# Active balancing?



## Jerry Liebler (Feb 1, 2011)

Active balancing is where energy is moved from higher voltage cells to lower voltage cells, as opposed to passive balancing where energy is dissipated to lower the cell voltage on higher voltage cells. Is anyone using active balancing? Who makes active balancing hardware? How expensive is it? My reason for asking is I have an idea on how to build an active balancing system. The system I've designed (paper tiger) balances whenever 12 volts is applied (with a 12volt load of under 0.5 amp). This means balancing can occur during charging, discharging, and storage selectively. If the 12 volt input is not there the loads on the individual cells will be 0.2ma or less. When the 12 volts is present the balancing effect is equal to connecting all cells in parallel to a common "node" through a 50 milli-ohm resistor in series with each cell yet the cells remain connected in series.


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## jddcircuit (Mar 18, 2010)

Jerry Liebler said:


> Active balancing is where energy is moved from higher voltage cells to lower voltage cells, as opposed to passive balancing where energy is dissipated to lower the cell voltage on higher voltage cells. Is anyone using active balancing? Who makes active balancing hardware? How expensive is it? My reason for asking is I have an idea on how to build an active balancing system. The system I've designed (paper tiger) balances whenever 12 volts is applied (with a 12volt load of under 0.5 amp). This means balancing can occur during charging, discharging, and storage selectively. If the 12 volt input is not there the loads on the individual cells will be 0.2ma or less. When the 12 volts is present the balancing effect is equal to connecting all cells in parallel to a common "node" through a 50 milli-ohm resistor in series with each cell yet the cells remain connected in series.


Sounds interesting but unfortunately usually futile from what I have seen. Perhaps post a block diagram or concept schematic.

The need or benefit for on board balancing in itself is debatable whether or not it is dissipative or active.

Dissipative seems to always win on simplicity and cost of implementation. There is usually little capacity mismatch between cells so trying to redistribute charge to increase range of an EV becomes a mute point in almost all cases. There may be some exceptions any may be more as battery packs age.

So lets assume that you are trying to align SOC to protect from over discharge and over charge.

Having a dissipative or active balancing method based on voltage differential has some pitfalls with these cells. The only time the voltage has any significant difference is when the cells are at the extremes of the their state of charge.

Staying away for the upper and lower ends of the SOC may be one of the ways to increase the cell cycle life. There is also some concern that maintaining the voltage differential at the upper end during balancing could have some unknown side effects. Time spent above open circuit voltage is a consideration. We do not trickle charge.

If you have a way to selectively remove (dissipative) or distribute(active) charge independent of cell voltage then you have a balancing system that I would consider. This charge control feature would then be available to the controller that predicts and manages the SOC of each cell.

Regards
Jeff


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## Jerry Liebler (Feb 1, 2011)

Jeff,
To get the usual cost consideration out of the picture assume the acive balancer is free. With that out of the way, Consider what would happen to a pack if it was active whenever the pack was either being charged or discharged. The effect would be perfect top balance at the end of each charge AND perfect bottom balance at the end of full discharge. This means that the FULL capacity of all cells,between the voltage per cell limits chosen, is recovered while either top balancing or bottom balancing leaves some capacity un-recovered unless the cells are perfectly matched.


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## pm_dawn (Sep 14, 2009)

In theory it sounds really good !!!
But when you start to look at the numbers then it really starts to get out of hand. If that shuttling BMS is to do any good in the lower parts of the SOC area it needs to be able to shuttle ALOT of current. It needs to be able to move just as much as the motorcontroller can take from the battery. 

Probably not going to play.

Just so much simpler to match the capacity of the cells in the pack.

Regards
/Per


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## jddcircuit (Mar 18, 2010)

Jerry Liebler said:


> Jeff,
> To get the usual cost consideration out of the picture assume the acive balancer is free. With that out of the way, Consider what would happen to a pack if it was active whenever the pack was either being charged or discharged. The effect would be perfect top balance at the end of each charge AND perfect bottom balance at the end of full discharge. This means that the FULL capacity of all cells,between the voltage per cell limits chosen, is recovered while either top balancing or bottom balancing leaves some capacity un-recovered unless the cells are perfectly matched.


