Bottom balance cells?

[Cuemaker]

The more I study the issue of building a battery bank & balancing the cells the more confusing it gets.
I have EVE 3.2v 280ah Grade-A cells, 16s-48v
Just watched 2 videos where Will says to bottom balance the cells for most home solar systems.
They were from 4 years ago, has something changed.
I top balanced the cells & had no problems at higher voltages cells stayed within 15mv.
But at lower voltage cells below 3.1v they started to get out of balance a lot, almost 100mv.
Which is best, top or bottom balance?

 

[upnorthandpersonal]

Just watched 2 videos where Will says to bottom balance the cells for most home solar systems.
They were from 4 years ago, has something changed.

Yes. If your batteries are mostly sitting full (like most solar applications these days) you maintain a top balance. If your batteries are most often sitting near empty, you bottom balance. Two things changed: batteries became cheaper (larger capacities), and solar power became cheaper (more generation). Unless you do high power applications where you need the last bit of power out of the cells or more often sit at empty than full, you top balance (or rather, have the BMS/active balancer take care of it). Most applications don't need bottom balancing these days, especially solar power related applications.

almost 100mv.

Normal, and not an issue. Below 3V there is practically no energy left in the cell. I use 3.0V as a cut-off for my packs.

 

[Warpspeed]

Look at it this way.
Your battery (hopefully) reaches full charging voltage every single day.
Unless your battery is hopelessly undersized, it should NEVER reach full discharge.

As it spends most of its life in the upper half of the curve, it makes more sense to top balance.

 

[Cuemaker]

I was wrong on the cell voltage saying it was off 100mv, actually it was off by 388mv
Low cell = 2.650v & high cell = 3.038v, the system shut down.
I use this system for off-grid with shore power as backup.
Been working pretty good, I went for over a month & never needed shore power.
But late fall less sun + a few days rain & heavy cloud cover, just wouldn't charge full & started switching to shore power.
This last time SumVolts = 44.8, SOC = 38.3%
Took it apart to top balance & started watching videos on the subject, thinking about changing BMS.
I have Daly 16s-48v-250a, works great if cells are 3.2v or higher, difference between cells usually less than 10mv.
Low end is the problem & it just baffles me.
Like to set it up like Will suggests, run the battery bank SOC from 10% to 90%.
Just can't figure it out, thanks for your help.

 

[Warpspeed]

If one or more cells go weak at the knees long before the others, they have simply lost capacity.

It's probably time to make an executive decision about ordering some replacements.

The whole battery is only as strong as the weakest link in the chain.....

If you are a cheapskate like me, and not too worried by aesthetics, you can get extra life, maybe a few years of extra life out of weak cells by connecting a pair in parallel.

 

[upnorthandpersonal]

Low cell = 2.650v & high cell = 3.038v, the system shut down.

That's empty. Even the cell at 3.038 won't have much left in it. Whenever you come to this point, don't start thinking about getting every last bit out of the pack. Yes, you can replace the one cell, but what do you really gain? A couple Ah? Instead, think about adding capacity. With the price of cells dropping, now is a good time to do that.

This last time SumVolts = 44.8, SOC = 38.3%

No, that's 0%. 44.8V total is an average of 2.8V per cell. That's empty. Keep in mind that voltage is not a good state of charge indicator with LFP, except in the knees. 2.8V is well past the knee: