# Shunt Balancer



## bruceme (Dec 10, 2008)

In researching "old-school" balancers used in the early days of Lead-Acid, I return to the zener shunt.

Here is my take on a LiFePO4 shunt:









The zener will have to be tightly matched (tested and returned). It would be much easier to match if I could use pairs, but I can't find power zeners at 1.8v. 3.3 is the smallest in this series.

I am building a cost-scaling system for very high voltage packs. The system uses fixed balancers and Lee Hart's "Batt Bridge".

Watch this good YouTube on batt bridge

Any thoughts or critique?


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## Qer (May 7, 2008)

bruceme said:


> Here is my take on a LiFePO4 shunt:


Well, after the transistor blow up (the diode between base and emitter will short the zener) it will probably work somewhat at least. However, you'll have a pack that will always discharge slowly due to the nature of zeners (the knee is kinda soft so they will pretty much always draw a bit of a current), but if you live in a cold climate maybe that's not only bad since it will keep the pack warm in the winters...


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## bruceme (Dec 10, 2008)

Qer said:


> Well, after the transistor blow up (the diode between base and emitter will short the zener) it will probably work somewhat at least.


What's the right circuit?



Qer said:


> However, you'll have a pack that will always discharge slowly due to the nature of zeners (the knee is kinda soft so they will pretty much always draw a bit of a current), but if you live in a cold climate maybe that's not only bad since it will keep the pack warm in the winters...


The specs for this closed zener is 500ohms;

I=V/R=3.4V/500R=0.0068A

40Ah cells, 

40/0.0068 = 5882 hours or 214 days, 

Even I can remember to charge in that time frame 

As for heat... I^2*R * 80 (cells) = 1.8watts, hardly a space heater. But good things to consider.

Thanks,

-Bruce


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## bruceme (Dec 10, 2008)

bruceme said:


> What's the right circuit?
> 
> 
> 
> ...


Ok, re-reading your post and viewing the graph I saw what you where trying to say. It's not "binary", the further you are from the knee the lower the current, and it's not by a little, it's by a lot. So yeah, I could calculate this, but it's more than the spec numbers.


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## Qer (May 7, 2008)

The right schematics.... Well. You probably have to do it a lot more complicated with some kind of comparator and a voltage reference etc. That's when things start to get so complicated I kinda prefer to put it all in a micro controller instead. I'm the software nut, remember?


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## EVfun (Mar 14, 2010)

A very simple idea for a shunt regulator if minimal shunt current is required (between 100ma and 140ma.) Based on my experience with and without regulators I suspect that a very tiny nudge each cycle would be more than enough provided you initially top balance the cells. 

View in fixed width font:

```
D2 // 
+ ---------->|--
    |     |    |
    |     >    >
 R2 >  R3 > R4 >
    >     >    >
    >     |    |
    |     |-----
    |    --     
    |----/\     
    >   D1|     
 R1 >     |     
    >     |     
    |     |     
- ---------
```
R1, 2700
R2, 1100
R3, 33
R4, 120
D1, LM431B
D2, red LED


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## bruceme (Dec 10, 2008)

EVfun said:


> A very simple idea for a shunt regulator if minimal shunt current is required (between 100ma and 140ma.) Based on my experience with and without regulators I suspect that a very tiny nudge each cycle would be more than enough provided you initially top balance the cells.
> 
> View in fixed width font:
> 
> ...


Ok, I get it, voltage splitter driven transistor to a shunt. makes sense and it's simple enough.


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## bruceme (Dec 10, 2008)

EVfun said:


> A very simple idea for a shunt regulator if minimal shunt current is required (between 100ma and 140ma.)


Two issues:

- The circuit described by EVfun is likely for 12v...[Edit: I was wrong! I swapped R1/R2 by accident] if you're reading this, you need to adjust the resistors for your battery cut offs. I will post an update later, but My R1 is 1.2k, R2 is 2.5k, producing the required 2.5v for reference trigger at 3.65 cell volts. If you have other chemistry, you will need to adjust.

I used this site to calculate the voltage splitter that drives the trigger.

