Lithium cell balancing is it a scam

[robint]

Hi Guys
Ive been researching these magic balancers and Yes - you need to balance cells - but only occasionally starting with new gr A cells IMHO.  Probably in practice once/month for a daily Power wall pack.  There are users who want to extract the max Wh from their packs so they should ignore my rant hereon.

I work on the principle of having reasonable headroom in my usage and specs for kit. Say 80 - 20% DoD - give very long life.

So we have 2 type pf balancer - passive and active.  I wont dwell on the detailed working here as there's plenty on Google to stew your mind on.

Loads of hype around these magic devices and some are big money so are they worth it?  IMHO no way (unless you are a big user with critical kit - then you should probably be using a professional supplier and Not in a DIY board here)

The Passive Balancer is a simple resistive selective discharge, putting a resistor across higher voltage cells to bring all down to the lowest  cell.  You can then put the pack back on charge to your defined cut off level.  Pro - very simple and cheap.  Con - slow (but you only do it once a month at the top end of a routine charge - may take several hours or even over night).  Forget about so called Green credentials what krap - this is a trivial concern here compared to your other inefficiencies in your system.  OBTW, just run over it with a quality 5dig DMM for accuracy at the end

The Active Balancer (and OMG Smart - I hate that BS word).  This shuffles cell charges up and down the Cell string till it all evens out ie no energy loss (but forgets about the heat in the balancers).  IMHO forget the inductive types nuff said. We are left with the swinging capacitor type.  This repetitively switches a large capacitor between neighbouring cells to average out charge.  This is done very rapidly and charges throughout the bank will even out to within 10mV or less given a few hours - at the top of the pack charge cycle.

Thats the theory - does it work ok? Well visit Andy's Off Grid Garage site, he has lots of actually tests and the jury is out still.  IMHO if you cant trust the device to do its job reliably then you are going to have to devoted time to investigating its obscure behaviour.  Its a complicated set of switches for all that juggling.

My concern is that these bucket capacitors are usually cheap electrolytics 6.3V for 4.2 volt duty (any switching spikes?).  They can fail to short circuit across a cell - disaster.  They get hot and have high peak currents (5A+) repeatedly so their operating conditions all indicate a short life span - just to save $.  So that alone make me uneasy

BTW the Smart thingy adds a BT app and a few other goodies.  eg OVD stop,  long term standby cut off (these have a significant standby draw of 10+mA), remote operation, programming etc. all at $$$.  Does it help peace of mind?

So what do you guys think  Worth the extra grief or KISS

Robin

 

 

[rteodor]

So, if I understand your concern correctly the grief is with active balancer (AB) ? Andy mentioned this more than once in his videos that usually a passive one is "good enough" in most situations. But he also mentioned when an AB is needed:
- during long periods of low PV supply (winter) cells drift and need to be rebalanced. Sometimes that should happen fast because of rapid ramp-up of production.
- cells age over time. After some years that makes cells drift more and the PB might not keep up, especially if its a low current one.

So it is a yes an no. It depends. Andy only has his Power Wall 1.0 for how long ? 2 years I think ? Let him tell us again in 5 or 10 years ... if he doesn't redo his "Power Wall" in the meantime.

And yeah, "smart" used to mean something good about a person or a situation. "innovation" used to mean something good (see Louis Rossman rant). "experience" used to mean something won with hard work and perseverance, now experience means ... whatever the hell marketeers want it to mean. This power of twisting and tuning daily vocabulary is bigger than money.

[DavidAlfa]

Does it matter? Definitely, the battery pack will stop working way sooner when unbalanced, I just remembered FU Dyson firmware, an open source project fixing exactly this.
Dyson didn't enable the balancing, I don't know the full details but let's say one cell might reach 4.3V while others had 3.9, tripping the safety check and disabling it.
"It'll be $100 sir, thanks for your money."

