# Rethinking what a BMS should be!



## boekel (Nov 10, 2010)

So you have to empty your battery to balance...

a good battery doesn't need much balancing, so I don't see any positive things about this idea. Please tell me where I'm wrong..


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## Jerry Liebler (Feb 1, 2011)

boekel said:


> So you have to empty your battery to balance...
> NO! NO! NO! it is being continuously kept balanced throughout the entire process of charging or discharging with per cell balance curents of up to 6 amps.
> 
> a good battery doesn't need much balancing, so I don't see any positive things about this idea. Please tell me where I'm wrong..


Agreed, but this assures that it'll stay good and even as cell capacity mismatch happens the absolute maximum capacity will remain available. Every cell will remain within +/-20 milliolts of the others to the extent that 6 amps can keep it that way.


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## Kevin Sharpe (Jul 4, 2011)

Jerry Liebler said:


> even as cell capacity mismatch happens


Does this *really* happen? Damien's experience would suggest not (watch from 3m30s);


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## kennybobby (Aug 10, 2012)

i like the idea of being able to disconnect the parasitic drain of the BMS from the cells--i have studied this in ryobi 40V power tool packs and found that to be the biggest source of pack "failure". Most "dead" packs had fully functional and good cells, but the BMS drained them. 5 Ahr 18650 packs of a smaller size but same principles apply.


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## Jerry Liebler (Feb 1, 2011)

kennybobby said:


> i like the idea of being able to disconnect the parasitic drain of the BMS from the cells--i have studied this in ryobi 40V power tool packs and found that to be the biggest source of pack "failure". Most "dead" packs had fully functional and good cells, but the BMS drained them. 5 Ahr 18650 packs of a smaller size but same principles apply.


To get to the very low drain this "BMS" would need to be "turned off" by removing the power. However, even with out powering down the parasitic load on the "cell tap" can be below 5 micro amps per cell module when cell voltage falls below 2.1 volts, assuming a 12 volt (4 cell) power situation.


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## boekel (Nov 10, 2010)

I've seen OEM modules (VW, Tesla, Mitsubishi, Kia) that were on the shelf for sometimes 2+ years, without inbalance...so the parasitic drain in 'off' state if obviously -very- low, and at least 'well balanced' on a good bms...

You'r free to build anything you want, but the only usecase would be a battery pack made from different capacity cells / modules....but...

when you bottom balance like this...what happens when you charge the pack fully? yes...smalles capacity cell arrives at 'charged' voltage first...and your 'active' bms has to drain this cell into the other cells of the module.

Also, the different modules (groups) become unbalanced from each other and how are you going to tackle that?

Luckily, more and more OEM bms systems become usable (Tesla, Mitsubishi, Volt?, etc) so everyone can use high quality stuff for very little money.


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## Jerry Liebler (Feb 1, 2011)

boekel said:


> I've seen OEM modules (VW, Tesla, Mitsubishi, Kia) that were on the shelf for sometimes 2+ years, without inbalance...so the parasitic drain in 'off' state if obviously -very- low, and at least 'well balanced' on a good bms...
> 
> You'r free to build anything you want, but the only usecase would be a battery pack made from different capacity cells / modules....but...
> 
> ...


Frankly I do believe this is better than any OEM system available. The retained effective and availale capacity after lots of use should be substantially higher. And almost all the OEMs insist on using more fragile and dangerous chemistry. LIFEPO4 doesn't have the highest energy density but is far and away the most forgiving and safest.


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## Jerry Liebler (Feb 1, 2011)

I'm surprised by the lack of response. I'll offer a bit of explanation of my reasoning in suggesting "charge only" active balancing. From a pragmatic view it is much easier to build, but does it work without ever needing to remove charge? Here is why it does: In use the pack is either being charged or discharged, during pack charging this BMS will add additional charge to the "stronger" cells making them fill to their actual capacity wile during pack discharge it will add charge to the "weaker" cells boosting their effective capacity allowing the stronger cells to deliver their actual capacity. Basically forcing all cells to have the same voltage through out the entire cycle of pack charge and discharge strong cells get added charge during the pack charge and weaker cells get added charge during pack discharge. Once I realized this the design became much simpler. It's easy to add an active rectifier to a "flyback" type of DC to DC converter if it's "half wave" and only transfers energy in one direction, much more challenging to get a truely bidirectional energy transfer. However, it still was a challenge to adapt available IC's to do it. they are all made to deliver an output voltage that is fixed by design and do not support the very necessary function of "tracking" an input voltage (the cell average). But after much head scratching I've "invented" a way to do it. I'm now thinking that the extra wire to stop the charger should go away as any pack charger will have a pack voltage setting. Or is redundancy really important? My 4 cell module is 4"x 2 1/2" x 1/2" 


.


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## boekel (Nov 10, 2010)

Well, do the math... see attached image and calculate how many amps you have to pull on your (12v?) bms supply if you have 100 cells and you need to charge 99 of them with up to 6A


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## Jerry Liebler (Feb 1, 2011)

boekel said:


> Well, do the math... see attached image and calculate how many amps you have to pull on your (12v?) bms supply if you have 100 cells and you need to charge 99 of them with up to 6A


It's true that the 6 amps comes from the cell tap and limits the available charging of all cells combined. This is why a higher voltage cell tap is desirable. With a tap at 16 cells, still within the components ratings, there would be 6/16 amps or 0.375 amps per 6 amp load allowing 15 cells to simultaneously receive 6 amps. 
The effect is the cell tap power limits the number of number of places the full 6 amps can be applied (bad cells) to 3 for a 4 cell volt tap 7 for an 8 cell tap and 15 for a 16 cell tap any of which is far better than traditional BMS or either top or bottom balancing!!


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## boekel (Nov 10, 2010)

ok I'll try something else:

when a weaker cell drops voltage...you can charge it with 6A, but say your car wants 50A...no-go...

and now you say you -do- use cell groups to get the power from... so group1 can get out of balance with group x

Plus, when charging 15 cells from a 16 cell group...well..when not counting the losses...you're basically discharging the 1 cell. Losses make it so that lots of heat is put out and this cell-group will drop below other groups.

An active BMS -might- make sense in a 4 cell battery...but there also the cells should just be the same, not needing active balancing.