- 140ma is a very small amount of shunting. LiFePO4's don't need much, but this is tiny. Many other examples online, use the LM431 (aka TL431) to drive a mosfet/resistor and drive much more current. Not saying either is right, just more research is needed. I'm always in favor of simple!

Here is a working simulation of the circuit described above for LiFePO4.





Thanks,

-Bruce


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## Qer (May 7, 2008)

That is a better circuit since you use a voltage reference rather than resistor dividers that has a percentage error and a transistor that will have a Vbc between, ehm, some and some more depending on temperature, current, the value of the Wall-Mart stocks and some crazy chaos butterfly in China... 

Biggest problem as I see it is that you have no overheat protection. Shunting means wasting voltage to heat and, well, things can get pretty hot if something goes wrong. It seems like a simple problem, shunting some current when the cells start to get fully charged, but it isn't. It's a can of worm...


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## EVfun (Mar 14, 2010)

bruceme said:


> Two issues:
> 
> - The circuit described by EVfun is likely for 12v... if you're reading this, you need to adjust the resistors for your battery cut offs. I will post an update later, but My R1 is 1.2k, R2 is 2.5k, producing the required 2.5v for reference trigger at 3.65 cell volts. If you have other chemistry, you will need to adjust.
> 
> ...


My circuit was drawn for 3.5 volt shunting. Notice that R1 is under R2 in the schematic. Specifically, if 1% resistors are used and an LM431C voltage reference is used then the range should be about 3.48 volts to 3.55 volts turn on. Only a few LM431 types are rated for 150 milliamps, most only for 100ma. In that case the R3 should be 56 ohms and R4 150 ohms to make the device survive up to 4.5 volts (after that you have ruined the cell so who cares about the reg.)


```
D2 // 
+ ---------->|--
    |     |    |
    |     >    >
 R2 >  R3 > R4 >
    >     >    >
    >     |    |
    |     |-----
    |    --     
    |----/\     
    >   D1|     
 R1 >     |     
    >     |     
    |     |     
- ---------
```
Of course the gate of a 2m3907 transistor could be connected between R3 and D1 with about a 240 ohm gate resistor. A 10 ohm load resistor and the LED with R4 could be connected between transistor and ground. Then, for hysteresis, a high value resistor could be connected, one end between transistor and the load with the other end connected to the reference pin of D1. I had a test rig similar to that working with a 2n3906 because that is what I had (very similar behavior, much lower load rating.) This resulted in a reg that sharply turned on, and then off a couple hundredths of a volt lower (helps keep the heat in the load resistor and not the reference or transistor.)

```
+ -------------------  
    |     |         |  
    >     >         |  
 R1 >     > R4      |  
    >     > R5      |  
    |     |--^^^--|/ T1
    | R3  |       |\   
    |     |         |  
    |-^^^-)------------
    |     |     |     |
    |    --     >R6   |
    |----/\     >     |
    >   D1|     >     >
 R2 >     |     |  R7 >
    >     | D2 \/     >
    |     |    --     |
    |     |     |     |
- ---------------------

R1 1500 ohms
R2 3600 ohms
R3 ? kohms
R4 240 ohms
R5 240 ohms
R6 120 ohms
R7 10 ohms 5 watt
T1 2N2907 PNP transistor
D1 LM431C Voltage reference
D2 red LED
```
For now I haven't been running with any regulators and I have only had to do a little rebalancing one time. I kinda put that work on the back burner but may revisit it.


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## bruceme (Dec 10, 2008)

EVfun said:


> My circuit was drawn for 3.5 volt shunting. Notice that R1 is under R2 in the schematic.


Yes! I am not worthy. I swapped them. I want to simulate your other circuit next... I like! Now does anyone sell this?


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## drgrieve (Apr 14, 2011)

Devils advocate question:

If charging voltage != SOC, why do you want to shunt a random cell at a random SOC?

It seems like you are trying to build a device to unbalance your pack.

What's the point of that?


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## EVfun (Mar 14, 2010)

Charging voltage is a good indicator of the SOC with LiFePO4 cells provided you consider the cell temperature and the charging current. Since the charging current is the same within a series string, and hopefully the temperature of the cells is close too, the voltage will make a fine indicator of the SOC. I don't know who told you otherwise -- voltage and current is how the manufacturers define a full charge (typically around 3.65v at 0.05C.)

