# Thought for a "minimal" BMS



## Jerry Liebler (Feb 1, 2011)

I'm about to build what I consider the absolute minimum BMS. Basically a bunch of tiny boards, 0.4"x 0.6", that connect across individual cells, one board per cell. The circuit on the boards is a switched shunt that draws 60 micro amps or less if the cell voltage is below 3.420 volts +/-2millivolts. For voltages higher it switches a 12 ohm 1 watt resistor across the cell. That's it! No wires between cells! No added wires from the cells! No head end!, No monitoring! No alarms! No computers! No 12 volt requirement! What it does is a fully automatic top balance every time the pack is fully charged with a "proper" charger that has a constant voltage output. The tolerance of the shunt switching voltage is one tenth that in the best BMS, I've seen, on the market. The idle curent is half of the best and a few percent of the worst offenders I've seen in the BMS market. Yes I probably should go with no BMS. This is my "compromise" with my "paranoia" about cell voltage drift.


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## Caps18 (Jun 8, 2008)

Would you always be losing some battery life with this setup, or am I not picturing it correctly (which is very possible)?


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## rwaudio (May 22, 2008)

Jerry Liebler said:


> I'm about to build what I consider the absolute minimum BMS. Basically a bunch of tiny boards, 0.4"x 0.6", that connect across individual cells, one board per cell. The circuit on the boards is a switched shunt that draws 60 micro amps or less if the cell voltage is below 3.420 volts +/-2millivolts. For voltages higher it switches a 12 ohm 1 watt resistor across the cell. That's it! No wires between cells! No added wires from the cells! No head end!, No monitoring! No alarms! No computers! No 12 volt requirement! What it does is a fully automatic top balance every time the pack is fully charged with a "proper" charger that has a constant voltage output. The tolerance of the shunt switching voltage is one tenth that in the best BMS, I've seen, on the market. The idle curent is half of the best and a few percent of the worst offenders I've seen in the BMS market. Yes I probably should go with no BMS. This is my "compromise" with my "paranoia" about cell voltage drift.


No comment on the BMS itself, however be careful with the resistor. 1 watt isn't enough for reliable operation, you would be operating at 1 watt 100% of the time when shunting, that doesn't provide any safety margin.


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## EVfun (Mar 14, 2010)

That is similar to how I'm using my EV Power BMS modules right now. The down side is the shunt level is 3.62 volts; I'd prefer about 3.5. The standby current is around 3 milliamps, not a big deal as it would take about 2 years to discharge my 60 amp hour cells.

I've considered building a simple shunt reg with a low bypass current. I was thinking along the lines of a shunting about 1/3 amp at 3.6 volts. I was looking at adding a little hysteresis, so the resistor carries the load instead of transistor switching losses, and a small red LED that lights when the shunt is on for user feedback.


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## frodus (Apr 12, 2008)

I made some of these same things using a voltage detector chip and a few other small components. I built it for lead acid but it would work for lithium very easily. It's a dumb balancer and worked quite well in bringing the back into balance. I have some boards still and the schematic somewhere if you want to mess around with it. Might have some chips too. We used a small light bulb instead of a resistor so you could see the ones balancing. Same idea though.


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## Jerry Liebler (Feb 1, 2011)

rwaudio said:


> No comment on the BMS itself, however be careful with the resistor. 1 watt isn't enough for reliable operation, you would be operating at 1 watt 100% of the time when shunting, that doesn't provide any safety margin.


Once the pack is balanced the shunts will, essentially, never be needed. or if needed wll operate in the "linear" region of the "switch". While I've said it is a "switch" it is a transistor and the transistor will begin conducting at 3.42 volts and not be "saturated" until the voltage reaches about 3.67 volts. But it's certainly easy enough to pick a little higher value resistor and since the shunts are just there to correct the cell imbalance that others have proven simply doesn't happen they will rarely be used and should never be fully on. Summary: I've changed my schematic to show 2 paralleled 26.1 ohm 1% 1/2W resistors This will take a voltage of 3.6 to reach the resistor's dissipation rating.


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## EVfun (Mar 14, 2010)

I don't generally consider a resistor to be stable for more than about 1/2 its rated power without forced cooling. I'd think more along the lines of a 2 watt resistor (or 2, 1 watt resistors.)

The only issue with operating the load transistor in the linear region is that they generally don't handle as much power, for their size or cost, as a resistor. I created a little hysteresis by adding a high value resistor, one end between the load transistor and the load resistor, and the other end in the middle of the voltage divider. I did it to make some Volt Blocher shunt regs operate without the transistor getting so flaming hot.