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## Jerry Liebler (Feb 1, 2011)

boekel said:


> ok I'll try something else:
> 
> when a weaker cell drops voltage...you can charge it with 6A, but say your car wants 50A...no-go...
> 
> ...


The active balancing systems I've seen are much less capable, most offer balancing currents in milliamps. This should cost about what the "Mini BMS" cost.


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## boekel (Nov 10, 2010)

Jerry Liebler said:


> The active balancing systems I've seen are much less capable, most offer balancing currents in milliamps. This should cost about what the "Mini BMS" cost.


normally milli-amps should be enough.

but you're not answering my questions...


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## Jerry Liebler (Feb 1, 2011)

Thank you boekel!
I have a way of "alarming" when any cell is pulled below say 2.5 volts and in my mind this is a much more valuable alarm than a charger cutoff. To have both a low cell and a high cell alarm would add more cost but can be done. Are both needed?


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## boekel (Nov 10, 2010)

Yes you need both, or you'll destroy cells the same way many diy projects without these functions have destroyed cells..

that's the main funcion of a bms: stopping discharge when a cell reaches minimum voltage, and stop charge when a cell reaches maximum voltage.
Battery Monitoring System is the minimum you need to prevent damage (and depending on chemistry: fire!)

balancing is next, to keep pack (top) balanced over time...


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## Jerry Liebler (Feb 1, 2011)

boekel said:


> Yes you need both, or you'll destroy cells the same way many diy projects without these functions have destroyed cells..
> 
> that's the main funcion of a bms: stopping discharge when a cell reaches minimum voltage, and stop charge when a cell reaches maximum voltage.
> Battery Monitoring System is the minimum you need to prevent damage (and depending on chemistry: fire!)
> ...


Why not keep it balanced full time with charge only active balancing? The cost impact of alarming on both conditions turns out to be about $0.10, truely negligible. Now my questions are what " feel good" leds should I incorporate?
I've also added one "jumper" to select 4 cell or 16 cell power, this is needed to take advantage of automatic power down which will reduce the power to 5 milliwats per cell but an external switch to control power is much better it would reduce the parasitic load to pico watts of "leakage" current. The automatic power down would drain the cells to between 2.1 and 2.4 volts before powering down but it only costs about $0.02 to implement a bit of added operator error "grace"..


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## boekel (Nov 10, 2010)

Just a little green led (not blinding) telling everything is ok...and a big red one for warning...followed by shutdown of the system.

'why not keep it balanced with charge only balancing'
well, I've made a nice drawing for you to explain why not...

But please trust me: bottom balancing is just not right, most of the time the battery will be 'full' (not driving, connected to the grid) and if necessary the bms has lots of time to balance.

balancing when battery is half-full doesn't work well, because the cells are on the 'flat' part of the curve, especially with LFP. bottom balancing can only be done with an empty battery....so why would you do that?
Then when charging, the weakest cell determines that your charger has to stop before the rest is full. and no, there is no way you can add enough current to the other cells to overcome this.

just top-balance, and throw out the cells that are bad. overall voltage will be lower, but usable capacity will increase.


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## Jerry Liebler (Feb 1, 2011)

boekel said:


> Just a little green led (not blinding) telling everything is ok...and a big red one for warning...followed by shutdown of the system.
> 
> 'why not keep it balanced with charge only balancing'
> well, I've made a nice drawing for you to explain why not...
> ...


I don't think you, quite yet, comprehend that what charge only full time active balancing does. As the pack is charged it is driven toward a "top balanced state" AND as it is discharged it is driven toward a " bottom balanced state"


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## boekel (Nov 10, 2010)

Jerry Liebler said:


> I don't think you, quite yet, comprehend that what charge only full time active balancing does. As the pack is charged it is driven toward a "top balanced state" AND as it is discharged it is driven toward a " bottom balanced state"


Than please explain it to me, with a nice drawing like I made, you could use the drawing and write some numbers on it...

say the 'good' cells are 100 Ah, and the 'bad' one is 90Ah..


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## MattsAwesomeStuff (Aug 10, 2017)

I'm not sure what the goal is here, but, I think what's being attempted is:


As the pack charges, those that are ahead of the others voltage-wise (higher), redirect their energy towards the weak cells.


As the pack discharges, those that are behind the others voltage-wise (higher voltage again), redirect their energy towards the weak cells.


I guess just constantly trying to maintain a constant voltage level for all cells at all times in use.


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## boekel (Nov 10, 2010)

MattsAwesomeStuff said:


> I guess just constantly trying to maintain a constant voltage level for all cells at all times in use.


I know...but the numbers just don't add up...charging 99 cells because 1 is less capacity doesn't work. more heat being made then you win.


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## Jerry Liebler (Feb 1, 2011)

boekel said:


> i know...but the numbers just don't add up...charging 99 cells because 1 is less capacity doesn't work. More heat being made then you win.


prove it!!!!


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## Jerry Liebler (Feb 1, 2011)

MattsAwesomeStuff said:


> I'm not sure what the goal is here, but, I think what's being attempted is:
> 
> 
> As the pack charges, those that are ahead of the others voltage-wise (higher), redirect their energy towards the weak cells.
> ...


You have it almost perfectly backwards! During charge the weaker cells rise in voltage faster than the stronger so the stronger cells are given added charge to maintain the same average per cell voltage.

Likewise during pack discharge the voltage of weaker cells falls faster so they are given extra charge to maintain the average per cell voltage.

Added charge goes to stronger cells during pack charging!
Added charge goes to weaker cells during pack discharge!
All cells see the same "cycle depth" or voltage excursion,
within the limitations of the equipment. 


Where is the advantage? let' assume you have a 100 cell battery with one cell with 80% capacity.
With "traditional" top or bottom balancing your pack has 80 % capacity!
The rest is unused because the bad cell limits the utilization of the good cells. 
With continuous balancing your pack will have 99.8% capacity if the system is lossless or with 50% efficiency you'll have 99.6% of rated capacity.


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## MattsAwesomeStuff (Aug 10, 2017)

Jerry Liebler said:


> You have it almost perfectly backwards!


Oops. Right right.

Weaker cells will finish charging sooner and those are the ones you have to redirect. Weaker cells will finish discharging sooner and those are the ones you have to redirect to.


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## Jerry Liebler (Feb 1, 2011)

MattsAwesomeStuff said:


> Oops. Right right.
> 
> Weaker cells will finish charging sooner and those are the ones you have to redirect. Weaker cells will finish discharging sooner and those are the ones you have to redirect to.