Ok, I am all for the hypothetical

I have a pack of cells with significantly different capacities and I don't want to be limited by my lowest capacity cell.

Lets see some details.


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## frodus (Apr 12, 2008)

I built a system for Lead with my buddies at Synkromotive a few years ago. We used 6 - 48V to 13.8V 75W DC-DC converters. We built a board that went on top that controlled the output voltage and the enable. We used another DC-DC that converted pack voltage to 48V. This would allow us to take energy out of the pack and put it into the 48V DC bus. We also used a 48V power supply that would hook to the 48V DC bus for charging. Everything was always balanced perfectly. I don't know of any commercial solutions that do this, as it can get spendy. I lucked out because I bought surplus.

On charging, the pack would be charged perfectly every time. On discharge, we could bump up one cell just a little bit while driving, but it didn't make a huge difference with only 5A output. While static, we could take energy out of the pack and put it into one of the batteries and self balance. It worked well for charging and while sitting, but discharging didn't really do too much.

If you want to do something like this, the power transfer needs to be much higher than a few amps. 75W (~5A) didn't do much for us while discharging. If I wanted to shuttle more energy into the cells, I'd have to have 4-5 times that current to make a dent. 

I got all of the DC-DC converters on ebay surplus and cheap. You'd need higher wattage DC-DC, which start to become more expensive and much bigger and require heatsinks. 

I used Vicor and look for them now and then on ebay. Right now there are some 48V to 3.3V 75W DC-DC converters on Ebay for $10 and he has more than 10. They can be adjusted up and down 10% or so, which makes them adjustable up to ~3.63V, which is perfect for Lifepo4. At $10 they're not bad to play around with. I've got a handful of them at home I use for charging single cells.



All that being said, if you wanted to shuttle from high cells to low cells, you'd need all of those DC-DC's to be bidirectional (to inject into the Bus, as well as charge from it), which adds a ton of cost and you'll likely need to design them yourself. Not really as practical as taking energy at the pack level IMHO.


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## Ziggythewiz (May 16, 2010)

Just stick them all in parallel and run a fat inverter off of it.


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## Jerry Liebler (Feb 1, 2011)

Travis,
You have seen the concept work, thank you for sharing that. You also pretty well understand what I'm planning. Basically a bidirectional DC to DC converter per cell with one side of each converter connected in parallel to a DC bus and the other side connected to it's associated cell. Without getting into a bunch of detail I can say there is NO cost to making the conversion(s) bidirectional. I can afford to sell a system of this design for about $15/cell, the cost of a Mini-BMS.

Per,
My initial design has, for cost containment, a compromised ability to shuttle current. Even so it's pretty substantial with a 0.1v difference resulting in 2 amps of balancing current.


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## jddcircuit (Mar 18, 2010)

Jerry Liebler said:


> Travis,
> You have seen the concept work, thank you for sharing that. You also pretty well understand what I'm planning. Basically a bidirectional DC to DC converter per cell with one side of each converter connected in parallel to a DC bus and the other side connected to it's associated cell. Without getting into a bunch of detail I can say there is NO cost to making the conversion(s) bidirectional. I can afford to sell a system of this design for about $15/cell, the cost of a Mini-BMS.
> 
> Per,
> My initial design has, for cost containment, a compromised ability to shuttle current. Even so it's pretty substantial with a 0.1v difference resulting in 2 amps of balancing current.


Jerry,

If your design rely's on terminal voltage then I have some concerns.

I am seeing a difference in voltage sag between cells under a discharge. So in this case I assume you would not be shuttling charge based on terminal voltage since the cells are not out of balance they simply have different internal resistances creating different terminal voltages.