Voltage is a poor indicator of SOC between around 20% and 90% SOC only because such small changes in voltage equal large changes in SOC. Using voltage to determine SOC only works well when the cell is at <20% SOC or >90% SOC.


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## drgrieve (Apr 14, 2011)

Well here is a graph by John Hardy. He was running a 550 cycle test on headways cells to measure cell drift. (No cell drift BTW)

This is a graph from cycle 500.










As you can plainly see, the charging voltage at high SOC is not relative to the pack.

I've seen others support this finding with their own data.

Perhaps you can supply some data which shows testing that proves that charging voltage = SOC?

Otherwise I remain convinced that only static voltage is an indicator of SOC, and charging voltage is not accurate to be useful.

Thus shunt balancing is unbalancing.


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## EVfun (Mar 14, 2010)

Cells 1 and 3 are suspect of being slightly high in internal resistance. Without the discharge curves there is nothing there to indicate that they really all at the same state of charge. Perhaps that difference is slight drift, where they initially balanced in some defined way? What was the method used, ending voltage at some specified current? Headway recommends CC/CV charging and provides graphs. Headway spec sheet from Manzanita Micro

I've actually used a pack of LiFePO4 cells with a BMS on it, and the system was only being used for the 1/2 amp shunt regulation function and low voltage alarm (no control over the charger or controller.) I can assure you, from experience, that the pack worked just fine with no indication that voltage at same current left them at different states of charge. I managed 36 miles on a 32 cell pack of 60 amp hour cells. I couldn't really drive the car anymore because the pack would sag to 80 volts under light load and the controller would limit current. Still, all the cells where over 3.0 volt resting. I'm not really seeing any difference now that I am running without a shunt reg except that the cells are slowly drifting apart at the end of charge. It used to be that they all hit 3.6 volts within 90 seconds. This ending drift cannot be allowed to go past 4.3 volts because the electrolyte will be damaged and capacity lost. 

The cells in that graph are all between 3.58 - 3.77 volts at just under 3.5 amps. That voltage difference at 6 amps represents a fraction of an amp hour on my 60 amp hour pack. Since 60 amp hour cells vary by more than that in capacity it can't really be considered imbalanced. Charging with shunt regs and charging without hasn't changed the what cell is the smallest when testing capacity.


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## EVfun (Mar 14, 2010)

bruceme said:


> Yes! I am not worthy. I swapped them. I want to simulate your other circuit next... I like! Now does anyone sell this?


Funny Bruce  If you do simulate it I would be interested in seeing your results. I'm guessing that R3 would end up being around 150k ohms. I think R1, R2, and R3 could be replaced with higher value parts since the load on the LM431 would be slight and the hysteresis can cover up slight impedance shifting in the reference input. 

I don't know of any shut regs using the same parts, but the old lead acid Rudman regs used some method of creating hysteresis. I came up with that method of introducing hysteresis when I was trying to make some Volt Blocher (remember those) regs not heat up the power transistor so badly.


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## PStechPaul (May 1, 2012)

Here is a very simple shunt charge limiter, but it depends on the turn-on voltage of the MOSFET and I don't know how stable that is with temperature and variation among devices. R4 and R5 attempt to simulate the internal resistance of the charger and the cell.

The MOSFET could be replaced with a PIC10F320 which has an A/D converter and PWM and on-board voltage reference, for about 50 cents apiece.

In either case the circuit draws only microamps until it starts limiting. And the PIC draws only 25 uA, and much less (20 nA) in standby.


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## drgrieve (Apr 14, 2011)

EVfun said:


> Without the discharge curves there is nothing there to indicate that they really all at the same state of charge. [/URL]


Ummm the lines cross over when charging. How does that relate to SOC?

I just don't understand why you want to shunt the cells when charging - it makes no logical sense.

What are you trying to achieve?


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## bruceme (Dec 10, 2008)

Ok, so I built the more complex circuit suggested. It works if you set R3 above about 10k-ohm.