I dissagree: During charging MORE charge has to go to the STRONGER cells so that the weaker and stronger finish at the same time!
During discharge the weaker cell cannot supply the amp hours needed so it must be charged to add amp hours so it finishes with the rest.


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## boekel (Nov 10, 2010)

Jerry Liebler said:


> prove it!!!!


I'm giving up...sorry.

figure it out by yourself.

ok one more hint: if 99 cells have to be charged..from...100 cells...your basically draining the one cell that's weak...but with a lot of extra parts...


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## Frank (Dec 6, 2008)

Sounds like a bigger version of the Lee Hart Battery Balancer. It can work but not sure if it's necessary. Maybe with really old and/or crappy cells, but you'd be better off getting good cells in the first place.


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## summetj (Mar 30, 2011)

Why not take this concept all the way, and use it to replace a single series charger with one charger per cell.


That way, every cell would get charged (topped up) to the same ending voltage every time you charged the vehicle. 



No need to communicate between cells, just have each charger charge it's own cell until it reaches whatever stopping voltage is appropriate for your chemistry.


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## Jerry Liebler (Feb 1, 2011)

boekel said:


> I'm giving up...sorry.
> 
> figure it out by yourself.
> 
> ...


Simply NO! During pack charging what is happening is the weak cell is not being charged, the charge that it would be getting, without the balancer, is being put into the stronger cells.


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## Jerry Liebler (Feb 1, 2011)

Hopefully everyone can agree with "conventional" battery management a top balanced or a bottom balanced pack's watt capacity is is limited to the capacity OF THE WEAKEST CELL. So if there is a single weak cell it controls the amount of charge that the rest of the pack can accept or deliver during a cycle. 
EDITED TO CORRECT confusing wording!
.
Now what seems like magic happens when full cycle active balancing is employed. Within the capabilities of the equipment all cells will be driven to the same limits so all cell capacity is available, from all cells and pack wat hour capacity is NOT limited by the weak cell or cells but is the sum of all the individual cell watt hour capacities.

Earlier I gave the example of a 100 cell pack with 99 "good" cells and one weak one cell with 80 % capacity. That single weak cell controls the pack capacity and forces the rest to accept or deliver no more than it can, with "conventional" battery management. Forcing the user to accept 80% of what he thought he had.
HOWEVER with full time active balancing, with sufficient capacity, the pack wat hour capacity is only degraded to 99.8% assuming no losses or 99.6% if the charge "redistribution" is 50% efficient.

In addition with "conventional" battery management the weak cell is cycled more deeply every cycle and it's life is shorter than the good cells which are not as deeply cycled while with full time active charge redistribution all cells are subject to the same cycle depth and the strong are far less likely to outlast the weak.


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## brian_ (Feb 7, 2017)

Jerry Liebler said:


> Hopefully everyone can agree with "conventional" battery management a top balanced or a bottom balanced pack's capacity is is limited to the number of cells times the capacity OF THE WEAKEST CELL. So if there is a single weak cell it controls the amount of charge that the rest of the pack can accept or deliver during a cycle.


No, not everyone.

All cells in a parallel group see the same charging voltage (by definition of "in parallel"), but you seem to be assuming that they all get the same current. I don't see that as obvious at all, since parallel cells at different states of charge and different internal conditions (e.g. internal resistance) will flow different currents.

I don't have an opinion on the BMS design, but this seems to be a problematic part of the assumptions underlying the design.


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## kennybobby (Aug 10, 2012)

Jerry Liebler said:


> ...so all cell capacity is available, from all cells and pack capacity is NOT limited by the weak cell or cells but is the sum of all the individual cell capacities.


How can this be? are you talking about parallel cells? a drawing or schematic block diagram might be helpful because this doesn't make any sense. For cells in series--the capacity is not the sum of all the cell capacities.


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## Jerry Liebler (Feb 1, 2011)

brian_ said:


> No, not everyone.
> 
> All cells in a parallel group see the same charging voltage (by definition of "in parallel"), but you seem to be assuming that they all get the same current. I don't see that as obvious at all, since parallel cells at different states of charge and different internal conditions (e.g. internal resistance) will flow different currents.
> 
> I don't have an opinion on the BMS design, but this seems to be a problematic part of the assumptions underlying the design.


I'm not talking about paralleled cells at all, to my knowledge no bms deals with paralleled cells individually, the concern is with series connected cells which would get the same current unless modified by an active balancer. In fact one way to "balance" cells is to connect them in parallel.


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## Jerry Liebler (Feb 1, 2011)

kennybobby said:


> How can this be? are you talking about parallel cells? a drawing or schematic block diagram might be helpful because this doesn't make any sense. For cells in series--the capacity is not the sum of all the cell capacities.


NO I'm not talking about paralleled cells. By capacity I mean amp hours.
I've edited the original post that was confusing hopefully it's clear now.


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## boekel (Nov 10, 2010)

Simple question not answered yet:

car needs 50A, weak cell needs charge...what happens?


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## Jerry Liebler (Feb 1, 2011)

boekel said:


> Simple question not answered yet:
> 
> car needs 50A, weak cell needs charge...what happens?


It depends on the ability of the weak cell to supply current, it's internal resistance and state of charge. The car gets the 50 amps from the weak cell 44 amps will be drawn, the remaining 6 come from the BMS module, if the weak cell's voltage drops below 2.5 volts the "alarm" line will be "activated" (pulled low).

Similar thing happens during charging with 50 amps stronger cells will be getting about 55 amp charge weak get 50, if or when the weak cell hits 3.6 volts it will cause "alarm" . The cell modules generate the "alarm" they don't act on it what the user does with it isn't part of my design.


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## boekel (Nov 10, 2010)

Jerry Liebler said:


> It depends on the ability of the weak cell to supply current, it's internal resistance and state of charge. The car gets the 50 amps from the weak cell 44 amps will be drawn, the remaining 6 come from the BMS module, if the weak cell's voltage drops below 2.5 volts the "alarm" line will be "activated" (pulled low).


Once it sags below the rest of the cells, thus being supported by your bms, the cell is just about empty and won't supply this 44 Amps. You can try to 'add' this 6 Amps, but it won't help much...