The only way that I can imagine doing effective charge shuttling during discharge would be to reference the Ah counting and have a known look up table for the capacity of each cell. This way your algorithm could align the % SOC of each cell even though the absolute amount of charge is different.

Seems complicated but you may have a simple solution.

Edit:
Or perhaps as soon as your pack is not completely full or has enough head room your system could redistribute charge so each cell is the same Ah from empty. This would be like having a bottom aligned pack on the fly.

Regards
Jeff


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## frodus (Apr 12, 2008)

That wouldn't really matter though.... think about it. If one cell is lower than any of the other cells, it gets balance current from the bus. I agree, you don't want it to work on a pre-calculated terminal voltage, but a delta between that cell and the next lowest. All you care about is that the cell is lower than the rest.


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## jddcircuit (Mar 18, 2010)

frodus said:


> That wouldn't really matter though.... think about it. If one cell is lower than any of the other cells, it gets balance current from the bus. I agree, you don't want it to work on a pre-calculated terminal voltage, but a delta between that cell and the next lowest. All you care about is that the cell is lower than the rest.


I am not exactly sure what you are referring to but I thought a little more about it.

If one cell voltage is lower than the others during discharge due to higher internal resistance and as a result receives charge from the bus to compensate for the reduced voltage, then it may be receiving charge that could potentially be pushing it out of SOC alignment.

In this case the difference in voltage does not correlate to SOC difference so I would be cautious having a design that assumed it to be.

I think.
Jeff


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## frodus (Apr 12, 2008)

So what if it drifts out a few mAh on that cell? It'll rebalance upon charging, or when the vehicle is stationary. Bottom and Top balancing is done by balancing top or bottom voltages, not SOC.

Its unlikely that while discharge the power is actually going into the cell.....That current from the DC-DC will not likely go into the cell, but into the series path.... and reduce the current that the cell is actually discharging, lowering the power being dissipated through heat due to a mismatched IR, and voila, your voltage sag is reduced and the cell pops back up.

Is it a perfect system? No, but no BMS that I've seen measures current/voltage of every cell and calculates individually the SOC. There's really no point. Watch the top and bottom voltages while driving and estimate SOC at the pack level based on historical data.


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## Jerry Liebler (Feb 1, 2011)

Travis,
You have described how it'll work, balance currents will flow to raise the lower voltage cells while lowering the higher voltage cells until they all have the same terminal voltage. What that terminal voltage is is a function of the state of charge but simply does not affect the balance process. 

Jeff,
During discharge balance currents will flow to reduce the sag of the cells with higher internal resistance, without any complex calculations or controls, just by the nature of the bidirectional DC to DC process. While my initial designs has a "balancing resistance" of 50 milliohms it could be considerably lower for more cost. Thinking of the effect on discharge slightly differently, the effect on sag is equivalent to shunting the internal cell resistance of the highest resistance cell with 50 milliohms.so if all but one of the cells were say 1 milliohm and the outlier were 1.02 milliohm the result would be equal sag on all cells including the outlier.


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## frodus (Apr 12, 2008)

Jerry Liebler said:


> Travis,
> What that terminal voltage is is a function of the state of charge but simply does not affect the balance process.


Exactly, You're not trying to make an SOC gauge, you're trying to make a BMS, and all you care about is which cell is too low, and which cell is too high so that you can shuttle charge between cells that don't need the energy to cells that do. Actively balancing the cells will slow that low cell from reaching the lower limit.


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## Jerry Liebler (Feb 1, 2011)

Travis,
You keep referring to an active balancer as a "BMS" I prefer to call it a "NON BMS cell balancer". What has come to be known as a "BMS" is a collection of monitoring and some control circuitry that may or may not even include balancing hardware. To me the balancing function should be primary, that's what I've designed. When an active balancer is operating it becomes rather difficult to gather data on individual cells, just like when cells are paralleled, but really what good is all that data?


