LFP/LiFePO4 Top vs bottom balancing on sailboats

[Singularity]

I'll try to squelch this thread if there is a specific thread that addresses this question. Clearly this topic has come up transiently in many threads, but I'm trying to get a narrower discussion to understand the gestalt taken by those on a cruising sailboat who choose top balancing vs the logic of once-and-forget bottom balancing (NOT active bottom balancing!).

The following thread from a household solar forum site makes a compelling argument (in my mind) for "once and forget it" bottom balancing (+/- periodic manual balancing/testing).

https://www.solarpaneltalk.com/forum...-balance/page2

The compelling-est argument to me (FWIW author sites MSEE/PE credentials): [bold is my emphasis]
"[with bottom balancing]....only the weakest cell ever reaches 3.5 volts or roughly 90% SOC. No cell will ever see 100% SOC. All others will be slightly lower. But all cells will have the same capacity in a Bottom Balanced system. The only time the voltage will be equal is at the bottom. What kills lithium batteries is the stronger cells driving the weaker cells into reverse polarity when fully discharged. With all the cells having the same capacity makes it almost impossible for that to happen. With Top Balanced systems every cell except 1 or 2 gets overcharged every time and those 1 or 2 cells go to 100%. You do not want any cell going to 100%. They are not lead acid batteries and thus no need to go to 100%. "

and...
"The weakest cell goes to higher SOC than the other 12 in your example. If the lowest cell is say 100 AH and all the others are 115 AH. You would charge to 90 AH. Weakest cell is at 90% SOC and all others are 78% SOC. You have not sacrificed any capacity, and gained cycle life."

Someone proposes annual capacity checks, just to be safe:
If one bottom balanced yearly while recording the voltage differences at both LVD and full charge, thenI expect to find the same battery # being the odd one out at both ends (true?)

Response:
False. When you Bottom Balance all batteries will have the same voltage of 2.5 volts, and the same capacity of 0 AH. Think of it as say ten slightly different sized buckets with no water in the, When they are empty, you know how much water is every one of them. When you charge, everyone gets the exact same amount charge, so you know exactly where how much water is in each bucket. The smallest bucket gets filled first and you stop.

In Top Balance the only place where anything is equal is at the TOP at 3.6 volts. But that does not tell you how much capacity you have. Back to the slightly different sized buckets. Everyone of them is completely filled up. You do not know exactly how much charge is in them except 100% SOC. On the discharge side, all drain equally but the smallest bucket is falling faster than the others. As you get near the Bottom you can have a cell or two go below 2.5 volts and never know it because your Inverter or Gizmo LVD only sees Total Pack Voltage and thinks all if fine when in fact your destroying cells from over discharge. At the bottom of a Top Balanced battery voltages are all over the place. You can have some of the stronger cells still up around 3.2 volts and others well below 2.5 volts which destroys them. If you look at the Total Pack Voltage everything looks good when in fact it is not.

The author (in blue) opines that the top-balancing concept really was imported from the lead thinking world and his argument is compelling to me. Top-balancing requires a BMS for reliability whereas bottom-balancing uses a BMS for fidelity, a very important distinction with respect to redundancy, cost, etc.

Any anecdotes of failure with the bottom bottom balancing? Why not bottom-balance LFP cells in cruising sailboats?

[200MPH]

Two simple points:

Why go to 3.6 V upper and 2.5V lower per cell? Don't chase that potential last percentage or two of capacity and live a largely worry free life as your cell voltages shouldn't diverge worth a bean.

Boats generally don't have a huge amount of cells in series, so the comparison to household systems is not typically altogether similar.

[john61ct]

Yes, if you are diligent about never approaching the shoulders in the first place, then the concerns about either method pretty much go away,

assuming you bought brand new top quality prismatics from a good source that delivered a pretty well matched set in the first place.

My 4s banks never drop below 11.9x (3.0Vpc is my 0%)

nor get charged past 13.8 (3.45Vpc), zero absorb (my 100%)

And they still test at or over rated AH capacity, so I give up nothing.

Therefore could top- or bottom- balance, and in both scenarios no cell ever gets near danger at either end.

People say you "can't really tell SoC" until you hit the shoulders, their "end of charge" is up in the 3.6-3.7 or even higher range, where the cells' differences may start to show.

But I say going up there hurts longevity, so don't go near there at all. Imbalance issues then just become a theoretical non-problem.

With that operating philosophy, the answer to the OP becomes, "it literally doesn't matter".

[nebster]

Here's how I like to think about it, assuming (a) we are dismissing the idea of using a BMS for "active" management and (b) we are comfortable with staying out of the SOC curve's knees in normal use and sacrificing those last 1-3% of energy storage (two points made succinctly by the posters above):

What's the more likely disaster in your system: a runaway charge, or a runaway discharge?

In my setup, I figure it's substantially more likely that I lose all my charging sources (PV, generator, and shore) by accident than that one of my two charging devices somehow suddenly goes whacko and starts overcharging the pack.

So, for me, bottom balance was the obvious choice. It provides a small additional safety net if the pack goes really low for an extended period by keeping all the cells at a similar voltage in that event. Assuming my LVD circuit works correctly, then, I come back to a stable(r) scenario with hopefully a lower chance of one or more cells being totally nuked.

That said, I carry a few extra cells stored at 50% SOC in case something does happen one day. There's no problem inserting a healthier cell into an older string; it'll balance just fine.