Here's my own interpretation of a simplified gated shunt. The LED used is large 3v with a large internal resistance, it would cost more. The gate would switch between 1A at 3.4v to 1.4A at 4.2v.



I know it's much simpler, but it appears to do the job.


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## bruceme (Dec 10, 2008)

EVfun said:


> If you do simulate it I would be interested in seeing your results.


Click the first image link in my last post and press F5. 




EVfun said:


> I'm guessing that R3 would end up being around 150k ohms.


I guessed at R3 till it started working. Higher values are probably better.



EVfun said:


> I don't know of any shut regs using the same parts, but the old lead acid Rudman regs used some method of creating hysteresis. I came up with that method of introducing hysteresis when I was trying to make some Volt Blocher (remember those) regs not heat up the power transistor so badly.


I have a box of old Rudman v1's in my garage... that's what I want to duplicate. The one modification, I don't want it adjustable, they should be preset and precise. 

My current project is a 10kwh Prius PHEV. I need to balance 78 cells. That's $2,000 in miniBMS [BEM Update: it's $768, not 2k]. I can build the shunt circuit x78 for $50 in parts. The trick is the boards. I like the gang boards maybe 4 3"x5" boards with arrays of 20 balancers on each. I built one and got it quoted at $334  ! Any thoughts? 

[Update] - I got the price down to $138 for a 32-circuit array board if I removed the silk screening and solder masks. Simple circuit over and over, I can do without.


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## dimitri (May 16, 2008)

bruceme said:


> I need to balance 78 cells. That's $2,000 in miniBMS.


78 x $11.99 = $935.22 , not $2000 and you also get a feedback signaling circuit, not just plain balancing.

Not trying to bash your fun project, just correcting your facts


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## bruceme (Dec 10, 2008)

dimitri said:


> 78 x $11.99 = $935.22 , not $2000 and you also get a feedback signaling circuit, not just plain balancing.
> 
> Not trying to bash your fun project, just correcting your facts


Hey you're right and 4x16-boards brings that down to $768. That's not bad at all, and I know it works. Cause I've been driving it for year. miniBMS was always my backup plan. Btw, the mix up was cause I was thinking of a friend's 380v system for a Twike. They run very high voltage, very small current. Different kind of balancing problem. My system would be more cost effective.

Thanks,

-Bruce


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## Siwastaja (Aug 1, 2012)

My opinion is;

Get a small MCU for every cell module and carefully design a digital BMS. There is no price difference compared to proper voltage reference. A simply zener diode system will not work as it will drain the cells quickly.

If you get an MCU and can write code for it, you can add cell-level LVC which is, IMO, pretty important, maybe even more important than the shunt balancer feature.

Some small MCUs (e.g. ATTiny25/45) include integrated temperature sensors, too. You can add this kind of extra features basically for free.

You can make the communication (cell-LVC and cell-HVC) by using optocouplers, for example.

I would just skip "very simple shunt-only non-communicating" BMS altogether. People seem to have surprisingly good experience without any cell-level BMS at all. Otherwise, do a really good design. Include cell-level LVC and HVC, communication with motor controller and charger, and pay special attention to reliability and safety.

It's not difficult. Include a fuse (directly soldered to PCB) that will blow if the shunting gets stuck. Test that it blows. Include some kind of heartbeat signal that notifies you and shuts of the charger (and possibly drive) if any cell module goes offline. Etc. This kind of simple stuff.


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## bruceme (Dec 10, 2008)

Siwastaja said:


> My opinion is; Get a small MCU for every cell module and carefully design a digital BMS


Sounds cool, have fun with that.


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## dougingraham (Jul 26, 2011)

Siwastaja said:


> Get a small MCU for every cell module and carefully design a digital BMS. There is no price difference compared to proper voltage reference. A simply zener diode system will not work as it will drain the cells quickly.
> 
> If you get an MCU and can write code for it, you can add cell-level LVC which is, IMO, pretty important, maybe even more important than the shunt balancer feature.
> 
> ...


You will still want a precision reference on each MCU on each cell.