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## QuietCar (Jan 3, 2013)

I am using a "Mini BMS" By Clean Power Auto LLC. (*analog*) I am completely satisfied with it. 

It also uses those small boards (one to a cell) and also can use a small extra board to monitor your pack voltage to either "Warn" you or shut off the charger on the top and "Warn" you or shut down your controller on the bottom. It also will shunt each cell separately to provide a top balance each charge cycle.

It sort of scales up or down as you see fit. 

The cell level boards were around $12/ea. (quite reasonable)

If you wanted a "loop" to monitor top/bottom voltage, add $48.

There is out now a similar *digital* system by another maker. It does offer some small other features into cell logging (If I remember right) for about the same price too.

QC


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## Jerry Liebler (Feb 1, 2011)

EVfun said:


> I don't generally consider a resistor to be stable for more than about 1/2 its rated power without forced cooling. I'd think more along the lines of a 2 watt resistor (or 2, 1 watt resistors.)
> 
> The only issue with operating the load transistor in the linear region is that they generally don't handle as much power, for their size or cost, as a resistor. I created a little hysteresis by adding a high value resistor, one end between the load transistor and the load resistor, and the other end in the middle of the voltage divider. I did it to make some Volt Blocher shunt regs operate without the transistor getting so flaming hot.


Actually I didn't describe what I'm building correctly, calling it a switch is misleading, it's more corectly described as a synthetic Zener. which draws only 70 micro amps below 3.42 volts. Above 3.42 volts it draws current that increases as it the voltage it sees increases. The current rises at 2 ma per mv, a zener impedance of 0.5 ohm. At 3.62 volts 400 ma is flowing through a transistor that has resistors in it's collector so the power is shared about equally between 3 components, 2 resistors and a transistor each of the 3 parts is dissipating about 1/2 W the resistors are 1/2w units, I want them to fail first as they fail open, they act as fuses in a severe overload. The transistor is rated for over 1 watt so it is still rather lightly loaded.

In operation these modules should never see a voltage more than a few milli-volts greater than 3.42 so the power dissipation will be very small.
Putting larger resistors in is counter productive they will no longer be the fuse! Similarly putting an led in is useless as it simply shouldn't ever light.
Keeping the "standby" current as low as possible is what prevents the middle letter of BMS from representing the word MURDERING! A post above mentioned a cell current of 3 milliamps, about 50 times as much or a discharge rate of one AH every 2 weeks. Another mentioned the "MiniBMS" with a parasitic drain that's even worse.

I'm NOT building a cell balancing module rather I'm building a balance maintainer module! Excess "design safety margin" costs money, size and weight.


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## QuietCar (Jan 3, 2013)

Jerry: My thoughts for choosing the Mini over the others is simple:
*1*-I am an electronic idiot. I lack any skills in the field.
*2*-I had no BMS for the first year. I was spending too much time keeping my cells any where near balanced, using manual methods.
*3*-Reality had set in as to the use of lithium cells in an automotive situation. I had finally acquired enough experience with the bigger 
cells to make a reasonably correct decision. 
*4*-My criteria was simple: low cost and still protect the expensive cells.
Back then, Mini's fit the bill and also fit within my operating environment.
(The parasitic loss was well within my usage statistics- I drive almost 
every day and do not sit more than a month-so far.)

When my car sat over a month, the aux. (Pb/Acid) battery gave out first.
The car started giving off a low volume squeal. After some system checks, it was determined that the BMS-the ZEVA sensor board and the J1772 spoofer board, being on full time, had drawn the Aux. battery down below the BMS head board's tolerated voltage. It was giving the alarm for me to perform corrective actions (as designed I assume). 

This situation was unique and not to be repeated. It was a fluke........(right) 

It has happened 4 times in 2 years, so I guess I need to place this in the "Normal" part of my car's operating scheme....LOL

My opinion: a small parasitic draw will happen in every system regardless as to how it is designed. Some are just smaller than others, plan for it.

QC


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## Jerry Liebler (Feb 1, 2011)

QuietCar said:


> Jerry: My thoughts for choosing the Mini over the others is simple:
> *1*-I am an electronic idiot. I lack any skills in the field.
> *2*-I had no BMS for the first year. I was spending too much time keeping my cells any where near balanced, using manual methods.
> *3*-Reality had set in as to the use of lithium cells in an automotive situation. I had finally acquired enough experience with the bigger
> ...


I heartily agree with your last statement! That is why I'm building my own low drain cell modules. I'm dealing with much smaller cells (10 AH) because I have a higher voltage pack. Also I'm near the other end of the electronics IQ scale.