Just remove the bad cell(s) and go on with life. Keep them (top) balanced so you can charge them all without cutting out the charger when a cell hit's max. first.


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## Jerry Liebler (Feb 1, 2011)

I'm now thinking that any "alarm" condition should be latched in the cell module and an LED tuned on to indicate which cell went out of bounds. The latched condition will be cleared by power cycling. I can do a kind of latch by adding a single diode per cell module but with that implementation I won;t store which bound was exceeded. I've set the cell voltage limits at 3.6 and 2.5 volts with a tolerance of +/- 50mv. How does this sound?


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## brian_ (Feb 7, 2017)

Jerry Liebler said:


> I'm not talking about paralleled cells at all, to my knowledge no bms deals with paralleled cells individually, the concern is with series connected cells which would get the same current unless modified by an active balancer.


Okay, that's more clear. So when you say "cell" you mean a group of paralleled cells. Especially since you won't provide any diagrams, clarity of wording is critical.


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## Jerry Liebler (Feb 1, 2011)

brian_ said:


> Okay, that's more clear. So when you say "cell" you mean a group of paralleled cells. Especially since you won't provide any diagrams, clarity of wording is critical.


YES, if that's the structure of the battery.


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## Jerry Liebler (Feb 1, 2011)

I have changed the design yet again. Now only the low cell voltage alarm is " latched" and it lights a red LED, High cell voltage alarm is not stored. If there is interest I would " publish" the schematic and/or PWB layout of the 4 cell module.


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## Jerry Liebler (Feb 1, 2011)

summetj said:


> Why not take this concept all the way, and use it to replace a single series charger with one charger per cell.
> 
> 
> That way, every cell would get charged (topped up) to the same ending voltage every time you charged the vehicle.
> ...


That sure could be done but it is a different design. The components I'm using could support a 6 amp charger, not enough for most situations.


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## steveob (Nov 10, 2017)

Jerry Liebler said:


> capacity is is limited to the capacity OF THE WEAKEST CELL. So if there is a single weak cell it controls the amount of charge that the rest of the pack can accept or deliver during a cycle.


Yes, and normally the difference isn't anything to worry about, for a variety of reasons. If it is different by a lot, replace the cell.


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## sholland (Jan 16, 2012)

I designed a stackable board for 5A active balancing here, and there is example code here.

This puts a shared, isolated bidirectional buck boost on every 16s module.


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## Jerry Liebler (Feb 1, 2011)

sholland said:


> I designed a stackable board for 5A active balancing here, and there is example code here.
> 
> This puts a shared, isolated bidirectional buck boost on every 16s module.


Very impressive! Mine is MUCH less sophisticated, all analog (NO SOFTWARE) and much more brute force. The big difference(s) is I use a buck (flyback) only converter on each cell and an analog average of cell voltage. 
Do you have any user experience?
Why did you choose 5 amps?


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## sholland (Jan 16, 2012)

The TIDA-00817 reference design was for balancing of all cells in an entire pack (16 of the those 16s boards can be stacked) charging or discharging any one cell at a time from/to the chassis referenced 12V battery. It can be varied in software from 2.5 - 5.5A.

This type of solution is usually found in one of the following applications:
- BIG capacity batteries, like in a bus or grid storage
- Poor quality cells. Only active balancing can compensate for capacity mismatch
- Performance, like in a motorcycle, where the battery is always too small for the application, and maximum capacity at all times is needed. Active balancing can allow deep depth of discharge, right out into the knees of the SoC curve.
- Fast time to balance is needed, such as fast DC charging. Typically, passive balancing only occurs near the top of charge, as range is being thrown away as heat while balancing. If the time on the charger is short, the pack may never actually be balanced, leading to an ever growing mismatch or overall capacity loss. This is already starting to be shown in Model S that are mostly charged on superchargers. Another application where fast time to balance is needed is residential solar storage. Those batteries can go through a full charge and discharge cycle twice a day. Drones are another application - not on the drone, but in a fast charge station.

This last reason is why I've always been a proponent of active balancing. As we get faster and faster charge rates, a form of efficient balancing will become necessary. The current may not need to be too high, but it will need to be efficient, e.g. not 100% loss thrown away as heat in passive balancing.

I used a precursor to this board to babysit my used Thundersky 160Ah cells. Without balancing, the mismatch would grow pretty quickly. With just a single shared 5A bidirectional balancer always balancing the cells to average of all cells, I was able to keep all cells in the entire pack within +/-2mV.

There are other solutions, such as the LTC3300 which puts a dedicated bidirectional DC-DC on every cell, and also the LT8584, which puts a unidirectional (discharge) DC-DC on every cell.


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## Jerry Liebler (Feb 1, 2011)

sholland said:


> The TIDA-00817 reference design was for balancing of all cells in an entire pack (16 of the those 16s boards can be stacked) charging or discharging any one cell at a time from/to the chassis referenced 12V battery. It can be varied in software from 2.5 - 5.5A.
> 
> This type of solution is usually found in one of the following applications:
> - BIG capacity batteries, like in a bus or grid storage
> ...


I suspect you are in some way employed by what once was National Semiconductor Company, now the silicon valley division of TI. I've designed yet another approach to active balancing using a charge only DC to DC on every cell which ideally is powered by the whole pack, through another DC to DC converter if the pack is over 60 volts, or directly if the pack is 60 volt max or less. My key components are the LM46002 a 2 amp switcher with on-board power FETs and PA3855.004 a flyback transformer with a 3 to 1 turns ratio and an auxiliary winding suited for gate drive of a synchronous rectifier FET. I should be able to deliver 6 amps @ 3+/- volts @ 90+% efficiency. In addition to the above I have a low power shunt reference and a quad op amp and 2 bipolar npn transistors and a diode & LED for the "alarm" plus a bunch of 0805 resistors and several capacitors. 4 cells worth fit on a 2 1/2"x4" PWB. A salient feature of my design is that nothing is powered from the cells and when the "BMS" is powered down the cells see only the leakage current through an off power FET. My intended use is for a 48 volt solar "energy bank", interesting that you include this use as one that needs active balancing.


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## sholland (Jan 16, 2012)

Yes, I once worked for those companies... Very exciting industry to be a part of! 

It was especially fun in the beginning when no one knew what was next for batteries, or what the effects of aging might be. We once brainstormed many possible ways to manage batteries. There is now some maturing of the industry, but batteries are still finding homes in more and more exciting places. Next it will be airplanes and rockets, who knows where else.

