balancing

Sorry if this is a stupid question.
But my mother told me: the only stupid question is the question never asked!

Can anybody please explain what balancing is, in simple english please!

What is it purposeand for what.

My packs will be running between 3.1v and 3.9 volt (80%????)
All my cells in my packs are above 92% soh.
I have batrium, 72longmons.(more coming as needed)
I have only packs and nothing connected, i just want to understand, why what and how.

Thanks in advance, sorry if this is a stupid question, but i would like to know

An overly simplistic explanation, but essentially all you need to know is that balancing is the process designed to ensure all cells within a multi cell battery maintain identical states of charge.

Think of each parallel set of cells are water tanks of a set volume.

During charging, it's possible for 1 tank to get a little higher in volume, or little lower, than the others. Balancing will start to bleed off the capacity of the fuller tank until the others catchup, or all the others until the slower one catches up.
The end goal of balancing is so that all packs (tanks) have the same capacity (volume) at the same time at the end of charge.

Suppose you have a 14s1p battery, and start off with all cells at the same voltage (3.9V) and keep cycling them between 3.1v ~ 3.9 volt (in total 43.4v ~ 54.6v)

Suppose half of the cells have a self discharge rate just a little quicker than the others, so that eventually, those "bad" half of your cells will be cycling at 3.0v ~ 3.8v. Since the inverter/charger is set to keep the total voltage between 43.4 ~ 54.6, the "good" cells will now be cycling between 3.2v ~ 4.0v. Not a big issue.

Suppose you keep doing that. After a long time, you end up with a battery where half the cells cycle between 2.5v ~ 3.2v (not so good) and the other half cycles between 3.7v ~ 4.5v (definitely not good, unless your aim is to start a fire).

The aim of balancing is to keep the voltage of all cells at the same level, usually by discharging "good" cells to match the "bad" cells.
Most super cheap BMS start to balance only when a cell reaches 4.2?v. If your top voltage is going to be 3.9v/cell, then you need a BMS with configurable balancing function.

For those that like to charge their packs right up to the top - balancing becomes critical because some may be at 4.1v while others are at 4.2v and the 4.2v packs can be pushed over into 4.3v kind of thing as the 4.1v continue to charge up to 4.2v. A closely balanced battery becomes more critical if you insist on 'max charge'.

On a different note - I have read many times that a properly balanced pack will make it 'more efficient' and 'have more power'? But maybe all folks mean by that is that a finely balanced battery can be charge higher etc. Do you folks know what people are thinking of when they say things like this?

Experience so far... using Batrium... I have 42packs and after a bit of adjustment (adding a few cells to weak ones) they are running 30mv-70mv max difference thru the daily charge/discharge cycle with no balancing for several months at a time. My system's hi is 4.03v/cell and low is 3.46v/cell. So it looks like a finely tuned battery does not need constant balancing BUT its nice to be able to balance them as they age or when doing maintenance to add/adjust packs. Batrium works really well for 'day 2' operations as a resource to identify things that need maintenance and then adjust things when adding new packs etc. I hope to have this battery for 10-15years... to get my money back! so long term is a key part of my thinking.

OffGridInTheCity said:

On a different note - I have read many times that a properly balanced pack will make it 'more efficient' and 'have more power'? But maybe all folks mean by that is that a finely balanced battery can be charge higher etc. Do you folks know what people are thinking of when they say things like this?

Click to expand...

I would imagine this means that there isn't a lot of power wasted trying to bring the high pack(s) down. So you take 100% of your solar (or other alt energy) and dump into your batteries, and then you can pull out 95+% or so (there's always a little loss in storage/retrieval) instead of 88% or less. (%'s are just estimates and not hard set)

Another way would be, the battery could be fully charged in, for example, 5 hours, so by 2pm or so they are good and topped off. Whereas a battery that needs constant balancing, it might take 8hrs or more to do the job. This equates to lost power, and time. And more importantly, if you have overcast weather, can drastically effect how fast/efficient you can charge the batteries.

A properly balanced pack gives you more usable capacity.

Or rather, an unbalanced battery reduces the usable capacity, because the BMS will cut off discharge as soon as the lowest cell drops to a little under 3.0v, and also cut off charging when the highest cell reaches a little over 4.2v. (values configurable with a decent BMS)

Also, the cells that repeatedly touch those extremes will over time degrade at a faster rate, possibly exacerbating the problem.

Thank you all for those answers.
It makes the balancing act/part very clear.

Is this also the reason that your packs must be at the same capacity?
For instance 13 of them are 300A and one is 250A.
If one pack(barrel) is 250A and its full,it will be flooded, so the other packs will never reach there full state?
Or they are all full but because one barrel can not hold more than 250A the otherswill not increase to 300A?

Or is it more when you reach the depleted state of that 250a pack?
Won't they balance like fluid in barrels? this would be my guess.

I think that i am missing the point on the same capacity per pack, and it is a completely different story.

Thanks in advance.

Its Ah not A.... A = Current and Ah = capacity...

No they wont balance when in series. Only oin paralell.

The reason you top balance is that you dont want them to overcharge. So lets say you got 250 250 and 300 and top balance.

What happens is when you drain you will end up with 0 0 and 50.. But when charged up again all is full.

If barrels = cells

The bypass voltage equals a hole drilled into the side of the barrel (at the point you deem it to be full)

If you dont slow down the rate of filling the barrel, at the point when it starts spilling fluid via the hole, you'll be wasting fluid (energy) and risking an over filled barrel.