You don't want to place any load on the cell being measured. Buffer with an op-amp and power the MCU with a DC-DC converter to isolate each board from the 12V if necessary. Microchip has devices with 12 bit A/D converters and built in op-amps. I dont think 10 bits is really enough because the last bit is mostly noise anyway. With a 12 bit you get 11 usable without resorting to tricks. With a 4.096V reference you could read to 1 millivolt plus or minus a millivolt and with a tenth percent reference and tenth percent resistors on the op-amp you should have really good relative accuracy from unit to unit. A few millivolts at most. The 10bit A/D converters are good to 9 bits plus or minus a bit which would let you see a 4.096 volt in 4 millivolt increments plus or minus 4 millivolts. This would be good enough for shut balancing or giving a charger a hint as to when to turn off or catching a low cell to drop the controller into limp or disable completely. The 12 bit would be needed if you wanted to log the data with the idea of watching resting state of charge. And it is just barely good enough for that.

To make this work you would need to run three wires between boards. Two of the wires would be 12V vehicle power and return (to power the board via an isolated DC-DC converter 12->5 volts) and the other wire would be a serial data stream (opto isolated). The board would then connect to the positive and negative terminal of each cell. Since you have gone to so much trouble you could run a 4th wire to the adjacent board to measure the voltage drop across the cell to cell interconnect. A second op-amp would be used to amplify this signal to obtain a measurable number. This would let you know if there was a problem developing such as a loose or corroded connection. A temp sensor on one of the battery terminals would be useful as well.

I've been thinking on and off about BMS's since I did the software for the first shunt balancer for the RC hobby market (around 2003 I believe). I came to the conclusion after we started selling the product that you only needed it during the initial charge and then if you abused your batteries. Otherwise the batteries self balance a tiny amount on each charge cycle. It is a complete myth that LiPo batteries unbalance on their own. This self balance seems to occur during the rapid rise near the end of charge. My best guess is that the charge efficiency is slightly less when the cell is in the rapid rise at the end. So the one that gets there first gets slightly less charge than its neighbors. This may not happen to a measurable extent with large cells but on the 1AH cells I tested it was clear at 30 cycles (2C discharge and 1C charge) that the cells were converging. It could be completely different with LiFePo4 chemistry but I wouldn't bet on it.


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## Siwastaja (Aug 1, 2012)

Doug,

There is no problem running the MCU from the cell voltage. Any design will use some amount of power from the cell, so there are no absolutes here. You just need to calculate how much. It can be a problem in a small laptop battery with 1-2 Ah cells.

Many small MCUs can run with a slow internal oscillator and use around 0.5 mA even in the active mode. You can sleep the processor when not driving/charging to practically eliminate the current, but it's not usually needed.

Even the active-mode current is usually low enough for a car-sized cell so that the module will not drain the cells even if they stay untouched for _several years_. Apparently _some_ BMS modules on the market use in the range of 3-5 mA which is quite a lot in my opinion and _can_ destroy cells is the car is left away without charging it for a long time. Even some production EVs are said to have this problem. OTOH, there are claims that production EV's would not use cell-level BMS. Either one is a lie.

So, the simplest passive digital BMS is IMO just a small MCU with ADC, a small FET to drive a resistor, the resistor, and an optoisolated communication link, the communication being the hardest part. And add a fuse.

An active BMS could consist of two MCU's with isolated balancing DC bus side and a cell-side powered from the cell and a small bi-directional isolating transformer in-between.


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## dougingraham (Jul 26, 2011)

Siwastaja said:


> Many small MCUs can run with a slow internal oscillator and use around 0.5 mA even in the active mode. You can sleep the processor when not driving/charging to practically eliminate the current, but it's not usually needed.


You havent looked at the Microchip parts recently. They have extremely low power levels. 30 microamps per megahertz while not sleeping. An imbalance of only 12 microamps in a system that will have a 10 plus year life is an amp hour too much. And not isolating with an op-amp and not powering the mcu's from the cell is required to do this right. If you don't power it separately from the cell then you have created the need for cell level balancing with your own circuit.