There are many ways to provide solutions, including discrete circuits. As with most things, the best engineering solution is not always what makes it into products, but rather the best solution for a cost target. There are few applications to date that have justified active balancing, but that is changing. Just like not every person needs or wants an iPhone 10, so there is always a market for something that just makes do, or can accept a compromise. Many of today's hybrids and EV's are designed exactly that way, with compromises.

The devil with these circuits is in details... 
- Can it survive hot-plug?
- Can it survive for 10 years?
- Can it work reliably and accurately?
- Can it not affect the battery cells?
- Can it not affect any other systems in the car?
- What cost is allowed for the product?
- and many, many more considerations...


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## Frank (Dec 6, 2008)

My experience is that as cells age they can drift apart on discharge but still be together at the top. Is it worth it? i.e. shuttle electrons around to keep cells somewhat even during discharge and then have to do it again during charge? If you're running your pack so low this is a factor you're probably hurting it anyway.


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## sholland (Jan 16, 2012)

Are you running just single cell groups, e.g. no parallel cells? I see a lot of folks here that are together at the bottom and apart at the top.

It can depend on pack construction. Many packs are built with many cells in parallel, with fuses to remove a cell that forms an internal short. If a fuse blows, then there will always be a capacity mismatch, and the cell groups with less parallel cells will be the first to reach top of charge and the first to reach bottom of discharge. The one cell group will limit the capacity of the entire pack and there would be a lot of wasted capacity that never be used. High selective discharge would be a big help.

Every pack design can be different, and the balancing method used can also be different. Passive may be enough, but some use cases may need more.

High current only excaserbates the small mismatches. It's interesting times with high current charging becoming available.


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## steveob (Nov 10, 2017)

5 amps isn't going to cover for a cell dropping out of parallel though for your typical ev module. And yes, pushing it to %100 charge/discharge has a disproportionate decrease on number of cycles as frank had noted.


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## sholland (Jan 16, 2012)

Why so literal? I didn't say anywhere that 5A was the number everyone should use. I DID say every pack's requirement can be unique. FWIW, I have seen designs that the engineers thought needed just 2A, and other's that thought they needed 10A or more. It all depends how they are intended to be used, along with all the other requirements such as cost, premium product, etc. Most modern generation battery packs being designed with 150kW+ CCS charging in mind have passive balance current of 200mA+ and most typically 300mA. This would seem like the upper limit to have to dissipate thermally.

I mentioned compromise... Maybe just a smaller amount of balance current, but the fact that active balancing is used will allow balancing beyond just when attached to the charger. With an efficient converter maintaining the energy in the system instead of just throwing it away as heat, the algorithms can be opened up significantly.

I don't think I ever mentioned going to 100% capacity use, that's just not possible. But, there is so much margin between available capacity and advertised capacity in some packs... some get it right, some don't and have issues of noticeable capacity loss well before the expected lifetime of the pack is done.


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## steveob (Nov 10, 2017)

sholland said:


> I don't think I ever mentioned going to 100% capacity use, that's just not possible.





sholland said:


> - Performance, like in a motorcycle, where the battery is always too small for the application, and maximum capacity at all times is needed.


It is exponential as you approach %100, really from about %80 and up. And I wouldn't design even a motorcycle to push the cells to "maximum".


the more complicated a system, the more chance there is for a failure. I think more than a few folks have looked at charge shuttling, looked at how it really complicates things without really being able to make a difference in the real world, and reasonably decided it wasn't a good idea after all. I mean even simple shunting bms's have killed their share of cells.


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## sholland (Jan 16, 2012)

I have not seen a case where an OEM BMS has killed cells. There are some pretty sketchy hobby grade BMS systems out there though, so I'll agree with you there.

That's all the more reason to use the ASIC's that are designed for this task. I would be careful before I trust my batteries to a home-spun MCU based or discrete BMS.

These ASIC's weren't designed for no reason. There are obviously folks out there that think they are necessary.

And nowhere did I say 'maximum' = 100%. Maximum is a definition for each pack designer, some may say that is 60%, some might say 80%, it's up to the application.


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## steveob (Nov 10, 2017)

sholland said:


> I am pretty sure no OEM BMS has ever killed cells.


That assertion might be statistically impossible. Indeed there have even been a few fires, how is the BMS not culpable (in this imaginary discussion of perfect machines)?!? 

I mean perhaps your work experience gives you a feeling of superiority and authority, but you still make some unjustified claims.


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## sholland (Jan 16, 2012)

Wow, why did even bother trying to share anything here... You already know everything.


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## steveob (Nov 10, 2017)

Are you sharing or selling?!?


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## sholland (Jan 16, 2012)

Only sharing. I do not stand to gain anything, other than your disrespect.


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## kennybobby (Aug 10, 2012)

The BMS chip, OZ8940ATN, in the Ryobi 40V power tool packs has a permanent failure mode in which it drains the cells and disables the pack--that is one example of an OEM BMS that kills the cells.

The company that makes the chip, O2 Micro, seems to be a very secretive holding company of patent whores.


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## sholland (Jan 16, 2012)

kennybobby said:


> The BMS chip, OZ8940ATN, in the Ryobi 40V power tool packs has a permanent failure mode in which it drains the cells and disables the pack--that is one example of an OEM BMS that kills the cells.
> 
> The company that makes the chip, O2 Micro, seems to be a very secretive holding company of patent whores.


They describe that as a feature.


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## steveob (Nov 10, 2017)

sholland said:


> Only sharing. I do not stand to gain anything, other than your disrespect.


I only ask because you make statements like "always" and "maximum" and make gross assumptions, i.e. failure rate, and how people are incapable of provide sufficient headroom in a motorcycle or whatever, doesn't seem like you are trying to have an objective discussion about it as much as selling an idea. And when I point out that 5 amps isn't enough to make up for, say, losing a leaf or volt pouch, you get offended Indeed we don't even know what sort of circuitry OP has in mind.

And still the solution is obvious, if they are only a little out on capacity, it doesn't matter, if they are a lot out of wack, you need to replace the cell. The important thing is having the information and taking action, perhaps automatically if you assume a braindead driver, which in its own way is disrespectful.


