

dtbaker said:


> I had them in parallel at 3.8v in one big string with pos and neg coming into string from opposite ends for almost a month w a Mastech power supply.... actually I had it cranked up for the first couple weeks to get the most out of the mastek I could, and then cut back to 3.8v untill current dropped to about 2 amps, then cut back to 3.6 and held for a couple days. Then unhooked power and let them sit in parallel for a couple more days.
> 
> my target series end of charge is 169v, which is at an average of 3.52vpc if all cells are perfectly in top balance.
> 
> I would LIKE to see an initial balance with all cells plus/minus .02v of that value at end of charge....(3.50-3.54 at end of charge.)


I thought a resting (10 days) voltage of 3.38 was considered fully charged.


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## dtbaker (Jan 5, 2008)

Joey said:


> I thought a resting (10 days) voltage of 3.38 was considered fully charged.


at resting voltage, you have NO idea what the state of charge is. if you are top-balancing, the goal is to have all the cells hit the target voltage a little up the knee as close to the same as possible. It HAS to be above 3.4 so the charger can 'see' the voltage rise and switch to CV. A good conservative target is 3.5 vpc. You might get a little more total capacity with a target for 3.6 or 3.7 vpc, but run increased risk that one might shoot too high if it drifts out of balance over time.


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## EVfun (Mar 14, 2010)

If you have resting voltage with a day or more of rest you have a good idea of the state of charge. If they have been resting with absolutely no load for days and you can accurately measure temperature and voltage to the ten-thousandths of a volt you have a really good idea of the state of charge.


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## Ziggythewiz (May 16, 2010)

Joey said:


> I thought a resting (10 days) voltage of 3.38 was considered fully charged.


That's what I've heard...which is why I don't bother to charge them over that.

Some of mine drifted a bit after switching to series, ~.1V. Nothing to worry about, I don't take them high enough for it to matter.


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## dougingraham (Jul 26, 2011)

corbin said:


> What? This is not at all correct. If all the cells are at 3.300v, they may not all be at the same SOC. Voltage is a bad indication of state of charge. However, it is a good indication of when the cell is fully discharged or fully charged.
> 
> corbin


If the cells are at the same temperature and have rested for several hours then the resting voltage is a reasonable indicator of state of charge.

State of charge spreadsheet with scatter plot

The chart is not tempertature compensated and was generated from testing I did with my GBS 100AH batteries using a Fluke 489 meter that can display to a resolution of 0.0001 volts. One of these days I will fill in the gaps in the chart but there is plenty there to see the trend. It is good to better than a couple of percent which is similar to about what you can get by keeping track of SOC with a hall sensor. The down side of using voltage to determine the state of charge is you have to wait too long for it to be of much value. Adding in a temperature compensation factor will probably clean up the chart a bit. The temps in the spreadsheet are the air temp in my garage at the time of the test. It would have been better to measure the cell temp. The cell# is the last three digits of the cell serial number. The serial numbers have a date code as part of the number so I know what day they were serial numbered at least. These all have the same date code. The randomness of the state of charge came from the mechanism I used to recharge to an approximate half charge. I used a Vicor Megapac with a 5V module trimmed down to 3.6V with a booster module paralleled. This would do a little over 80 amps so I charged them for around 40 minutes to get them back to that middle state of charge. But it was just clock watching so some were short and some were long.

This chart applies only to GBS cell chemistry but other LiFePo4 types should be similar.


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## tomofreno (Mar 3, 2009)

Hi Dan, At 3.52 vpc you are up on the exponential part of the curve. Small differences in cell voltage could be due just to differences in cell ir at that point of charge. I think you are doing well if you see less than 0.04V spread there. I see spread of this order regularly with shunt balancing, and as others have said, it moves around, difference in spread each charge and different cells come and go from the group of highest cells. Heck, the shunts have a turn-on voltage spread of that order. And the further you drive the cells up that curve, the steeper the slope and the more spread you will see. If my cell voltages are all within a range of 3.45 to 3.52V near end of charge I'm quite happy.

As I've said a number of times, I wouldn't pay much attention to this. The differences in cell SOC are miniscule if any. I expect that cell V is very sensitive to number of intercalation sites remaining at that point and how difficult they are to access, and that changes from charge to charge, cell to cell. I think cell voltage differences are as much or more due to these kinds of effects as they are slight differences in SoC when on this part of the charge curve. If your cells were at 3.44V or so and you saw that spread then I would say you had maybe on the order of 1 to 2 Ah of imbalance, but not up on that part of the curve.

I disagree with a sweeping statement like cell voltage is not an indication of cell SOC when cell voltages are on the "flat" part of the curve. It clearly is not completely flat, and cell rest voltages do give an indication of SoC there. It is just not very precise. I would say voltage readings to the nearest mV indicate SoC to within something on the order of +/- a few Ah or so. So yeah, you can't determine if all cells are at exactly the same SoC, but you can see if they are in the same ballpark.


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## EVfun (Mar 14, 2010)

I have a top balanced pack without a BMS. I have an end of charge scatter of 0.09 volts, with a range of 3.47 to 3.56 volts. Without a BMS that scatter is consistent, I can tell you which cells will be near the top of the range and which will be near the bottom. 

Two years ago I ran with a BMS and the scatter moved around. Cells which tagged a shunt earliest on one cycle rarely did so the next cycle. The scatter range was smaller, 3.61 to 3.64 volts, by force of the reg function. The difference in SOC between a cells in series charging at 3.50 volts and another at 3.60 volts is less than the SOC difference between cells resting at 3.32 volts and at 3.31 volts.


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