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## PStechPaul (May 1, 2012)

I don't have experience with cell balancing, but there seems to be some confusion about the need for extremely low current for the balancer, and extremely accurate measurement. ISTM that LiPo and LiFePO4 cells, or any cells for that matter, are made with reasonable manufacturing tolerances which will create differences in their capacity. I contend that such normal variations will be greater than 1%, perhaps as much as 5%-10%, and certainly not in the realm of 0.1% or better that would be a reason for 12 bit A/D converters and current sensing in the nanoamp range. 

Apparently balancing is needed more for Lithium chemistry than for NiMH and Lead Acid, because excess charge is simply converted to heat and after a while all cells in a series charging circuit will reach their maximum SOC. But Lithium cells do not have such self-leveling mechanisms and can be damaged by overcharging. 

So, given my assertion that the cells in a pack will differ in capacity by, say, 1%, a small but equal load on each cell will eventually discharge the weaker cells more than the stronger ones. If the SOC of individual cells can be determined by its open circuit voltage, and the voltages of all cells start equal, it might be possible to maintain equal SOC during storage by taking measurements at accurate time intervals and adjusting the current drain on each so as to maintain equal voltages at each time tick, based on a maximum self-discharge rate.

If the cell monitors can communicate with each other, the SOC can be measured and adjusted and balanced even better. This would probably require an additional "master" battery pack monitor. But it could measure the overall pack voltage and send that value to all of the individual cell monitor/balancer elements. There are various ways to do this, but it's not difficult, complex, or expensive, and the cells only need to "listen". The control signal could transmit the total number of cells and the total pack voltage, and if any cell is higher than the average per-cell voltage, the load element would be switched on. This should work for charging and discharging.

Of course, commercially available BMS ICs are available and rather inexpensive, and are probably the best option. But I think inexpensive individual cell monitor/balancers would work fine for charging and would allow the entire pack to be charged to full SOC for each cell, and they could also monitor discharge and flash an LED as a warning if discharged below a dangerous level. The LED could be replaced with an optoisolator that could trigger an alarm or shut down the controller as desired.


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## GizmoEV (Nov 28, 2009)

PStechPaul said:


> Apparently balancing is needed more for Lithium chemistry than for NiMH and Lead Acid, because excess charge is simply converted to heat and after a while all cells in a series charging circuit will reach their maximum SOC. But Lithium cells do not have such self-leveling mechanisms and can be damaged by overcharging.


My experience with LiFePO4 in my rig and that of others supports the exact opposite of your assertion. While my cells are only a little over 2.5 years old and still a long way from their end of life so I can't say for sure about 5-10 years from now, I'm not seeing any drift between cells. With lead acid and NiMH some form of balancing needs to be done, appropriate overcharging for the chemistry is one way. With LiFePO4 they just don't drift so why bother balancing after an initial balance?


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## drgrieve (Apr 14, 2011)

Yeah there is a lot of theory bandied around with absolutely no proof.

The only scientific tests I've seen assert the following for LiFeP04.

1. No measurable cell self discharge over a period of several years (4 years and counting from on report).
2. Cell drift on a active pack is non-existent or not measurable within noise level when cell static voltage is checked from the point they where balanced at.
3. Charging voltage is not an accurate indication of SOC.

Shunt balancing is for the uniformed and / or the stupid.

You think it would so easy to do a test for any of the above - self discharge, cell drift, and charging voltage = SOC. But there is nothing out there I can find on this side of the fence. Just made up crap which sounds great but no proof, no tests, nadda nothing.


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## EVfun (Mar 14, 2010)

Keep typing yourself smart. You should have some personal experience before you start telling others how to manage LiFePO4 battery packs.



drgrieve said:


> Yeah there is a lot of theory bandied around with absolutely no proof.
> 
> The only scientific tests I've seen assert the following for LiFeP04.
> 
> ...


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## drgrieve (Apr 14, 2011)

True.

But I await any proof that contradicts the evidence.

I had my ass handed to me regurgitating the so called conventional wisdom. Perhaps you can supply some? I'd like to hand some back but I don't think you can help.

It's quite crazy to have so many people at divide. It's not like a AC vs DC debate only one can be right.

I go with those that can actually supply tests backing up their talk.

What can you provide but talk?


