# Active Battery Balancer - Ideal design? Never seen



## dcb (Dec 5, 2009)

Ive seen it or similiar.

Consider that 
1. Batteries don't really go out of balance on the scale of weeks/months/???
The need for post-initial balancing is rare.

2. Manufacturers can capacity match cells.

3. Most batteries are routinely not charged to absolute full capacity, or discharged to absolute empty anyway because it shortens lifespan.

4. Active balancing needs to move a lot of current (bulky and $$$) to be of any real practical value. Lets say one cell has 10% less capacity than the others. The system has to shuttle that charge equally from all the other cells to that cell while the car is driving, which could be less than an hour, to get the most out of the pack, which nobody does anyway. The balancer would have to move 1/10 the discharge rate to the lowest cell in that example. if one cell is really much lower capacity than the others, replace and rebalance. If you have 180ah cells and can burn through them in an hour, then you need to be able to pump ~18 amps into the lowest cell continuously.

cell level monitoring is nice, if it doesn't increase the chance of failure or add imbalance, shunting can be an inexpensive add on once you figure out cell level monitoring/communications, though it is often used as a "brain dead" top balancer, would be better as a smart bottom balancer, and used only when there appears to be a problem.


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## psron (Jun 19, 2012)

Well, that's the nice thing about a public forum... we can all decide for ourselves what we think is important for our own projects.

If it were a project that I was retaining responsibility for all maintenance over, and could check regularly to see if cells were going out of balance and resolve issues when they occur, then your comments might be applicable.

This is not that case. 

Some of us do projects for other people, and in such a situation, as the system designer/installer, I need to ensure long-term performance without my ever touching that system again... unless the owner asks me to.

Sorry if I seem coarse, but I tire of people who like to offer opinions instead of answers, when it was a very specific inquiry... which was:

*Has anyone ever seen this implemented in a real product?*

I have to believe there are even DIY projects where the owner wants ultimate reliability and "hands-off" after it's done... possibly in the case of a family car, not just used by the person who installed it. If I put my wife and family into a project car, I would be very concerned that it would perform flawlessly, or at least as close to that as possible, without burdening my wife to "keep watch" over it.

This charge balancing scheme is easily the most powerful one I have run across... it can be designed to transfer up-to 100 Amps, if so desired... or more, and doing so in real-time, even during nominal discharge is an incredible benefit... there will no longer be one single cell that will be the determining factor on those last few usable miles.


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## dcb (Dec 5, 2009)

The only active BMS's I know of are used on ES bicycles, to unknown effect. Even tesla uses a passive system.

You don't see them because it is far far far simpler and cost effective to add another battery if you need a bit more range (or cant reasonably match cell capacities in a pack).

And I provided realistic examples of why active isn't worth the complexity/expense/risk/time. I've considered active balancing for a long time, and it seems a solution in search of a problem. If you need hands free then passive makes the most sense, and there bottom balance makes the most sense. The only active part should be monitoring for out of balance condition and prompting the user that it is time to press the bottom balance button or something like that.

You're welcome, no charge.

Edit:


psron said:


> I have to believe there are even DIY projects where the owner wants ultimate reliability and "hands-off" after it's done... possibly in the case of a family car, not just used by the person who installed it. If I put my wife and family into a project car, I would be very concerned that it would perform flawlessly, or at least as close to that as possible, without burdening my wife to "keep watch" over it.


I need to clarify, you seem to think that active is the only automated way to balance a pack, or that the pack should routinely be run %100 to %0. Both of these are incorrect assumptions. Or that an active system will somehow (despite the increased complexity and obvious failure modes available) be more reliable. 

If you bottom balance, and routinely discharge to %20, you still have %20 for emergencies without much danger of trashing cells.

If you want reliable, you won't charge %100 and discharge to %0 on a routine basis.


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## psron (Jun 19, 2012)

> I need to clarify, you seem to think that active is the only automated way to balance a pack, or that the pack should routinely be run %100 to %0. Both of these are incorrect assumptions. Or that an active system will somehow (despite the increased complexity and obvious failure modes available) be more reliable.


Now _*I*_ need to clarify... 

1) Where did I say anything that made you say "you seem to think that active is the only automated way to balance a pack"??

2) What did I say that indicated to you that I think "that the pack should routinely be run %100 to %0" ???

Both of these are incorrectly attributed to me.

This is typical of many of your posts... sorry to inform you. You form an opinion (which you certainly are entitled to), but then you presume thoughts on other people... you create your own arguments, for the sake of arguing.

You seem to have a complete aversion to even considering the merits of anything except what you have formed your own strong opinions on... if it's not what you like, it's not worth considering.

All I was doing at the onset was bringing to light a new balancing concept that I found technically interesting, so as to get responses back on THAT idea.

Passive/shunt balancing is based on the complete opposite main principle of making EV's - which is to be more "green" and save energy. Shunt balancing results in wasting energy that was put into the battery pack. 

That's one of the main reasons that I, personally, support active balancing methods.

Is it "the only" method that accomplishes the goal of balancing? Certainly not... but it eliminates most of the energy waste.

*Yes, I do believe we should expect to use more than 60% of our battery pack capacity* (or 56% as stated in your last response). You stated we should charge no higher than 76% SOC, nor discharge no lower than 20% SOC, which results in only using 56%.

The prevailing reasons NOT to use much higher percentage of the rated capacity (from white papers I have read from battery/cell manufacturers) are:
1) to reduce possibility of overcharge, and plating-out lithium metal, which causes serious instabilities, and
2) to reduce the possibility of cell dropout, where a cell goes to zero and then into reverse polarity... which also is extremely dangerous

With a properly designed and sized active redistribution system, this is no longer an issue, and far more of the pack capacity is usable.