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## Jerry Liebler (Feb 1, 2011)

Deleted duplicate post


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## frodus (Apr 12, 2008)

Jerry Liebler said:


> Travis,
> You keep referring to an active balancer as a "BMS" I prefer to call it a "NON BMS cell balancer". What has come to be known as a "BMS" is a collection of monitoring and some control circuitry that may or may not even include balancing hardware. To me the balancing function should be primary, that's what I've designed. When an active balancer is operating it becomes rather difficult to gather data on individual cells, just like when cells are paralleled, but really what good is all that data?


I keep calling it a BMS because that is what the EV industry calls it, and I don't care what you prefer to call it...... its still a system that manages your batteries. Just because it doesn't have shunts or monitor the cells doesn't mean its not a type of BMS.

I've built active balancing, shunt balancing, charge shuttling (with capacitors) and simple monitoring systems and know all about the different types out there, not to mention having worked with places like Synkromotive, Manzanita Micro and Elithion.

You're still managing your batteries.


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## Jerry Liebler (Feb 1, 2011)

Travis,
You are right, I stand corrected. It certainly is a BMS, I'll work on a new name,NON BMS is simply not appropriate. Right now I kinda like "Active Battery Management System" or ABMS


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## jddcircuit (Mar 18, 2010)

If you don't like the term BMS then call it a CVE (Cell Voltage Equalizer). Sounds like a slick way to do the top or bottom SOC alignment on a string of cells. I would consider this part of my BMS.

We all agree that this terminal voltage correlates to SOC very well at low currents at both ends of the SOC region. Not so much under heavier loads or during fast charging throughout the in between region.

I am just getting started cycle testing some LFP cells and it is becoming increasing more clear from analyzing the cell voltages through the cycle that a BMS that solely uses the terminal voltage to dynamically align the SOC through the cycle could potentially have detrimental effects on SOC alignment.

Best Regards
Jeff


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## Frank (Dec 6, 2008)

Do a search on "Lee Hart battery balancer" to see info on a similar device for PbA cells. This is a fun idea IMO but if you're using your batteries so hard that you have to rely on one or two AH you'd be better off buying more capacity. Keep working on it though: you never know where these things lead.


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## Jerry Liebler (Feb 1, 2011)

Jeff,
Are you really saying there is a problem operating cells in parallel? What I'm building does just that through transformer isolated bi-directional DC to DC converters.


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## Jerry Liebler (Feb 1, 2011)

Jeff,
Why bother with state of charge alignment? It certainly is not a concern when cells are operated in parallel (which is the effect of a CVE as I've described). The real objective of any BMS is to prolong the life of the pack even when it is subject to abuse. Over discharge and overcharge are cell killers so it's the role of the BMS to prevent them.Theoretically it's possible to design a CVE that allows any cell to be removed and still support peak discharge currents ,it's however not practicle or reasonable economically. What I'm building will keep all cells within 0.25 volts of each other for ANY load or charge current below 5 amps. Regardless of the cells characteristics if it's not shorted. In situations where more than 5 amps are involved it might be prudent to detect over or under voltage conditions as well. Regardless, protecting the cells is totally unrelated to state of charge or it's alignment!


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## jddcircuit (Mar 18, 2010)

Jerry Liebler said:


> Jeff,
> Are you really saying there is a problem operating cells in parallel? What I'm building does just that through transformer isolated bi-directional DC to DC converters.


I am sorry if it sounds that I was saying that there is a problem operating cells in parallel. What I am saying is not that at all.

What I was saying was that in my situation if I had a system that moved charge between the serially connected cells that I am using based on their cell voltages alone it would undoubtedly create a misalignment in the SOC of my cells during normal operation at any given point in time. I understand that this self imposed SOC imbalance could be considered temporary and hopefully benign if it is remedied at the ends of their SOC when the cell voltages started to correlate more towards the actual SOC. SOC alignment is the primary concern of my battery management strategy so I may be over sensitive to it.