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## EVfun (Mar 14, 2010)

drgrieve said:


> True.
> 
> But I await any proof that contradicts the evidence.
> 
> ...


Real hands on experience with an on-road EV LiFePO4 battery pack. There is quite a bit of that being shared by a number of people in this thread and other threads here. Just because the information doesn't fit on a graph or in a paragraph doesn't make it any less factual. In fact, I think real world data is more relevant than lab tests because in the real world the current, temperature, depth of discharge, and time are not tightly controlled to be the same cycle after cycle. 

I suspect it will be like the AC vs. DC debate because the answer chosen will depend on the needed life, acceptable failure modes, maintenance expectations, and other factors.


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## drgrieve (Apr 14, 2011)

Ok let say a person has a goal of top balancing their pack

They A

Spend 2 days top balancing their pack to 3.350V (this is only an example) static per cell to a 1/1000 of a volt. The pack is absolutely top balanced (@95% SOC). They now never have to touch the pack again (unless they want to add more cells) for the life of the car (but reports are only 2 to 3 years old so this is not confirmed but so far so good).

Or B
They put together the pack as from the factory or semi top balance using charging voltage. Place shunts on the pack in a vain attempt to top the balance the pack. Never achieve a true top balance. The BMS places an unbalanced load on the pack, causing imbalance over time and hopefully doesn't burn down the car. The shunting day after day causing unnecessary cycle life wear at the worst time. And the pay money for it instead of more cells.

And you think there is an impasse on the decision?

It's quite clear that charging voltage is not accurately equal to SOC so again I say why do you want to randomly shunt a cell at a random SOC?

What's the point of that?


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## PStechPaul (May 1, 2012)

Here is what seems like a good article about cell balancing:
http://www.mpoweruk.com/balancing.htm

Some points to take home are:
(1) all cells are not created equal, due to mfg tolerances
(2) balancing can be enhanced by matching cells in a pack
(3) microcycles as encountered in EVs tend to accentuate imbalance over time
(4) multiple cells in parallel/series arrangement have some self-balancing effect
(5) differences in temperature and pressure and other factors can cause imbalance to occur even in perfectly matched cells

There are various methods offered for balancing and protection. I would think there must be some consistent way to determine ideal SOC for a battery of a given chemistry, most likely by accurate measurement of voltage and temperature. So any system that allows charging only up to that parameter should produce an ideal maximum charge without harm. 

Proper lab tests should be able to determine this, but must account for normal variations as experienced in the field. I find statements like "no self-discharge was detected" and "state of charge cannot be determined by cell voltage" to be highly suspect. 

There are also other tests that can be conducted by individual cell BMS devices, such as running a brief discharge pulse and measuring the voltage drop, and thus the internal impedance, which should be a good indicator of the capacity and condition of the cell. I think there is too much magic and rumor and inaccurate testing and observation going on, but I would welcome some very well documented lab results as well as field experience.

I have some LiFePO4 cells now and I plan to do some testing and perhaps design and build a simple individual cell BMS.


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## EVfun (Mar 14, 2010)

drgrieve said:


> Ok let say a person has a goal of top balancing their pack
> 
> They A
> 
> ...


A and B are versions of the same thing. In both cases you are talking about balancing the cells based on voltage! Your version of events in "B" indicates to me that you need to study shunt regulation more before trying to come across as knowledgeable on the subject. 

Shunting day after day doesn't cause any unnecessary cycle life wear. Shunting represents only a slight decrease in charging current (and so peak cell voltage) during the last part of a charge. I was balancing the pack in my car with 1/2 amp shunts that turned on at 3.62 to 3.63 volts. My finishing charge current was 3 to 4 amps (charger uses time at target voltage for shut off.)

This year I took the system off and added 6 more cells. I want to see if the shunting was necessary. I lowered the finish charge to 3.50 volts (133.0 volts for 38 cells) and hold for 13 minutes (about 6 amps.) This has not signicantly effected the capacity order in the original 32 cells -- the largest and the smallest cells are still the same. I have seen some drift in the added cells, even though the cells came from the same batch as my original 32. I have seen some increase in scatter at finish in the original 32 cells, but it isn't significant at this time.

