# Charge shuttling BMS chip



## Elithion (Oct 6, 2009)

Well, well: Linear Tech beats Texas Instruments.

TI first offered such a solution, though it was fundamentally flawed. It then quietly withdrew it. 
It looks like LT got it right.

I'll add it to the list of Li-ion BMS chips.


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## Ziggythewiz (May 16, 2010)

Any idea what this bit means?

"The part utilizes a nonisolated bidirectional synchronous flyback topology to balance up to 6 series-connected cells. Charge can be transferred between a selected cell and 12 or more adjacent cells. "


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## ga2500ev (Apr 20, 2008)

Ziggythewiz said:


> Any idea what this bit means?
> 
> "The part utilizes a nonisolated bidirectional synchronous flyback topology to balance up to 6 series-connected cells. Charge can be transferred between a selected cell and 12 or more adjacent cells. "


In short it's a DC/DC setup where a cell that is fully charged can send power to cells that are not yet fully charged. This paper describes the process:


ga2500ev


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## Elithion (Oct 6, 2009)

Ziggythewiz said:


> Any idea what this bit means?


It means that, you can use two such ICs with a 12-cell battery; you can transfer energy from any one cells to the entire battery (cell-to-battery balancing); you can also go the other way: battery-to-cell.










That's what LT did right: cell-to-battery (or battery-to-cell) is more efficient for larger batteries (compared to cell-to-cell, which is what TI tried to do and failed).

That only works for a 12 cell battery.

But here at DIY we have much bigger batteries. 

Here comes the 2nd way that LT got it right (and TI didn't).
You can use this scheme to do high voltage battery packs.









It combines the idea behind cell-to-cell (bucket brigade) but applies it to a larger scale (6-cell block to 6-cell block).

Pure genius!


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## frodus (Apr 12, 2008)

This is awesome! A real active BMS, not passive like shunts. This is super cool!

DC-DC's can be pretty high efficiency. Rather than shunting and wasting all that energy as heat, the DC-DC here is bidirectional, so it can shuttle charge to and from cells actively.


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## Ziggythewiz (May 16, 2010)

Elithion said:


> It means that, you can use two such ICs with a 12-cell battery; you can transfer energy from any one cells to the entire battery (cell-to-battery balancing); you can also go the other way: battery-to-cell.


I just didn't understand why they threw "12 or more" in there while talking about "the device" instead of saying "with multiple balancers working together charge can be transferred between any number of other cells".


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## Elithion (Oct 6, 2009)

frodus said:


> Rather than shunting and wasting all that energy as heat, the DC-DC here is bidirectional, so it can shuttle charge to and from cells actively.


That is not the whole picture.

For the typical Li-ion application needing only maintenance balancing, all non-dissipative (active) balancers that I analyzed before waste more power overall than plain old dissipative (passive) balancing, because they are on stand-by 24/7, while dissipative draws power only while balancing.

Not in the case of these ICs, though.

The typical dissipative balancer, used with the typical traction pack, generates 600 J of heat a day to balance a cell.

These LT ICs draw 16 uA all the time, which is 5 J a day from each cell.
Plus, 7 % of 600 J (due to inefficiency) = 42 J of heat.
So, a total of 47 J of heat from LT non-dissipative balancers vs 600 J for dissipative balancing: 12 times better. 
Does the extra complexity warrant the savings?

For gross balancing, when we're talking of 50000 J or so, then yes, these ICs do save a lot of energy! But this done once or twice in the life of the pack. Does the extra complexity warrant the savings for once or twice the life of the pack?

Some think so. For those people, this IC is great.


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## frodus (Apr 12, 2008)

Ahh, good point... Sorry Davide.


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## Lauris_K (Feb 25, 2013)

Well efficiency of active balancers not the only point. And I rather would say not the major also. Basically in my point of view, best thing about active balancing is top and bottom balancing possibilities. So in every charge you can get all energy in pack what is in it. And in older battery pack when cells became more disbalanced and specially when weak links starts to appear, sometimes you might get in situation when one discharged cell stops you just few kilometers away from your destination where rest of the cells could easily cover that distance.


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## Elithion (Oct 6, 2009)

Lauris_K said:


> So in every charge you can get all energy in pack what is in it.