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## Jerry Liebler (Feb 1, 2011)

QuietCar said:


> Jerry: My thoughts for choosing the Mini over the others is simple:
> *1*-I am an electronic idiot. I lack any skills in the field.
> *2*-I had no BMS for the first year. I was spending too much time keeping my cells any where near balanced, using manual methods.
> *3*-Reality had set in as to the use of lithium cells in an automotive situation. I had finally acquired enough experience with the bigger
> ...


I heartily agree with your last statement! That is indeed why I'm building these. I only have 10 AH cells and the parasitic drain of the BMS needs to be planned for. The planing means making it as low as practical. Finding nothing close to "the state of the art" in low power electronics in the BMS market means I'll have to "roll my own" . Fortunately,for me, I'm near the other end of the electronics "IQ scale".


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## mk4gti (May 6, 2011)

not sure if this can help

http://www.diyelectriccar.com/forums/showthread.php/homemade-bms-88012


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## rwaudio (May 22, 2008)

Jerry Liebler said:


> Actually I didn't describe what I'm building correctly, calling it a switch is misleading, it's more corectly described as a synthetic Zener. which draws only 70 micro amps below 3.42 volts. Above 3.42 volts it draws current that increases as it the voltage it sees increases. The current rises at 2 ma per mv, a zener impedance of 0.5 ohm. At 3.62 volts 400 ma is flowing through a transistor that has resistors in it's collector so the power is shared about equally between 3 components, 2 resistors and a transistor each of the 3 parts is dissipating about 1/2 W the resistors are 1/2w units, I want them to fail first as they fail open, they act as fuses in a severe overload. The transistor is rated for over 1 watt so it is still rather lightly loaded.
> 
> In operation these modules should never see a voltage more than a few milli-volts greater than 3.42 so the power dissipation will be very small.
> Putting larger resistors in is counter productive they will no longer be the fuse! Similarly putting an led in is useless as it simply shouldn't ever light.
> ...


That makes more sense and has a better chance of success. What voltage do you plan on charging the pack to? 3.42v Seems very low if you plan on never hitting it. I charge my 100 series cell pack to 3.45v per cell and that is quite low and many of my cells are in the 3.5x range and one or two in the 3.6x range at the end of charge (bottom balanced, not top balanced)

Personally I think Excess "design safety margin" is a basic requirement or don't build the BMS at all. A board that fails and drains a cell will cost how much to replace? A board that fails and burns down your bike/car/house will cost how much to replace?

I'm not trying to put down your project, but it might be a good idea to torture a few boards and learn their failure modes, nothing worse than a BMS that either destroys the cells they are trying to protect or burns down the whole vehicle.


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## Jerry Liebler (Feb 1, 2011)

rwaudio said:


> That makes more sense and has a better chance of success. What voltage do you plan on charging the pack to? 3.42v Seems very low if you plan on never hitting it. I charge my 100 series cell pack to 3.45v per cell and that is quite low and many of my cells are in the 3.5x range and one or two in the 3.6x range at the end of charge (bottom balanced, not top balanced)
> 
> Personally I think Excess "design safety margin" is a basic requirement or don't build the BMS at all. A board that fails and drains a cell will cost how much to replace? A board that fails and burns down your bike/car/house will cost how much to replace?
> 
> I'm not trying to put down your project, but it might be a good idea to torture a few boards and learn their failure modes, nothing worse than a BMS that either destroys the cells they are trying to protect or burns down the whole vehicle.


To answer the question " what voltage do you plan on charging with. 3.35 volts/cell. The tests I have done on the cells says that there is less than 1 % of cell capacity gained by charging higher. I'll forgo the last percent in favor of greater cell life.

Thank you for your critique! Because of it I've changed the design to give a bit more margin at higher voltage(s) I found that I can get 1/3 watt resistors in an 0805 package and replaced 2 ea. 1/2 watt 1210 packages with 4 ea 0805 so instead of 1 watt of resistor I have 1 & 1/3 With the revised design the dissipation limit is at 0.6 amps or 3.72 volts. 

I do believe you are shortening the life of the cells that hit 3.6 volts. With 100 cells charged to 345 volts what is your pack voltage after it sits, unloaded for an hour or 2?