But! You can't properly balance a battery pack by external means, must be managed by the BMS itself, controlling each cell individually.
This only happens in series battery packs, cells are not 100% identical and also won't age in the same way, thus if not balanced eventually one cell will get overvolted while others won't get fully charged.
The most common way is to connect a load (A resistor) on the cell exceeding the others until they all reach the same voltage level, maybe some more advanced chargers can completely bypass the cell, never heard about the switched capacitor thing.

Just like two capacitors in series connected to 12V, if identical they'll hold 6V each, but any difference in capacitance will change the voltage distribution.

[rteodor]

... never heard about the switched capacitor thing.

See this explanation on capacitive active balancers:
How Does An Active Balancer Work?, Stuart Pittaway
https://youtube.com/watch?v=VR-G-3D82e0

[robint]

Does it matter? Definitely, the battery pack will stop working way sooner when unbalanced, I just remembered FU Dyson firmware, an open source project fixing exactly this.
Dyson didn't enable the balancing, I don't know the full details but let's say one cell might reach 4.3V while others had 3.9, tripping the safety check and disabling it.
"It'll be $100 sir, thanks for your money."  Nice one, didnt know that back then and I had a Dyson fail from battery problem - what a Bstard.

But! You can't properly balance a battery pack by external means, must be managed by the BMS itself, controlling each cell individually.
This only happens in series battery packs, cells are not 100% identical and also won't age in the same way, thus if not balanced eventually one cell will get overvolted while others won't get fully charged. Well this is a slow gradual diversion. Like I say once a month balance session perhaps.
The most common way is to connect a load (A resistor) on the cell exceeding the others until they all reach the same voltage level, Yes this can be done manually, its a bit time consuming
maybe some more advanced chargers can completely bypass the cell, never heard about the switched capacitor thing. Does this work with a dc source?
https://julianilett.youtubersblog.com/flying-capacitors-balancing-module/  A 4s type is ca £5. Its self power thru the string, so you can swap to another string as per a 24V system. Doesnt need permanent connection

Just like two capacitors in series connected to 12V, if identical they'll hold 6V each, but any difference in capacitance will change the voltage distribution.

[Siwastaja]

A few quick comments:

I suggest you choose different words than active vs. passive. Passive suggest usage of passive components, and that kind of passive balancing is used for balancing capacitors, so it's kinda relevant. Passive balancer is just a resistor, or maybe a TVS. It significantly increases discharge and obviously is no-go for batteries, but OK for capacitors where full discharge in tens of seconds is acceptable because intended energy storage period is in tens of milliseconds. Better terms are dissipative and redistributive balancing. Both are active circuits.

Redistributive almost never makes sense.

High-power redistributive could increase usable pack capacity in presence of large cell capacity differences, but it would need to be really high power (full rebalance during every single half-cycle), and usually it's simple to minimize cell capacity differences to begin with, so maybe there is 1% energy capacity to gain. Better save on those large expensive DC/DC converters and buy a 1% larger battery.

Low-power redistributive doesn't increase usable pack capacity any more than good old dissipative. It still "saves energy" but probably less than it wastes due to having more complexity. It is orders of magnitude too little to make any sense.

So dissipative balancing is what everybody uses. Effectively, in long term it takes the self-discharge of the worst cell, and make the others discharge at the same rate. Self-discharge of li-ion cells is usually in range of 0.5-10%/year depending on product and conditions (temperature, state-of-charge), 3%/year is a good rule-of-thumb. So if you have two 10Ah cells and one self-discharges at 3%/year (0.3Ah/year = 34µA)  and another 2%/year (23µA), the better one needs to have added 11µA average balancing load over it, wasting let's say 0.1Ah*4.2V = 0.42Wh of precious energy per year, with energy cost of 0.30$/kWh that would cost $0.0001 each year. This is what you are trying to optimize when you design an "active" (redistributive) balancer. Let's make it a large EV battery instead and the energy wasted on dissipation resistors jumps to maybe $0.1 per year, or $1 during the lifetime of the vehicle.