I bought 42, 60 amp hour Thunder Sky LiFeYPO4 cells in February 2010 and used 40 for 6 months in one EV. I moved 32 to another car with shunt regs and a low cell voltage buzzer and used them for a year. I removed the shunts, added 6 cells, and top balanced the 38 cells to the point where charging would end with all of them within 0.06 volts. I recently noticed the added 6 creeping up in voltage to much at the end of charge, and knocked them all back 0.2 amp hours so the highest one wouldn't go right to 4 volts and all of them better follow the rest of the pack. The end of charge scatter right now is 0.08 volts with none of those 6 defining the top or bottom of that range.

This may not fit a theory or a graph, but this is reality. I do not over-charge or over-discharge these cells but I am not kind to them. I regularly demand 6C peak amps and often hold 4C for 20 seconds while launching up a local uphill freeway onramp. I drive it like I stole it. The pack voltage says over 105 volts (2.76vpc) except in the winter when I have to back off to 5C peak and accept the pack sagging to 102 volts (2.68vpc.) Winters in western Washington state are mild and the car is garaged.

We still know very little about end of life issues with LiFePO4 EV packs. No matter how an owner chooses to set up their pack I recommend they periodically check, to find any bad cells and eventually to detect any end of life changes. I would not consider any home built Lithium pack to be a build and forget system at this time.


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## dtbaker (Jan 5, 2008)

drgrieve said:


> Ok let say a person has a goal of top balancing their pack
> 
> They A
> 
> Spend 2 days top balancing their pack to 3.350V (this is only an example) static per cell to a 1/1000 of a volt. The pack is absolutely top balanced (@95% SOC). They now never have to touch the pack again (unless they want to add more cells) for the life of the car (but reports are only 2 to 3 years old so this is not confirmed but so far so good).



I would argue hard that an initial top balance to 3.35vpc may not be balanced at all as it is in a very flat part of curve..... manf instructions even indicate initial balance to 4.0 vpc, and then cycle charge to 3.65vpc is recommended and 'safe' as *all* cells will end charge within acceptable voltage even with a dumb charger.

without a decent initial top balance, there is huge risk that cell(s) would hit the top voltage and try to shunt while the charger is pumping out more amps than the shunts can handle.


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## EVfun (Mar 14, 2010)

I found a post of drgrieve over on another thread, it seems that he believes that the proper way to top balance the cells is by the *resting* voltage. Almost everyone seems to be charging based on a CC/CV scheme. Headway clearly specifies CC/CV charging in the spec sheet for the 10 amp hour cells.


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## dtbaker (Jan 5, 2008)

EVfun said:


> I found a post of drgrieve over on another thread, it seems that he believes that the proper way to top balance the cells is by the *resting* voltage. Almost everyone seems to be charging based on a CC/CV scheme. Headway clearly specifies CC/CV charging in the spec sheet for the 10 amp hour cells.



...after a short rest, even I look like I am full of energy... you could never guess how much work I've already done, or that I might keel over soon...

Using that same analogy, if we all stuffed ourselves at a Roman Orgy until we couldn't take another bite, then all rowed a boat together for a couple hours, then all got fed the same amount of food afterward.... would we all be full?


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## bruceme (Dec 10, 2008)

dtbaker said:


> ...after a short rest, even I look like I am full of energy... you could never guess how much work I've already done, or that I might keel over soon...
> 
> Using that same analogy, if we all stuffed ourselves at a Roman Orgy until we couldn't take another bite, then all rowed a boat together for a couple hours, then all got fed the same amount of food afterward.... would we all be full?


Colorful


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## EVfun (Mar 14, 2010)

I like that analogy Dan. In these parts running without a BMS seems to be the accepted norm, and I see it can work. Just out of curiosity I decided to check out the LiFePO4 powered cars over on the EValbum. I just looked as on-road passenger cars with LiFePO4 prismatic cells. The majority have some type of BMS installed. 

I'm not sure if I will leave my pack naked, put some type of newly built simple shunt reg system on it, or simply reinstall the EVworks BMS modules on the pack without wiring up the monitoring loop.


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