The question for me becomes what is the value of creating a SOC imbalance at one point in time and then fixing it at a later point in time. Perhaps this is a necessary condition to increase the available energy in a pack and some applications may want to use all the energy in the pack and others may have cells with high variation in capacities. So I support your design for those reasons.

Not to offend but if I have an application that does not use all the energy in the pack then it makes me think of that crazy character, Jack Rickard, calling it a Battery Masturbation System.

Jeff


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## jddcircuit (Mar 18, 2010)

Jerry Liebler said:


> Jeff,
> Why bother with state of charge alignment? It certainly is not a concern when cells are operated in parallel (which is the effect of a CVE as I've described). The real objective of any BMS is to prolong the life of the pack even when it is subject to abuse. Over discharge and overcharge are cell killers so it's the role of the BMS to prevent them.Theoretically it's possible to design a CVE that allows any cell to be removed and still support peak discharge currents ,it's however not practicle or reasonable economically. What I'm building will keep all cells within 0.25 volts of each other for ANY load or charge current below 5 amps. Regardless of the cells characteristics if it's not shorted. In situations where more than 5 amps are involved it might be prudent to detect over or under voltage conditions as well. Regardless, protecting the cells is totally unrelated to state of charge or it's alignment!


It sounds interesting. I am all about prolonging cell life.

My application is way over 5 amps which is why I will have to consider other factors.


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## frodus (Apr 12, 2008)

I'll meet ya halfway and call your product the CVE.

Regardless of the SOC, I see that your primary goal is to equalize the cell voltages across the entire pack. I'm sure secondarily you may be able to figure out a way to measure SOC by adding a current sensor coming out of the pack and measuring the pack voltage, but there are other devices that do that standalone.

I would suggest that you do have a way to sense cell voltage on each of the devices. If it goes above threshold 1, flash warning light. If it goes above threshold 2, have the master (or an add on board) cut off the charger. If the cell goes below threshold 1, flash a warning light, if the cell goes below threshold 2, force the discharge to either stop or ground out throttle to reduce the discharge. Overtemperature is another option.

Our system worked well. We used isolated RS485 to go between devices. Each board had a fuse, RS485 connection, DC-DC converter, Microcontroller, Status LED's and IIRC, a small shunt to monitor current into the cell.


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## Jerry Liebler (Feb 1, 2011)

Travis,
You mentioned your system had fuses. I'd like to incorporate a fuse per cell but have to decide what the purpose of the fuse actually is to locate it effectively. Does the fuse protect the DC to DC converter only (my current choice as the most likely cause of a blown fuse will be installation error)? Should the pack and system continue to function with one shorted cell and it's fuse blown? I think the answer is yes and have designed accordingly. An open cell will also blow a fuse but in that case the pack is definitely out of operation and, unless the fuses are rated for the full pack voltage, arcing and destruction of the fuse and it's surroundings are likely. BTW a fuse and holder add about $2 to the BOM cost & that's using automotive fuses rated at 32 VDC. If I were to add voltage limit detectors I'd place them on the DC to DC converter side of the fuses to protect them but that precludes detecting the shorted cell and blown fuse.


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## frodus (Apr 12, 2008)

The fuse protects the battery from a short in the DC-DC converter on mine.... but on yours, you'd be protecting both from eachother, since you're bidirectional.... so I'm not sure.


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## GerhardRP (Nov 17, 2009)

frodus said:


> If you want to do something like this, the power transfer needs to be much higher than a few amps. 75W (~5A) didn't do much for us while discharging. If I wanted to shuttle more energy into the cells, I'd have to have 4-5 times that current to make a dent.
> 
> I got all of the DC-DC converters on ebay surplus and cheap. You'd need higher wattage DC-DC, which start to become more expensive and much bigger and require heatsinks.
> 
> ...


This is what you want. http://cdn.vicorpower.com/documents/datasheets/vibrick/VI_BRICK_48V_4V0_200W_BCM.pdf
Gerhard


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