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## rwaudio (May 22, 2008)

Jerry Liebler said:


> To answer the question " what voltage do you plan on charging with. 3.35 volts/cell. The tests I have done on the cells says that there is less than 1 % of cell capacity gained by charging higher. I'll forgo the last percent in favor of greater cell life.
> 
> Thank you for your critique! Because of it I've changed the design to give a bit more margin at higher voltage(s) I found that I can get 1/3 watt resistors in an 0805 package and replaced 2 ea. 1/2 watt 1210 packages with 4 ea 0805 so instead of 1 watt of resistor I have 1 & 1/3 With the revised design the dissipation limit is at 0.6 amps or 3.72 volts.
> 
> I do believe you are shortening the life of the cells that hit 3.6 volts. With 100 cells charged to 345 volts what is your pack voltage after it sits, unloaded for an hour or 2?


That gives a bit more overhead, probably a good choice.

My cells come down to around 3.34v after charge. My charger terminates within a couple minutes of reaching the CV voltage, so basically there is just a CC charge up to 3.45v per cell. That's not 100% charged, but keeps all the cells within a decent range for a bottom balanced pack.


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## Jerry Liebler (Feb 1, 2011)

rwaudio said:


> That gives a bit more overhead, probably a good choice.
> 
> My cells come down to around 3.34v after charge. My charger terminates within a couple minutes of reaching the CV voltage, so basically there is just a CC charge up to 3.45v per cell. That's not 100% charged, but keeps all the cells within a decent range for a bottom balanced pack.


Have you ever tried setting the CV limit at 335 volts? It may take a bit longer to complete charging. But, I'll wager you'll have the same capacity. The cell that hit's 3.6 volts first controls your pack capacity, it is most definitely fully charged. Because you have bottom balanced,your pack is fully charged, yes 100%. To charge further will destroy the bottom balance and convert the pack to a top balanced condition.


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## dougingraham (Jul 26, 2011)

Jerry Liebler said:


> Yes I probably should go with no BMS. This is my "compromise" with my "paranoia" about cell voltage drift.


Instead of spending time on this why not just get over your paranoia about cell voltage drift since there isn't any.

Manually top balance your pack. Check the balance several hours after the charge finishes. Do this after the first day, first week, second week and then month. Feel good about all the time and money you saved now that you know there is no cell voltage drift. After that you can kick yourself for not bottom balancing the pack in the first place.

Best Wishes!


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## lithiumlogic (Aug 24, 2011)

Rather than shunt excess voltage , is there a way to optocouple a feedback signal to the charger to reduce current in the CV phase, to keep the highest cell below 3.6 or whatever your target is. That way you don't destroy the bottom balance of the pack - though in the end it is inevitable that it will be lost whenever you connect a bms of any sort - the Schrodinger's cat paradox - you cannot measure the voltage of a cell without taking current from it, different boards will take different amounts due to manufacturing tolerances, therefore periodic balancing is needed.


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## rwaudio (May 22, 2008)

Jerry Liebler said:


> Have you ever tried setting the CV limit at 335 volts? It may take a bit longer to complete charging. But, I'll wager you'll have the same capacity. The cell that hit's 3.6 volts first controls your pack capacity, it is most definitely fully charged. Because you have bottom balanced,your pack is fully charged, yes 100%. To charge further will destroy the bottom balance and convert the pack to a top balanced condition.


This is about your BMS, I'm not going to debate LiFePO4 charging theory here. My pack is under charged based on the simple fact I can charge it further by going up to the manufacturers recommended charging scheme (CC-CV to 3.65v and terminate at C/20), yours will be "more" under charged and potentially last longer than mine, lets leave it at that.



lithiumlogic said:


> Rather than shunt excess voltage , is there a way to optocouple a feedback signal to the charger to reduce current in the CV phase, to keep the highest cell below 3.6 or whatever your target is. That way you don't destroy the bottom balance of the pack - though in the end it is inevitable that it will be lost whenever you connect a bms of any sort - the Schrodinger's cat paradox - you cannot measure the voltage of a cell without taking current from it, different boards will take different amounts due to manufacturing tolerances, therefore periodic balancing is needed.


There is a point where for practical purposes you can call it "zero" current draw when measuring voltage if the input or load impedance of the meter is high enough. Common multimeters often have a 10meg input impedance which can be dismissed as zero in the real world. More to the point if you have a 10meg or 1meg or lower load on all cells the draw is the same and drift doesn't exist. 

My experience with these cells says that one cell reading 3.6v during charge does not affect the bottom balance. I won't say you're wrong yet, however you need to back it up with data, give us your log files of a few dozen cells charged at the manufacturers recommend charge rate under controlled conditions where bottom balancing to 1/100th of a volt was done. After x number of cycles the pack was taken down to the bottom balance voltage and there was a significant change in state of charge of only the cell that hit 3.6v during charge.