I am afraid you're confusing "balancing" with "redistribution".

"Redistribution is a technique that shuffles energy in a battery in such way that all of its energy can be used. While discharging, it involves taking additional energy from the cells with the highest capacity, so that the cells with the lowest capacity are no longer the limiting factor in the battery capacity. "


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## Lauris_K (Feb 25, 2013)

If I'm not mistaken active balancing goes toe to toe with redistribution, since whole method of active balancing is to transfer charge from more charged cells to less charged ones.
And in passive balancing case that is done by burning excess energy at end of charging to keep all cells in battery at same level.
While in active balancing since it can transfer charge from cells to cell it can do that on top (end of charging) and bottom (close to discharging), middle too, but I don't see any reason why to do that. so decently implemented active balancing method works as redistribution by default.

Or I'm missing something?


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## Elithion (Oct 6, 2009)

Yes, the hardware is the same for non-dissipative balancing and for redistribution (other than the fact that redistribution takes DC-DC converters that are one order of magnitude larger).

But the software (the algorithm) is significantly different. The correct implementation of Redistribution is rather more complicated (and well beyond the scope of this forum).


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## Lauris_K (Feb 25, 2013)

Well actually not that not that different, specially if balancer unit contains coulomb counter. Other case that is bit more tricky, since difference in voltage starts to appear too close to depletion and either you have to have really powerful DC/DC balancing converters, or you need to limit motor power a lot to keep battery in safe area of operation.

Laurynas.


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## JRP3 (Mar 7, 2008)

I don't see the point of redistribution since all cells should be very close in actual capacity. If not then you have a cell problem.


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## Elithion (Oct 6, 2009)

JRP3 said:


> I don't see the point of redistribution since all cells should be very close in actual capacity.


Yes, I tend to agree. Yet, we get a lot of requests for it.

That white paper about redistribution shows that there is a cutoff point beyond which it is cheaper to buy extra cells that to do redistribution:


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## sholland (Jan 16, 2012)

Elithion said:


> Well, well: Linear Tech beats Texas Instruments.
> 
> TI first offered such a solution, though it was fundamentally flawed. It then quietly withdrew it.
> It looks like LT got it right.
> ...


I wouldn't speak so soon... 

This solution looks compelling at first glance, but the devil is in the details... 
1. It's far from $1 per channel. With custom magnetics and other external components, figure more like $3 per channel.
2. The flyback converter will never get the quoted efficiency at higher currents.


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## sholland (Jan 16, 2012)

JRP3 said:


> I don't see the point of redistribution since all cells should be very close in actual capacity. If not then you have a cell problem.


Active balancing really comes into its own if you want get to 90-95% depth of discharge. The SOC curve 'knees' can come on at different points in even the closest matched cells. Factor in temperature variation across a pack and all of a sudden near real-time compensation for mismatch can become necessary.


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## Siwastaja (Aug 1, 2012)

sholland said:


> Active balancing really comes into its own if you want get to 90-95% depth of discharge. The SOC curve 'knees' can come on at different points in even the closest matched cells. Factor in temperature variation across a pack and all of a sudden near real-time compensation for mismatch can become necessary.


What you are describing is real-time redistribution. It's the best among the best, but you need to realize it needs quite powerful DC/DC converters to supply enough power to overcome the capacity differences at high discharge rate. For example, a 100 cell LiFePO4 pack in a typical car averaging 10 kW consumption, 10 kWh battery pack and 10% capacity difference would need 10 watts (3A) per cell to do real-time redistribution. 100 pcs of 10-watt isolated DC/DCs do cost something. If you drive city speeds and have a lot of stops, you can live with lower redistribution power, but it's still a lot.

The algorithm needs to remember which cells did get empty first and that way keep book of the approximated real capacities of each cell. Then the system can do redistribution during the drive, or even when stopped, as long as there are no full cells. It's not difficult on paper, but needs testing.

I prototyped small homemade flyback transformers with a very simple bidirectional topology which allowed me to balance at 5...10W. Look at the typical flyback circuit; it has one mosfet, the transformer and a diode in output. Replace the diode with another mosfet, and you have a bidir flyback. It works pretty well and I had around 80% efficiency, without litz wire and with some random ferrite cores I don't know anything about. 