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## Jerry Liebler (Feb 1, 2011)

steveob said:


> I only ask because you make statements like "always" and "maximum" and make gross assumptions, i.e. failure rate, and how people are incapable of provide sufficient headroom in a motorcycle or whatever, doesn't seem like you are trying to have an objective discussion about it as much as selling an idea. And when I point out that 5 amps isn't enough to make up for, say, losing a leaf or volt pouch, you get offended Indeed we don't even know what sort of circuitry OP has in mind.
> 
> And still the solution is obvious, if they are only a little out on capacity, it doesn't matter, if they are a lot out of wack, you need to replace the cell. The important thing is having the information and taking action, perhaps automatically if you assume a braindead driver, which in its own way is disrespectful.


I think I've pretty well described what I have in mind and have/do offer a schematic and PWB layout. Here is a quick summary: Each cell has a DC to DC converter consisting of a LM46002 and a PA3855.005 along with a quad op amp and support circuitry to implement a 4 wire bus that interconnects the per/cell electronics. The 4 wires are DC power, Ground, cell average voltage (@ 3 times actual), alarm. The alarm wire is an open collector "or" of cell over or under voltage events. The alarm events are any cell above 3.6 volts and a latched any cell under 2.278 volts. The latched alarm also lights a bright red LED to indicate which cell. Power for all the DC to DC converters can come from any DC source between 12 and 60 volts. The preferred situation is from the whole pack but taps could be used. When this BMS is powered down the cells will have virtually no parasitic load!


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## steveob (Nov 10, 2017)

I guess one concern is you have listed over $20 in parts per cell/voltage level, even after some quantity discounts, and more pins than I care to count, for a few watts of balance, when simple monitoring has been done repeatedly for around $1/cell, and not much more for balance shunts, with actual digital communication. I appreciate your enthusiasm, but that isn't what a BMS "should" be IMHO.


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## Jerry Liebler (Feb 1, 2011)

steveob said:


> I guess one concern is you have listed over $20 in parts per cell/voltage level, even after some quantity discounts, and more pins than I care to count, for a few watts of balance, when simple monitoring has been done repeatedly for around $1/cell, and not much more for balance shunts, with actual digital communication. I appreciate your enthusiasm, but that isn't what a BMS "should" be IMHO.


I don't know where you are getting pricing. I use Digikey and it's nowhere near that bad even at the totally impractical, single 4 cell module! The 4 cell module has a grand total of 162 components including the PWB and 3 connectors. Over 1/2 of the components are 0805 resistors and a quarter are ceramic capacitors. My schematic capture software reports 471 pins for the 4 cell module. Key features 6 amps of balance current to cure a 15 mv imbalance and no drain when off and <15 mw/cell drain when in balance plus very simple installation. No it's not digital, PURE ANALOG!


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## steveob (Nov 10, 2017)

Jerry Liebler said:


> I don't know where you are getting pricing. I use Digikey and it's nowhere near that bad even at the totally impractical, single 4 cell module! The 4 cell module has a grand total of 162 components including the PWB and 3 connectors. Over 1/2 of the components are 0805 resistors and a quarter are ceramic capacitors. My schematic capture software reports 471 pins for the 4 cell module. Key features 6 amps of balance current to cure a 15 mv imbalance and no drain when off and <15 mw/cell drain when in balance plus very simple installation. No it's not digital, PURE ANALOG!


or a 50 cent 8 pin cpu and an opto and some programming, maybe some extra passives if you want multiple cells per cpu or a sample and hold multiplexer. Not being analog means you can get cell by cell data, and send commands to specific cells, and of course lower part count/cost with trivial hookup. Sorry, I just don't see the benefits of the proposal. Auto balance is largely overrated in the DIY sphere as well (if you can build it, you can fix it).

FYI, there is a way to infer cell voltage through a transformer as well as charging, not new, just that magnetics (and caps and etc) are a bit of a PITA compared to digital and temperature/voltage calibrated references/adc. And it usually requires some processing power anyway to fit the curve or otherwise detect the threshold, and doesn't account for changes in temp, etc so pretty inaccurate typically. You can do something similar with capacitors instead of transformers as well. https://www.maximintegrated.com/en/app-notes/index.mvp/id/4553

There isn't much point discussing it without a schematic though, but I kinda get the jist.


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## Jerry Liebler (Feb 1, 2011)

steveob said:


> or a 50 cent 8 pin cpu and an opto and some programming, maybe some extra passives if you want multiple cells per cpu or a sample and hold multiplexer. Not being analog means you can get cell by cell data, and send commands to specific cells, and of course lower part count/cost with trivial hookup. Sorry, I just don't see the benefits of the proposal. Auto balance is largely overrated in the DIY sphere as well (if you can build it, you can fix it).
> 
> FYI, there is a way to infer cell voltage through a transformer as well as charging, not new, just that magnetics (and caps and etc) are a bit of a PITA compared to digital and temperature/voltage calibrated references/adc. And it usually requires some processing power anyway to fit the curve or otherwise detect the threshold, and doesn't account for changes in temp, etc so pretty inaccurate typically. You can do something similar with capacitors instead of transformers as well. https://www.maximintegrated.com/en/app-notes/index.mvp/id/4553
> 
> There isn't much point discussing it without a schematic though, but I kinda get the jist.


With a synchronus rectifier on the secondary and a known turns ratio the cell voltage is accurately measured on the primary (isolated) side of the transformer.


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## steveob (Nov 10, 2017)

Thanks for the picture.

I don't mean to belabor the point, but there is definitely room for improvement. The first thing that jumped out was the typo on r21, and I don't know how that chip is supposed to do anything with VCC connected via a capacitor.. Another issue is that this is basically mirroring the voltage stack, not even to the point of controlling or instrumentation (still needs to get digital to turn things on or off or to use anything but an analog meter). But stacking the output voltages is a common source of error as to compute the cell voltages you have to subtract the readings from the lower cells. Plus repeatibility is surely temperature dependent and no provisions for calibration, and it is rather busy for what it does, like it was designed 20 years ago or something.

No offense, but packing in functionality and easy hookup really is a job for adc to digital.


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## Jerry Liebler (Feb 1, 2011)

View attachment 106714


steveob said:


> Thanks for the picture.
> 
> I don't mean to belabor the point, but there is definitely room for improvement. The first thing that jumped out was the typo on r21,
> 
> ...