For typical let's say 20mA balancing current which is pretty sweet for not being able to do thermal damage if the dissipative balancer fails on, the balancer would need to run at 11µA/20mA = 0.055% duty cycle to be able to do its job. This sounds small but is something to take seriously, if packs are only rarely fully charged. The algorithm needs good enough conditions to assess the balance, you can't do it during large currents flowing in either direction, or at too low SoC; then it needs time to perform the balancing. The classic brute-force approach is to use enough balancing current so that balancer simply runs during the time some of the cells are already in CV phase. It works, except if you never fully charge.

[robint]

@siwastaja This is what I proposed in my OP.  Balance is an occasional job say once a month and a simple resistive device is good enough and very simple and cheap. 

All the rest is smoke and mirrors for the DIY man

[Siwastaja]

Yes, once a month is fine. Or even once a year! Just remember that the circuit you use must be able to do its job. Some balancing circuits are so weak (algorithm which can utilize only small amount of time, maybe just some minutes on each full charge cycle, combined with low balancing current) that they need to run more often than others. The longer you wait, the more difference has accumulated to be taken care of.

It is not unheard of to not balance at all. Many reputable manufacturers have done exactly that, including many 2s packs on older consumer gear, or 4s-6s packs on Bosch power tools from a decade ago. These do not even have cell-by-cell monitoring, just connected in series (with one low-voltage cutoff for the whole pack) in a way most internet sources say leads to explosions and fires. Yet, no one had any problems with these Bosch power tools. By choosing cells of high quality control, and with physically small pack so that cells are at the same temperature, amount of accumulated imbalance during the whole lifetime is just small enough. So if you balanced once a year, you'd be doing better than that already.

I'm not suggesting to go without balancing or monitoring, though, just to give idea about how little of balancing is actually needed. When designing the cell voltage monitoring and balancing system, your #1 goal should be making 110% sure your own circuit won't fail or cause imbalance, because in such case the pack would have been better off without any added circuitry! Only after the reliability and low leakage current are verified should you start adding fancy features - redistributive balancing being one of the last to make sense.

And remember the difference of balancing and monitoring. You can choose just to monitor the cell voltages and control charge / discharge such that the highest / lowest cell voltage does the decision. This is what you "must" implement anyway, and which gives you the safety. Balancing is an added functionality on top which just maintains the usable pack capacity to maximum possible (this is, capacity of the lowest-capacity cell is the capacity of pack. Without balancing, it can be worse than that.). The thing is, most of the complexity goes into monitoring - level shifting etc., so once you have that, you usually implement balancing as well.

[Faringdon]

I think , for (series cell stacks) a small, cheap, low power active balancer is best for a Tesla Power wall size thing.
Ayk, you need to detect individual cell voltages anyway and stop charging once any cell gets to its max voltage. (and vice versa bla bla with going low voltage).
Basically, the cell stack should be made from matched cells in the first place, and they should be factory tested with a few deep charge/discharges to check that
the cell voltages  really are matched.
Then you just need a small active balancer to make sure that if the cells are left unused for ages, they dont drift a long way apart over a long time...

I used to work in this area, and we could turn peoples relatively_high_power balancers on remotely whenever their cells went badly unbalanced, but it was rarely worth it....
It rarely worked out.
The cells would have needed a super high power balancer to bring them back properly, and it just woudlnt have been worth putting that into every battery pack.

Matched cells to start off with, and small active balancers is the way.

[indeterminate]

The 21KWH pack that runs the house " there is no grid power " has been running for the last 8 years.
it is top balanced every day that the sun shines by shunting 2A into a resister for any cell that is high
the older the pack gets the more drift you see.
it is simple and it works , letting a cell drift low will result in a blackout in the morning while you are  make toast and your partner is in the shower . Not a good start to the day.