The high-voltage active management strategy of most packs strikes me as a silly choice with batteries built from fresh, verified cells. You pay more, you add complexity and thus failure modes, you are forced to drive the cells high to squeeze in a little bit of extra capacity, and then you likely give up a lot of those tiny gains on the backend of the cells' lifecycle anyway because of all the extra time at high-SOC. Pretty crazy.

If you are designing the system yourself and you have control over the parameters (and you should, anyway), it's hard to argue with an initial balance and a passive approach.

[tanglewood]

Interesting perspectives on this.


I've elected to go with top balancing. The reason is that I will recharge on a regular basis, rising the batteries up to whatever level I decide is my operational full charge level. By top balancing, I think I create the best assurance that my recharge process won't drive any single cell over voltage.


In contrast, it's never my goal to drain the batteries way down, and I will have an auto-start generator kick in when they get too low. So I expect to infrequently visit the low charge limits.


Since I expect to regularly visit the high charge range, and infrequently visit the low charge range, I want to have higher confidence that a high charge won't create a nuisance alarm, or worse yet, a disconnect.


But in all honesty I don't think it really matters a whole lot which way you do it. What's more important I think is to monitor cells so you can start to see when one is getting ahead or behind others, and plan appropriate action.

[newhaul]

Wouldn't all of this be covered and made moot by doing an annual ( imo you should) ah capacity test ?

[tanglewood]

Originally Posted by newhaul

Wouldn't all of this be covered and made moot by doing an annual ( imo you should) ah capacity test ?

It might be a warning indicator if you find cells with diverging capacity, but that’s not directly what you need to protect against.

Even with cells that are exactly matched, they could be a different SOC. Most charge controls look at the battery bank voltage and stop when it hits some point. But they might not have visibility into each cell’s voltage. If one is a bit high and others a bit low, the high cell might go over voltage before the whole string reaches to target voltage. Top balancing would rectify that. And there is an analogous scenario for discharge and bottom balancing.

[nebster]

Originally Posted by tanglewood

Since I expect to regularly visit the high charge range, and infrequently visit the low charge range, I want to have higher confidence that a high charge won't create a nuisance alarm, or worse yet, a disconnect.

Yep, seems perfectly reasonable.

[CGirvan]

Originally Posted by john61ct

Yes, if you are diligent about never approaching the shoulders in the first place, then the concerns about either method pretty much go away,

assuming you bought brand new top quality prismatics from a good source that delivered a pretty well matched set in the first place.

My 4s banks never drop below 11.9x (3.0Vpc is my 0%)

nor get charged past 13.8 (3.45Vpc), zero absorb (my 100%)

And they still test at or over rated AH capacity, so I give up nothing.

Therefore could top- or bottom- balance, and in both scenarios no cell ever gets near danger at either end.

People say you "can't really tell SoC" until you hit the shoulders, their "end of charge" is up in the 3.6-3.7 or even higher range, where the cells' differences may start to show.

But I say going up there hurts longevity, so don't go near there at all. Imbalance issues then just become a theoretical non-problem.

With that operating philosophy, the answer to the OP becomes, "it literally doesn't matter".

How important in this scenario is balancing, if quality cells and a use pattern of avoiding the knees do we need to balance at all?

[tanglewood]

Originally Posted by CGirvan

How important in this scenario is balancing, if quality cells and a use pattern of avoiding the knees do we need to balance at all?

You definitely need to initially. When presented with a new set of batteries, you really don't know their SOC short of running them up to the top or down to the bottom. But once synchronized, people's experience seems to show that they are not prone to drift apart. I still personally like the idea of ongoing per cell monitoring to detect drift, but that's just me.

[john61ct]

Originally Posted by CGirvan

How important in this scenario is balancing, if quality cells and a use pattern of avoiding the knees do we need to balance at all?

Initial balancing yes.

Checking for imbalances yes, if manually then at first more frequently, if no problems shown, scale back to less frequently.

Active cell-level monitoring,

and especially balancing,

there is IMO danger from complexity, especially with inexpensive gear bought OTS.

Presumably less so if buying a packaged system at much higher cost per AH.

Not advising one way or the other though, up to each to balance their own preferences.

[Nicholson58]

This is the best boat tech help I know. It is a very good read and educational. Read from top to bottom and skip nothing. You will be an expert.

https://marinehowto.com/lifepo4-batteries-on-boats/

[nebster]

Originally Posted by CGirvan

How important in this scenario is balancing, if quality cells and a use pattern of avoiding the knees do we need to balance at all?

I think you can actually never balance at the top or bottom and use the "avoid the knees" strategy.

It just makes it very hard to characterize cell-level performance over time (since you have no steep-slope point on the voltage curve to make precise measurements and to compare to later), and it means you have the risk of an outlier cell experiencing an under- or over-voltage on both ends of the curve, instead of only on one end of the curve.

You could just try it as an experiment, if you've got the right tools to keep tabs on your cells and you watch carefully so as not to take any of them out of range. You might find that your "mid balanced" starting point is plenty enough for your needs.

More practically, I think you'll find, though, that you do want to parallel your cells to bring all their voltages equal first, no matter what initial balancing strategy you pick. And, if you're going to bother with doing that, it's only a little more work to go ahead and draw them down or charge them up to a nice bottom or top balance, respectively. (It's even less work if you have a computer charger to do it for you!)

[john61ct]

Saving up for this guy

iCharger 4010-Duo