Myself, and likely every other competent analog designer, find the obsession with digital solutions annoying and generally misguided. 
Quality analog design is certainly unusual today.
Edit I've attached a corrected schematic, in addition to correcting the aforementioned typo I dramatically reduced the time constant of the "reference" filters which should improve start up behavior.


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## PStechPaul (May 1, 2012)

I suppose the concept has some merit, but the implementation seems impractical. Just the LM46002 and the PA3855.003 transformer are about $4 in 1000 piece quantity, and there are about 40 components per cell. Once you add the cost of the PCB and assembly, you are probably looking at $10/cell, or more. 

There is a place for analog design, and it is still necessary for most digital implementations, but modern microcontrollers have a wealth of on-board peripherals, including voltage references, comparators, ADCs, S-R latches, timers, EEPROM, and op-amps. Tweaking performance and adding features usually involves just some coding, which is much easier and cost-effective than changing or adding components.

I think good battery packs should have cells that are matched better than 5%, and mostly will have excess capacity, so you should be able to run them from 10% to 90% SOC after an initial balanced charge (or discharge for bottom balance). The extra cost and complexity of a dynamic charge balancing system might only provide another 5% capacity. If a passive BMS detects a seriously low cell, it is probably better to just identify it and bypass it. For a pack of 50 LiFePO4 cells, one cell represents only 2% of total pack voltage. 

I have proposed a modular system that might consist of one charger/BMS per cell, or maybe as many as 16 for nominal 48 VDC modules.


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## Jerry Liebler (Feb 1, 2011)

PStechPaul said:


> I suppose the concept has some merit, but the implementation seems impractical. Just the LM46002 and the PA3855.003 transformer are about $4 in 1000 piece quantity, and there are about 40 components per cell. Once you add the cost of the PCB and assembly, you are probably looking at $10/cell, or more.
> 
> There is a place for analog design, and it is still necessary for most digital implementations, but modern microcontrollers have a wealth of on-board peripherals, including voltage references, comparators, ADCs, S-R latches, timers, EEPROM, and op-amps. Tweaking performance and adding features usually involves just some coding, which is much easier and cost-effective than changing or adding components.
> 
> ...


Where can I see your proposal?


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## steveob (Nov 10, 2017)

Jerry Liebler said:


> View attachment 106714
> 
> Myself, and likely every other competent analog designer, find the obsession with digital solutions annoying and generally misguided.



You should turn off your computer in protest then  Analoging all the things is a garbage ideology, you only analog what you have too. I'm sure at one point there was a guy saying "silicon is overrated"... You would do yourself a favor to understand digital better instead of whatever this is.


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## Jerry Liebler (Feb 1, 2011)

steveob said:


> You should turn off your computer in protest then  Analoging all the things is a garbage ideology, you only analog what you have too. I'm sure at one point there was a guy saying "silicon is overrated"... You would do yourself a favor to understand digital better instead of whatever this is.


I think we agree cost is an important consideration. Take the example of average cell voltage, here it is "computed" for the cost of a wire and a resistor per cell. The alternative with a micro-controller per cell would require each micro controller to acquire all the voltages then perform the calculations. 

Despite the attempted insult I understand "digital" quite well, well enough to have several hundred products, I designed, which incorporate micro-controllers being sold. 
Concentrating on the " whatever this is" Please show me the BMS alternatives that:
1. Do not impose a parasitic load on any cell when not in use ( can be turned off).
2. Are completely electrically isolated from each cell being monitored.
3. Have valuable other features like active balancing.


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## steveob (Nov 10, 2017)

Jerry Liebler said:


> 3. Have valuable other features like active balancing.


monitoring is the important feature, balancing is only marginally valuable, active balancing isn't valuable, you have bad cells before it becomes valuable, and they have to be bad by a fairly specific amount, and the energy it saves doesn't even register. If you hadn't noticed from the discussions, we are talking milliamps of balancing in production vehicles. You haven't demonstrated any value here. Low to no parasitic is trivial, as is isolation.


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## Jerry Liebler (Feb 1, 2011)

steveob said:


> monitoring is the important feature, balancing is only marginally valuable, active balancing isn't valuable, you have bad cells before it becomes valuable, and they have to be bad by a fairly specific amount, and the energy it saves doesn't even register. If you hadn't noticed from the discussions, we are talking milliamps of balancing in production vehicles. You haven't demonstrated any value here. Low to no parasitic is trivial, as is isolation.


Please show me your "better" BMS! EDIT with both "trivial" characteristics.


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## steveob (Nov 10, 2017)

why would I have to show you a microcontroller and an opto? I thought you were a genius digital and analog and battery expert?


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## Jerry Liebler (Feb 1, 2011)

It looks like abandoning the lm46002 and using the l7987 saves $1.80 @ quan 100 for essentially the same performance. It'll take a bit of redesign as there are several differences.


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## Jerry Liebler (Feb 1, 2011)

steveob said:


> why would I have to show you a microcontroller and an opto? I thought you were a genius digital and analog and battery expert?


I never said anything like that. I just want to see the accurate BMS with isolation and ultra low parasitic drain that you keep pushing.


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## boekel (Nov 10, 2010)

A couple of good bms modules with very low parasitic draw:




























(I added the batteries for scale)


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## steveob (Nov 10, 2017)

Jerry Liebler said:


> I never said anything like that. I just want to see the accurate BMS with isolation and ultra low parasitic drain that you keep pushing.



well you will have to look at the datasheet for the microcontroller in question. I'm content with zero draw $2.00 dual voltmeters for balance monitoring my junkyard batteries (switch controlled, each measures half the pack) so I haven't pursued it past a bunch of designing and figuring and confirmational experimenting.

i.e. if you are optimizing for cost/minimal parts, 
https://www.mouser.com/datasheet/2/268/40001723D-767332.pdf


it can measure DAC output internally, so you can get a measure of VCC, sleep modes in sub micro-amps, internal temperature and voltage reference, so it can be calibrated. Internal oscillator. Like most things, there are always compromises to be had for optimization, some folks also factor in for through-hole design (esp in the DIY sphere). 

plus you can get away with just one opto with the right topology and protocol, but you may sacrifice speed, but that isn't a big deal for the home gamer, again pick your compromises. Heck you can even try the home-brew fiber optic approach to connect the cells (this IS a DIY forum after all), then it is just 2 wires per cell and the fiber. https://makezine.com/projects/how-to-connect-optical-fibers-to-leds-and-sensors/, though a couple small screw terminals would be a lot easier to work with, maybe save that for the return signal from the top of the pack.