Good luck with your BMS


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## Jerry Liebler (Feb 1, 2011)

dougingraham said:


> Instead of spending time on this why not just get over your paranoia about cell voltage drift since there isn't any.
> 
> Manually top balance your pack. Check the balance several hours after the charge finishes. Do this after the first day, first week, second week and then month. Feel good about all the time and money you saved now that you know there is no cell voltage drift. After that you can kick yourself for not bottom balancing the pack in the first place.
> 
> Best Wishes!


FACT we cannot avoid: Cell characteristics change over time, both charge capacity and voltage at any charge state can and will drift over usage cycles. Operating any cell above some, manufacturer specified voltage will be destructive. Operating any cell below some, manufacturer specified voltage is destructive. To avoid cell destruction in use some form of battery management system simply must be used. We can make the battery management system either manual or automatic but it will exist or cell destruction is inevitable!

One of the battery management systems in popular use is fully manual, involving, "commissioning" and periodic "monitoring." With this system, during "commissioning" the cells are balanced at one end of their voltage range and monitored at the other end to set system parameters of maximum and minimum pack voltage. The periodic monitoring is typically only done at the "unbalanced" pack voltage. This system depends entirely on the initial balancing never changing!

What I'm building, automatically maintains the top balanced state so relative cell capacity can be confidently accessed during periodic manual measurements at the controller's low voltage cutout.


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## dougingraham (Jul 26, 2011)

Fact: I probably won't bother to check it again unless something happens. I've just gotten tired of never seeing anything drift.

The charger I have set to stop when it gets to 3.45 volts per cell. The highest cell is about 3.5 volts and it has been that way from the start. And yes I know I am undercharging my batteries a little but that is good for them. I have the Soliton set to limit at 90 battery volts (1.76 volts per cell). I would go lower but my DC/DC doesn't like that very much. And on warm days it never gets that low anyway. I ran it into the ground once and I was 0.2 mile from home. Because I was bottom balanced I only managed to get 50 more feet by waiting and trying it again a few times. I towed it home, charged it and took out one cell and cycle tested it to see what had been lost. And the answer was nothing measurable. My BMS is the charger cutoff for charging and the the bottom balance for the discharge. I've been living BMS free now for 1.5 years.

They are your batteries and it is your time and money and you can do whatever makes you feel good.


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## elevatorguy (Jul 26, 2007)

Doug, is that 1.76 under load? I have mine set much higher, like 2.6v, maybe I could get more range if I lowered it.
I ran it dead and when I got home the lowest cell was 2.37
I use the same method, slight undercharge and let the Soliton watch the bottom under load and I monitor the pack voltage gauge.


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## jddcircuit (Mar 18, 2010)

Jerry Liebler said:


> FACT we cannot avoid: Cell characteristics change over time, both charge capacity and voltage at any charge state can and will drift over usage cycles. Operating any cell above some, manufacturer specified voltage will be destructive. Operating any cell below some, manufacturer specified voltage is destructive. To avoid cell destruction in use some form of battery management system simply must be used. We can make the battery management system either manual or automatic but it will exist or cell destruction is inevitable!
> 
> One of the battery management systems in popular use is fully manual, involving, "commissioning" and periodic "monitoring." With this system, during "commissioning" the cells are balanced at one end of their voltage range and monitored at the other end to set system parameters of maximum and minimum pack voltage. The periodic monitoring is typically only done at the "unbalanced" pack voltage. This system depends entirely on the initial balancing never changing!
> 
> What I'm building, automatically maintains the top balanced state so relative cell capacity can be confidently accessed during periodic manual measurements at the controller's low voltage cutout.


I have some cells that have exhibited some interesting behavior at the top of the charge curve. I am monitoring each cell so I can switch to CV mode when the first cell hits a voltage threshold. As I hold this CV mode and current starts to drop another cell will switch places as being the CV limiting cell.

So which cell do I need to remove energy from the first one or the second one? Having a hardwired shunt that assumes cell voltage = SOC under these dynamic regions makes me think twice.

I also decided to roll my own BMS. My BMS only monitors cell voltages. It only draws 9.5uA +/- .5uA. So it will take 100 years to create a 1Ah imbalance.

I am close to finishing my cycle tester with the purpose looking for cell drift. I expect unequal capacity fade which will cause jagged bottom and top but will I find a situation where my lowest capacity cell is not limiting my pack at both ends.

I am all for building your own BMS but I have concerns about voltage regulated shunting during charging because of some of my observations. I haven't thought about the effects all the way through yet so no decision.