The replies above are great & I wanted to add a bit about these two parts:

100kwh-hunter said:

My packs will be running between 3.1v and 3.9 volt (80%????)
All my cells in my packs are above 92% soh.

Click to expand...

The reason you run a pack at say 3.1 to 3.9V & only over 80% of the full SoC range is to extend the cell life.
This also means you are less likely to hit voltage extremes with some cells going too low or too high voltage. Ie the lights won't cut off, keeps the wife happy!
Note that 3.9V/cell may be unnecessarily low for the top end, you could easily run up to 4, 4.05 or 4.1 like most on the forum do.

Re the cells having >92% SoH is great, it means you have cells which have only been lightly used & have most of their life to go!
It means you have approx 92% of the original manufacturers stated capacity available.
Over time & with use, this SoH number will drop, reducing the available capacity in your system.
The SoH (& available capacity) will drop faster if you push the cells harder eg use over a wider SoC range &/or draw/charge at higher currents.

Thanks for your replays,

With a overfilled barrel you mean, cells above 4.20 volt?
Makes sense, cause they keep on charging till they are all at(in my case) 3,9v.

The longmons make sure that the 250ah pack will not overcharge but give some more to the other packs or starts to bleed some energy.
Except when the charge is to much for them?, then the pack will be flooded?

So in both ends, you would have some energy in storage that is useless?
At the top end it could be even risky to have pack's that are so far away from each other?

Thanks

100kwh-hunter said:

With a overfilled barrel you mean, cells above 4.20 volt?

Click to expand...

No, not necessarily 4.2v - that's the very top end of the charge target range, where you need to think about setting your critical cell voltage at (in order to prevent over charging and hence damage)

In the context of this conversation overfilled means reaching a voltage above that which you've set the bypass voltage to - whatever that might be (upto a max of 4.2v dependent on chemistry etc)

100kwh-hunter said:

The longmons make sure that the 250ah pack will not overcharge but give some more to the other packs or starts to bleed some energy.

Click to expand...

Bypassing = energy wasted, so the aim should be to minimise it (hence being able to set a lower charge rate when in bypass with a compatable charger) - when some cells are in bypass, the charge rate for all other cells does not increase, so no the system will not "give some more to the other packs"

100kwh-hunter said:

Except when the charge is to much for them?, then the pack will be flooded?

Click to expand...

The mons can only bypass a fairly limited, but more than adequate current, that's assuming you thermally manage them by either active cooling or current limiting (either the overall charge rate, or the mons bypass current from within the toolkit) - or both.

100kwh-hunter said:

So in both ends, you would have some energy in storage that is useless?
At the top end it could be even risky to have pack's that are so far away from each other?

Click to expand...

I'm not entirely sure what this means - but no energy stored (in a system designed to store energy) can be deemed "useless" - unless you've allowed cells to reach a terminal voltage above your bypass limits, or out with it's safe working parameters - perhaps pointless or dangerous is a better word than useless in that scenario.

If you've got cells "at the top end" and others that are "so far away from each other" - I'll assume you mean in terms of states of charge - in that situation the role of the BMS is to manage the charging system such that those cells which are "at the top" stay exactly "at the top" while allowing all remaining cells to catch up "at the top"

Any system like longmons will not stop the cells overcharging (going too high in voltage) or being over discharged.
But they will tell you about an issue (cells gone high or low) and help maintain balance with other cells.

If cell voltage goes too high, you need the Watchmon to trigger "stop charging" or even disconnect the battery via a shunt trip breaker.
Similarly if a cell goes too low voltage, you need the Watchmon to trigger "stop discharging" (eg switch off the inverter) or also disconnect the battery via the shunt trip breaker.

Don't worry about the energy lost in bypassing - it's small compared to everything else but essential & the simplest way to do it.

Redpacket said:

Any system like longmons will not stop the cells overcharging (going too high in voltage) or being over discharged.
But they will tell you about an issue (cells gone high or low) and help maintain balance with other cells.

If cell voltage goes too high, you need the Watchmon to trigger "stop charging" or even disconnect the battery via a shunt trip breaker.
Similarly if a cell goes too low voltage, you need the Watchmon to trigger "stop discharging" (eg switch off the inverter) or also disconnect the battery via the shunt trip breaker.

Don't worry about the energy lost in bypassing - it's small compared to everything else but essential & the simplest way to do it.

Click to expand...

Batrium mons, or any other competently designed cell monitor will prevent a cell being charged to a voltage deemed to be excessive - that is one of their main design criteria, but they'll only be able to do that in a correctly configured system.

My comments regarding bypassed energy being wasted energy are correct, but you are right to point out it's a relatively small amount in the scheme of these things - however it's been mentioned elsewhere that stacking multiple mons in order to achieve 10A and above is a practical solution - bypassing that amount of current really is wasteful, the point at which bypassed charge current becomes wasteful is obviously somewhat subjective.

Sean said:

.... stacking multiple mons in order to achieve 10A and above.....

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If high bypass currents are wanted, maybe active balancing units would be a better answer, although they can tend to mask issues. 10A long term bypassing with stacked longmons is highlighting bad design of something!

Sean said:

Batrium mons, or any other competently designed cell monitor will prevent a cell being charged to a voltage deemed to be excessive - that is one of their main design criteria, but they'll only be able to do that in a correctly configured system.

Click to expand...

Yeah that's what I was pointing out, unless the Watchmon can trigger something else, a longmon won't protect a cell by itself.