It depends on your goals, but analog for the sake of analog isn't a goal. I tend to agree w/Johannes, it isn't about a specific design, it is about being reasonably accurate and performant and cost effective, and not introducing problems like imbalance (or killing cells) in the process


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## PStechPaul (May 1, 2012)

There are many threads where I have posted some BMS designs and concepts. Here are some links:

http://www.diyelectriccar.com/forums/showthread.php/12v-battery-soc-monitor-193905.html (This is specifically for a 12V SLA battery monitor, which I actually built and tested. Some principles also apply to multi-cell BMS)

http://www.diyelectriccar.com/forums/showthread.php/bms-design-guidelines-82646.html (A very long thread with many proposals and discussions)

http://www.diyelectriccar.com/forums/showthread.php/idea-distributed-charger-bms-one-per-191706.html (My proposal for modules with integrated charger and BMS)


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## steveob (Nov 10, 2017)

steveob said:


> it can measure DAC output internally, so you can get a measure of VCC



I should have said it can measure the internal reference using the DAC as vref, so that with a 1.1v ref and 10 bit adc, 2.2v would be 512, 4.4 would be 256, etc, and the resolution at ~4.4v is ~0.035v and at 2.2v is about 0.004v. Both of which are outside the realm of where your batteries should be, and is plenty good enough resolution for most diy applications. V=1024*1.1/ADC, plus calibration factors. 

And with a voltage range of 2.3-5.5v that pic is quite suitable for 3.75v batteries(0.0125v resolution), i.e. leaf cells, i.e. you *should* probably be in limp mode at 3.6v as you are at the knee, but would have to draw about 600+ amps to get the cell voltage down to 2.3v.
https://www.energy.gov/sites/prod/files/2015/01/f19/batteryLeaf5045.pdf
There are of course much more accurate and performant options, this is just sorting mcu by price and coming up with the cheapest yet functional through hole that I can think of (hardware wise, still have to wrestle w/assembly language, ah well, compromises)


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## Jerry Liebler (Feb 1, 2011)

steveob said:


> It depends on your goals, but analog for the sake of analog isn't a goal. I tend to agree w/Johannes, it isn't about a specific design, it is about being reasonably accurate and performant and cost effective, and not introducing problems like imbalance (or killing cells) in the process


Fine! My goals include: The ability to "power down" the BMS leaving insignificant loading on the cells. Isolation of the BMS from the cells.

A simple way to meet both of these goals is to use a "flyback" transformer per cell and a synchronous rectifier (FET) on the cell side. The PA 3855.005 is a small modest cost seemingly suitable transformer. It happens that this structure allows charging the cells as well. My first cut at a design was in many ways overkill and the balance charging excessively costly. There are many ICs that can drive the transformer and I didn't choose the lowest cost.
I've taken another look at suitable drivers and my new choice is the MAX17572 which will shave $1.88 off the cost per cell @100 cell quantities.
Choosing the MAX 17572 reduces the available "balance current" to 4.5 amps
which is still far more than most here think is needed.


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## bigmotherwhale (Apr 15, 2011)

Just FYI the Prius plug in hybrid with Li-ion batteries with the Denso BMS has active cell balancing using capacitors to shunt energy.


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## Jerry Liebler (Feb 1, 2011)

bigmotherwhale said:


> Just FYI the Prius plug in hybrid with Li-ion batteries with the Denso BMS has active cell balancing using capacitors to shunt energy.


Very interesting! Thank you! BTW I happen to own a 2013 PIP and really like it. Where can I learn more about the PIP?


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## bigmotherwhale (Apr 15, 2011)

You could try a Prius forum, I know this because i bought a battery pack from one to use in a project and had a good look at the BMS.


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## PStechPaul (May 1, 2012)

One of my designs used a "flying capacitor" which used an analog multiplexer to take a sample voltage from a selected cell and then apply it to the ADC at ground reference. When this same capacitor was applied to other cells, it would transfer charge to the cell if it was at a lower voltage.


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## steveob (Nov 10, 2017)

gets complicated at higher voltages. xformer or capacitively isolated charge pump designs less so if you want isolation, but at these voltage levels it is gonna be inefficient, and the energy saved isn't worth the complication and increased failure rate. Worth noting that most power and efficiency calculations I've seen ignore power factor.


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## bigmotherwhale (Apr 15, 2011)

A multilevel inverter controller could be used to drive the traction motor provide BMS functions and also be used as a charger, using a program you can select the cells which have the highest or lowest charge and either use them or not by connecting to different points along the pack, stepping an x amount of cells on each motor revolution. and skipping the lowest cells until they match, this way they can be all kept at the same level of charge, you could possibly do the same in reverse for charging using the motor as an inductor. 
There wiil have to be alot of switching devices but you would end up with a battery charger bms controller all in one unit which could be neat.


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## steveob (Nov 10, 2017)

so, just add 192 800 amp switches, great.


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## bigmotherwhale (Apr 15, 2011)

It was just a thought, I know its going to be a stupid number of switching devices for high voltages, but for low voltage operation the number of devices are not too bad, I had a look and found this.

http://ijesc.org/upload/b4eb5ee4fb8...Inverter with Battery Balancing Algorithm.pdf


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## piotrsko (Dec 9, 2007)

At this point I'm confused. We need to design build install all sorts of relatively expensive hardware to prevent me from being stupid?
Caveat: I'm a non bms volt pack accolyte. Been 4 years 10k miles now, they dont get very much imbalanced unless I'm drawing more than 300 amps a string or +5C, haven't had a problem with low cell voltages and they are generally within .02 volt fully charged using an Elcon. Might be the chemistry, or the design, certainly not anything I am doing.

Bring on the flames!!


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## boekel (Nov 10, 2010)

piotrsko said:


> Bring on the flames!!


I've stopped responding to this thread, the questions I asked were never answered.

I do read it for entertainment purposes though...

(this is not a response...it might look like one but it's not...ok?)


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## steveob (Nov 10, 2017)

at least monitor the pack half voltages is all I got. That is trivial to do and it is diminishing returns after that, but you have zero balance/failure info without at least that much.


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