These transformers were indeed small and easy to make, but I doubt it's worth the hassle anyway. The point I had is that we had the ferrite cores for free and the winding was easy and fun. But if you try to buy these things, it will cost a lot more than a passive BMS.

So, my idea is to utilize the redistribution if we, for some odd reason, could come up with a big lot of cells for very cheap which do differ in capacity more than normally but are otherwise good. Probably not going to happen.


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## sholland (Jan 16, 2012)

Siwastaja said:


> What you are describing is real-time redistribution. It's the best among the best, but you need to realize it needs quite powerful DC/DC converters to supply enough power to overcome the capacity differences at high discharge rate. For example, a 100 cell LiFePO4 pack in a typical car averaging 10 kW consumption, 10 kWh battery pack and 10% capacity difference would need 10 watts (3A) per cell to do real-time redistribution. 100 pcs of 10-watt isolated DC/DCs do cost something. If you drive city speeds and have a lot of stops, you can live with lower redistribution power, but it's still a lot.
> 
> The algorithm needs to remember which cells did get empty first and that way keep book of the approximated real capacities of each cell. Then the system can do redistribution during the drive, or even when stopped, as long as there are no full cells. It's not difficult on paper, but needs testing.
> 
> ...


You have very good understanding  

Solutions that provide exactly what you describe are already available to system designers. You will find this type of system in really high capacity packs servicing commercial vehicles, buses, grid storage, UPS, etc. These applications cannot be balanced by passive balancing...

And the BOM cost is not as high as you might think...


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## JRP3 (Mar 7, 2008)

sholland said:


> Active balancing really comes into its own if you want get to 90-95% depth of discharge. The SOC curve 'knees' can come on at different points in even the closest matched cells.


I don't buy this. A pack should have at least a 1% or less variance between cells and a 90-95% DOD won't cause an issue since you still have at least 4% of buffer.


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## Elithion (Oct 6, 2009)

sholland said:


> I wouldn't speak so soon...
> 
> This solution looks compelling at first glance, but the devil is in the details...
> 1. It's far from $1 per channel. With custom magnetics and other external components, figure more like $3 per channel.
> 2. The flyback converter will never get the quoted efficiency at higher currents.


I agree on both. Thanks.


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## sholland (Jan 16, 2012)

JRP3 said:


> I don't buy this. A pack should have at least a 1% or less variance between cells and a 90-95% DOD won't cause an issue since you still have at least 4% of buffer.


I don't think you could ever use 100% capacity, as the extreme areas in the SOC curve (example below) move so fast it would be dangerous to load the cell. You can operate into those regions if you can move enough current to compensate for that change in SOC. These big changes in slope of this curve are what I see varying from cell to cell, far more than middle of the curve SOC variation. With active balancing all cells can go down together following the same curve profile. 

With passive balancing, you won't see anyone operating into those regions as the mismatched loading can really affect the SOC mismatch for the entire subsequent charge cycle and top balance. It may even take several charge/discharge cycles to get to balance with large capacity cells. With active balancing you can balance at all times (not just near top) and babysit the cells through the entire SOC curve.


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## JRP3 (Mar 7, 2008)

I didn't say anything about using 100% of capacity, you were talking about 90-95% and I explained why that should not be an issue.


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## PStechPaul (May 1, 2012)

Something to consider is that cells larger than an individual pouch or cylinder are really multiple cells internally connected in parallel. The larger cells may appear to be well balanced with other cells in the same lot because the capacity is the average of all the cells in the unit. But the individual cells may exhibit a much larger variance due to manufacturing tolerances as well as the construction itself, where the inner cells may run hotter than those at the outer edge, and they are also subject to more compression. 

Also, even a single pouch or cylinder is essentially a parallel connection of an infinite number of individual cells that vary because of differences in the thickness and chemical composition of adjoining anode/cathode pairs, as well as the variance of the electrical current paths among various points in the cell. This may be why cells are damaged when used at more than 80-90% DOD, because at that point, many of the "virtual microcells" have been depleted and cause circulating currents within the cell.

This effect is probably proportional to the amount of current being drawn from the cell, and without external current the healthier "microcells" tend to charge their weaker neighbors which causes the cell to recover. 

A similar effect may occur during charging, which is why the current must taper off at the top of charge.


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