Regards
Jeff


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## Jerry Liebler (Feb 1, 2011)

jddcircuit said:


> I have some cells that have exhibited some interesting behavior at the top of the charge curve. I am monitoring each cell so I can switch to CV mode when the first cell hits a voltage threshold. As I hold this CV mode and current starts to drop another cell will switch places as being the CV limiting cell.
> 
> So which cell do I need to remove energy from the first one or the second one? Having a hardwired shunt that assumes cell voltage = SOC under these dynamic regions makes me think twice.
> 
> ...



Jeff,
I have a friend who, for complicated reasons, placed 150 brand new but tested for open circuit voltage of 3.10 +/- 0.05 16AH Headway cells in storage then a year later retrieved them and found 16 of them read ZERO volts, to his dismay. To those who totally forgo a BMS of any sort, good luck! It is a definite fact that individual cells differ and some form of re-balancing will eventually be needed. I choose to build that capability in from the start. FWIW I'm building these modules to be used in a " EV range extender" in a plug in Prius so I'm fascinated by your project and would like to know more about it.


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## rwaudio (May 22, 2008)

Jerry Liebler said:


> Jeff,
> I have a friend who, for complicated reasons, placed 150 brand new but tested for open circuit voltage of 3.10 +/- 0.05 16AH Headway cells in storage then a year later retrieved them and found 16 of them read ZERO volts, to his dismay. To those who totally forgo a BMS of any sort, good luck! It is a definite fact individual that cells differ and some form of re-balancing will eventually be needed. I choose to build that capability in from the start. FWIW I'm building these modules to be used in a " EV range extender" in a plug in Prius so I'm fascinated by your project and would like to know more about it.


3.1V open circuit is almost 100% discharge and a horrible way to store cells long term (perhaps you meant 3.30v?). A defective cell can have an internal short which will cause self discharge. A healthy cell can be stored for many years with minimal change in state of charge. I've used headways and they are like A123's, fairly high power, but not the same quality as Thundersky/Winston/Sinopoly/CALB etc. Perhaps it's easier to manufacture the prismatic style cells? Prismatics are not immune to internal shorts though, one of my CALB 60's had internal discharge and I knew it would need replacing eventually so I did. The rest of the pack doesn't discharge.

The moral of the story is don't mistake the behaviour of a defective cell(s) with normal behaviour of a healthy pack.


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## Yabert (Feb 7, 2010)

rwaudio said:


> The moral of the story is don't mistake the behaviour of a defective cell(s) with normal behaviour of a healthy pack.


I second.
I've stored few headway cells 2-3 years ago at 3.3v and I found, a month ago, one cell at 0v, another around 2.8v and all the other around 3.3v.


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## Duncan (Dec 8, 2008)

rwaudio said:


> The moral of the story is don't mistake the behaviour of a defective cell(s) with normal behaviour of a healthy pack.


I have also seen this with Headways -
They don't "drift" they just die

Saying that the company has been very supportive - replaced them free 
and they stopped making the 16Ah - 
I hope the replacement cells (15Ah) don't suffer the same


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## Jerry Liebler (Feb 1, 2011)

rwaudio said:


> 3.1V open circuit is almost 100% discharge and a horrible way to store cells long term (perhaps you meant 3.30v?). A defective cell can have an internal short which will cause self discharge. A healthy cell can be stored for many years with minimal change in state of charge. I've used headways and they are like A123's, fairly high power, but not the same quality as Thundersky/Winston/Sinopoly/CALB etc. Perhaps it's easier to manufacture the prismatic style cells? Prismatics are not immune to internal shorts though, one of my CALB 60's had internal discharge and I knew it would need replacing eventually so I did. The rest of the pack doesn't discharge.
> 
> The moral of the story is don't mistake the behaviour of a defective cell(s) with normal behaviour of a healthy pack.


Please share,with all here your SOC vs open circuit graph or a source for the assertion that 3.1 is "almost 100% discharged! The data I've collected,on the cells I'm using, admittedly not Headway cells,shows approximately 30% SOC @ 3.100V. My test data also shows 3.300 volts at 98%+ SOC.


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## rwaudio (May 22, 2008)

Jerry Liebler said:


> Please share,with all here your SOC vs open circuit graph or a source for the assertion that 3.1 is "almost 100% discharged! The data I've collected,on the cells I'm using, admittedly not Headway cells,shows approximately 30% SOC @ 3.100V. My test data also shows 3.300 volts at 98%+ SOC.