Are there other methods to prevent these two dangers? Sure! Monitor, and terminate use, or at least drastically reduce the load (turtle mode). But this doesn't address the _*problem*_, only the _*symptom*_! I say resolve the problem BEFORE it happens, so that you don't need a "limp home" mode (or at least not so crazy-early in the discharge cycle).

None of the circuits, topologies, or components are new... this is based on technologies that are decades old, but with the advantages of the latest in amazing semiconductor performance (who could have ever imagined a 211Amp MOSFET with 500*µ*Ω RDSon for less than $1???). The thing that's hard to understand for the non-EE, is that the circuit power level required to achieve the results is pretty small, because it's not moving "large chunks" (coulombs) of energy per "bite"... but rather very small bites, thousands of times per second.

It's similar to Toyota's bidirectional flyback Buck/Boost converter... the entire power of the battery pack flows through that one inductor (they use the term "reactor"), in both discharge (to boost the battery pack voltage) and charge/regen modes (reduce generated voltage down to battery pack level). One inductor, about 8 cu. in. in volume.

If the technology and components were not mature, we would not have major automobile companies producing extremely reliable electric vehicles... but they are... and have been for several years. In fact, from my observations, the predominant failures haven't been the electronics... it's been the batteries.

Eliminating individual cell stress on a constant basis, regardless of the level of that stress, goes a long ways towards extending the useful life of every multi-cell battery pack. 

Why throw more/bigger cells at it, if the "problem" can be designed out? Additional battery capacity is EXTREMELY expensive... and uses even more precious materials... electronics have neither problem. Both approaches can resolve the issue... one uses finesse, the other uses brute force, added weight, and more money.

Car manufacturers are forced to be ultra-conservative... due to the runaway lawsuits in this country. They have to test new tech for nearly a decade before releasing it to the public. The DIY community does not have these restrictions... we should be leading the way, not playing catch-up.

I just don't see where you don't "get it"... other than a closed mind.

NO... I'm not saying this is for you... several times I have said that for the DIYer who doesn't mind spending the extra time, effort, and money on maintaining his own system... that's great!! That's what made the DIY market what it is today!!! For some, it's the only way possible... but we should not try to prevent others from pushing the envelope, to advance the market.

Technology continues to advance... some chose to embrace it and advance the DIY market, others don't... and that's their prerogative.


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## dcb (Dec 5, 2009)

psron said:


> but it eliminates most of the energy waste.


Not really. If the self discharge is a few mA difference, it is going to be a waste of energy/expense to overengineer a solution (and it is a lot more complicated than you realize). If you have to move charge, one converter at a time in a 96 cell pack... Like I said, this thesis is the bubblesort of active balancing, it sucks, not really an opinion, just the way it is.



psron said:


> *Yes, I do believe we should expect to use more than 60% of our battery pack capacity*


The capacity diminishes at the cell level, active balancing isn't going to get you 4x the mileage like not beating the crap out of your cells will. Obviously there are tradeoffs, but if you want the pack to last a long time you stress it less. This isn't really a matter of opinion either.



psron said:


> But this doesn't address the _*problem*_, only the _*symptom*_!


Swing and a miss, if one of your cells capacity is way out of whack with the others, THAT is the problem.




psron said:


> Why throw more/bigger cells at it, if the "problem" can be designed out?


because you are putting a band aid on it. Individual cells don't like 0-100% cycling, period.



psron said:


> Additional battery capacity is EXTREMELY expensive... and uses even more precious materials...


So don't abuse them and shorten their useful life by a factor of 4. If an 80ah cell can go as far as a 60ah per charge and has 4x the cycle life... You are advocating premature death of cells. The very waste you think you are curing. You don't know enough yet to be designing this stuff. 



psron said:


> Both approaches can resolve the issue... one uses finesse, the other uses brute force, added weight, and more money.


Nope. You don't even seem to understand the nature of problem. There are large returns on cycle life for relatively small increments in capacity. You are (without any data) saying this can compete somehow, magically, if/when cell capacity decides to diverge, perhaps years down the road, maybe, after cell capacity has already diminished by a wide margin...


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## psron (Jun 19, 2012)

> You don't know enough yet to be designing this stuff.


Well... that's interesting. I've been designing electronics as a profession since late 1980. You must have more experience in design than I do... to make such a bold statement.


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## sholland (Jan 16, 2012)

Here you go:
http://www.ti.com/tool/TIDA-00239

I have it managing my 160Ah Thundersky's 24/7...

I have also seen this modified to be fully isolated to transfer charge to/from a 12V or 24V isolated DC source (e.g. lead-acid battery). In this implementation the BMS can act as a DC-DC for the 12V battery.


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## dcb (Dec 5, 2009)

psron said:


> Go to page *44* as numbered; Section 4.2 "Proposed BMS Design"


Went back and looked at page 48, MUCH more interesting than page 44 (which looks like a typical shuttle) 

Still pretty tricky to implement though. Lots of shoot through opportunities if something fails and leakage opportunities (might cause more imbalance than it fixes), etc, so definately not a no-brainer. Still won't keep cells from degrading faster if you overcharge/discharge. But otherwise it is an interesting circuit, definately the most cost effective redistribution approach I've seen (though not from looking at the TI implementation, and not considering gate drives).

The leakage currents in the bottom cell will be N times as much as the top cell, not accounting for voltage differential, just current flow.


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## dcb (Dec 5, 2009)

k, sorry for the confusion, with the value caveats in place:

the first thing that comes to mind is getting rid of half the battery switches and wrapping the inductor in an h-bridge of its own. Havent quite thought it through yet though, but it looks like you could turn on the - switch on the battery to charge from and the + switch on the battery to charge too. Or it immediately makes smoke. need at least 3 cells to break even on the switch count.

Edit, nope, I see the smoke.


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