It could easily have 30% or more if the measurement is under load, but not open circuit.
http://lithiumate.elithion.com/php/functional_descr.php

The point is 3.1v is not the voltage cells should be stored at long term based on all info I've seen. You want somewhere around 50% SOC give or take.
Almost all of the cells I've received from Calb/A123 arrived at very close to 3.3v and when I test them they are between 40-60% SOC based on the Ah's required to fill them up.

What cells are you using?


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## Jerry Liebler (Feb 1, 2011)

rwaudio said:


> It could easily have 30% or more if the measurement is under load, but not open circuit.
> http://lithiumate.elithion.com/php/functional_descr.php
> 
> The point is 3.1v is not the voltage cells should be stored at long term based on all info I've seen. You want somewhere around 50% SOC give or take.
> ...


I agree with 50% SOC for storage & should claim meter error because the cells were new, never charged or discharged, when they were stored they still had whatever charge Headway shipped them with, they only got an "is it alive" type of test.

I'm using cells from Liao so far I've only seen a few samples, I'm waiting for the bulk of them now. They are small 10 AH prismaticsi n aluminum cases with very low internal resistance (3.5 milliohm) that support 5c continuous discharge rates.


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## EVfun (Mar 14, 2010)

I test my cell capacity by discharging at 0.2C until the first cell hits 3.00 volts. That nets a measured capacity slightly greater than the name plate capacity of my cells. I can't find my bounce back data, but they are higher after 12 hours of no load rest.


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## jddcircuit (Mar 18, 2010)

Jerry Liebler said:


> Jeff,
> I have a friend who, for complicated reasons, placed 150 brand new but tested for open circuit voltage of 3.10 +/- 0.05 16AH Headway cells in storage then a year later retrieved them and found 16 of them read ZERO volts, to his dismay. To those who totally forgo a BMS of any sort, good luck! It is a definite fact that individual cells differ and some form of re-balancing will eventually be needed. I choose to build that capability in from the start. FWIW I'm building these modules to be used in a " EV range extender" in a plug in Prius so I'm fascinated by your project and would like to know more about it.


Jerry,
I hear you. I agree that there is evidence of the occasional occurrence of self discharge.

When you say you are building this capability in from the start I am thinking that you may be masking it from the start. Without some form of feedback on how much or how often rebalancing is needed we may never fully understand these cells. Are these early life failure symptoms or do most of the cells develop this behavior over time?

I don't have a position on whether balancing is needed or not yet. There is not enough statistical data that I can find. There are many that don't use it and have not detected a problem yet.

A minimalist approach in my opinion is monitoring only even if it is only to show that balancing is not needed. This data over time might enlighten us on best management practices and appropriate management circuits.

The other reason I am not sold on shunt balancing during charging is based on the dynamic terminal voltage of different cells near full SOC which seems to be very influenced by charge current. I have seen cells diverge in voltage during charging only to converge again after the charge current was reduced. This convergence happened without removing any energy from a cell so I assume it is not completely a SOC alignment problem.

Regards
Jeff


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## Jerry Liebler (Feb 1, 2011)

jddcircuit said:


> Jerry,
> I hear you. I agree that there is evidence of the occasional occurrence of self discharge.
> 
> When you say you are building this capability in from the start I am thinking that you may be masking it from the start. Without some form of feedback on how much or how often rebalancing is needed we may never fully understand these cells. Are these early life failure symptoms or do most of the cells develop this behavior over time?
> ...


The limit I have on CV charging Will establish an average cell voltage of 3.35 volts. My shunts begin conducting at 3.430 +/-14mv volts, I expect there will be negligible shunting. I am implementing an electronic version of the "Battbridge" using op amps and precision(0.1%) resistors and will force shutdown if the halves of the pack differ by 2%


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## dougingraham (Jul 26, 2011)

Yabert said:


> I second.
> I've stored few headway cells 2-3 years ago at 3.3v and I found, a month ago, one cell at 0v, another around 2.8v and all the other around 3.3v.


The two types of cells that I hear people have the most problems with are Headway and Hi Power. But it doesn't matter the brand, if they self discharge they were broken when they were manufactured and no amount of BMSing is going to fix them. It will identify them if it has the ability to monitor and report voltage centrally but all that can do is tell you which ones are bad.


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## rwaudio (May 22, 2008)

I want a logging cell level voltage and temperature monitoring system with no load on the cells (multimeter type 10meg input impedance is ok so the load is virtually zero)

I don't want to balance, I don't want to shunt, I don't want it to be powered by the cells, I want to be able to turn it off and store the car for a year without fear that I'll come back to a brick.

Seems simple enough, why doesn't it exist?

While I'm at it I want the car to talk to my home PC and send trend info from every drive and every charge and check it over time and tell me if I have a high impedance cell or something that needs to be replaced.


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## EVfun (Mar 14, 2010)

After using LiFePO4 cells for 4 years I'm starting at the beginning again. I want to qualify my cell behavior before I make any (more) balancing or BMS decisions. 

I've been playing around with some 60 amp hour Thunder Sky cells, I have 60 of them. I discharged some to 3.08 volts under load. The load was a 0.22 ohm power resistor. I removed about 50 amp hours from the cells. The resting voltage a week later is 3.24 to 3.25 volts. So that is the resting voltage at 15% to 20% SOC.

I did some charge and discharge testing on some top balanced 60 amp hour Thunder Sky cells with BMS modules that had sat for about 5 months. The cells where down around 3.15 volts (they started the storage at less than 50% SOC.) The BMS modules draw between 3.9 and 4.1 milliamps each, a difference of just 200 microamps. Over 5 months that pulled the pack balance off by up to 0.72 amp hours. That was way to much for the BMS, which shunts about 1/2 amp, to correct on a charge. The cells simply spend too little time over 3.6 volts where the modules turn on, and 1/2 amp doesn't pull the cells back much when charging at 10 amps. With a little manual intervention, by slowing the charged to 3 amps and manually helping with a 0.5 ohm resistor, I have the pack top balanced again. I pulled 12.5 amp hours out of the cells and then recharged the pack. If I chose to terminate the charge at 3.43 volts per cell with no hold time the cells would be 1.5 to 2 amp hours shy of where they are by charging to 3.64 volts and holding for 13 minutes. The drop in current, when the charger was holding the cells to 3.64 vpc, was dramatic. In just 13 minutes the current went from 10 amps to 1.3 amps. 

There is enough variation between cells, even after 3 cycles to bring the pack back together, with 1/2 amp of shunting that I'm not confident that less shunting would be a good idea. The cells still vary between 3.61 and 3.67 volts. I know from past tests that charging top balanced without shunts regs that 0.2 amp hour is roughly the difference between 3.5 and 4.0 volts at the end of charge. 

It does make me consider just charging to 3.43 volts with no shunt regs and no hold time at the target voltage. All the cells where at the same voltage up to 3.43 volts. They varied 0.02 volts at 3.5 volts. From 3.43 to 3.64 volts took less than 10 minutes at 10 amps. Holding the cells to the target voltage for 13 minutes resulted in the current dropping from 10 to 1.3 amps. At that point the charger shut off. 

I have considered a simple shunt reg (like originally proposed in this thread) but based on discrete components. Likely an LM431, a PN2907 transistor, 2 resistors for a resistor divider network for the reference. A couple resistors to limit LM431 current and 2907 gate current, 1 high value resistor from the reference to a point between the transistor and shunt resistor to create hysteresis, a load resistor of around 7.5 ohms, a 3mm red LED and 100 ohm resistor in series, placed in parallel with the load resistor. This could be made with all through hole parts and draw less than 500 microamps when off. That is low enough that it would take over 2 years to drain 10 amp hour cells.


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## Duncan (Dec 8, 2008)

dougingraham said:


> The two types of cells that I hear people have the most problems with are Headway and Hi Power. But it doesn't matter the brand, if they self discharge they were broken when they were manufactured and no amount of BMSing is going to fix them. It will identify them if it has the ability to monitor and report voltage centrally but all that can do is tell you which ones are bad.


I agree with this entirely - I have had bad cells - they die
No amount of BMSing would have saved them

I have not had any "drift" they have been rock solid or they have died

IMHO the only type of BMS you need is a BattBridge
http://www.evdl.org/pages/battbridge.html

This has the massive advantage of comparing each half of the battery under load - a much more telling test


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## EVfun (Mar 14, 2010)

Duncan said:


> IMHO the only type of BMS you need is a BattBridge
> http://www.evdl.org/pages/battbridge.html


What components did you use in your Battbridge? I was wondering if anyone made one using optical isolators in place of the red LEDs. They typically turn on at just a couple milliamps and would provide an isolated connection to a dash light (no traction pack potential behind the dash.)


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## Duncan (Dec 8, 2008)

*Hi*

I am not an electronics guy but these were the ones I used

*
*

*5mm Red High Flux LED*


Part Number: HF5-R5590
https://www.superbrightleds.com/mor...ed-90-degree-viewing-angle-5500-mcd/356/1303/
*5mm Green LED*


Part Number: RL5-G5023 
https://www.superbrightleds.com/mor...ed-23-degree-viewing-angle-5000-mcd/273/1200/


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