Self Balancing Li-Ion at lower float charge ?

[CrazedFox1313]

Hello everyone.

I’m building a li-ion 7S 40P “24v” bank for my BMS/power-conditioner unit.

The unit originally used small internal SLA batteries.
I converted it to 2x external deep-cycle lead acid.
Those did fine for years, but then they reached end of life as flooded lead acid does.

I currently have a small Li-Ion bank (18650 cells 7S-8P) in place for the moment to keep the unit content & working normally as I construct the much larger bank.
The UPS unit sits at a factory float charge of 27.2V. In turn will put each of the Lithium cells at 3.89v,
which will put the charge outside the set voltage points of active-balancers.
& I can’t find any “programmable” active balancers to where I can change the volt parameters to the lower charge levels.
(The very few that might work are too weak “1 amp” to function properly.)

So the question is;
With the lower float charge, will the bank’s cells just “self-balance” on their own with the constant-on low voltage ?

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[ULR]

Hello everyone.
So the question is;
With the lower float charge, will the bank’s cells just “self-balance” on their own with the constant-on low voltage ?

As I understand it:

No. Lithium cells require a battery management system (BMS) or risky interminable babysitting.

Lead-acid cells in a battery will self-balance when occasionally fed an "equalizing charge", a slight overvoltage charge that forces the higher charged cells to overcharge and leak current to bring up the undercharged cells. This works because slightly overcharging a lead acid cell causes it to waste the extra power electrolyzing water (inefficiently - which is good) into hydrogen and oxygen. Some of it escapes as gas, which you then replace by periodically adding water to keep the plates covered and the reservoirs above the plates full enough to keep the level from getting down to the top of the plates between maintenance sessions. (If you get fancy you can use cell caps that catalyze the recombination of the gases into water, stretching the time between need for level top-offs_

Lithium cells (of nearly all chemistries), when overcharged, tend to grow whiskers of metal that eventually penetrate the cell separator, shorting out the cell and perhaps causing a fire. They also don't have a lot of leakage-when-overcharging to work with. The BMS (depending on model) transfers power from one cell to another (really good ones) or wastes a little power from the higher-charged cells (cheaper OK ones) to get the cells to be balanced without ever being actually overcharged.

If you run lithium batteries without a BMS you're taking on the BMS' job as a periodic task and risking a fire if you screw up.

 

[CrazedFox1313]

ULR, thanks for responding.

But it is apparent you didn’t read the whole post.
I accept partial blame, due to the long explanation of it.
So let me paraphrase to shorten.

The UPS isn’t capable of charging the Lithium bank to full voltage.
Each cell will only get a max charge of 3.89v per cell, which roughly is 80% of cell’s full capacity.

To which why non-programmable active-balancers will-not work.
I’m aware of the usefulness of a BMS, but this isn’t about that. ( I do plan on using one.)
It’s about Active-Balancing, & the lithium battery not getting charged beyond 80%.

Lithium batteries are very content to live/stored at 80% charge.

•

 

[sunshine_eggo]

So the question is;
With the lower float charge, will the bank’s cells just “self-balance” on their own with the constant-on low voltage ?

No. There is no "self-balance" mechanism.

Balance is primarily a function of self-discharge. If they have identical self-discharge, they will not need balancing. If they do not have identical self-discharge, they will need balancing.

There are non-programmable active balancers that work throughout the range based solely on voltage deviation.

A BMS with programmable passive balancing should more than meet your needs.

 

[CrazedFox1313]

No. There is no "self-balance" mechanism.

Balance is primarily a function of self-discharge. If they have identical self-discharge, they will not need balancing. If they do not have identical self-discharge, they will need balancing.

There are non-programmable active balancers that work throughout the range based solely on voltage deviation.

A BMS with programmable passive balancing should more than meet your needs.

.
Where do I find these full-voltage-range active-balancers ?
( Hopefully without waiting for weeks to arrive from china.)

 

[sunshine_eggo]

Battery Equalizers-Lithium Batteries | Electric Car Parts Co

 

[400bird]

You probably don't need the extra current of an active balancer. You didn't mention pack capacity (at least that I saw) or if the cells are new.

But, if you think they'll self balance, why would an active balancer over 1 amp be required?

The passive balancing built into most BMSs should do fine, unless your cells are crap or capacity huge. After deployment, balancing is long term and doesn't need to happen instantly.

 

[Hedges]

"UPS" - what turns it off when battery gets low?
Is your cell type tolerant of over-discharge, or is it necessary (and do you have a BMS) to stop discharge at low cell voltage?

Since you say 3.89v is outside range of active balancers, I take that to mean not LiFePO4. So maybe a chemistry that doesn't have flat voltage in middle of SoC range, and balancing can work.

Always being charged by UPS, even the idea of Zeners and resistors comes to mind for balancing. Or just resistors.

 

[sunshine_eggo]

"Self Balancing"

The only type of "self balancing" that can occur is with coulombic inefficiency. This is how lead acid "equalizes" as well as NiMH.

As those chemistries increase in SoC, their charging efficiency takes a dive. For purely illustrative purposes, passing 5A through a cell at 50% SoC for an hour will take on near 5Ah. Another cell in that series string at a notably higher SoC (70-80%-ish) will take on notably less - maybe 3-4Ah.

Thus with subsequent cycles, the SoC of the cells eventually converge due to the higher SoC cells having lower coulombic efficiency.

Lead acid is about 85% charge efficient. NiMH is even worse at about 70-80% if you insist on topping them off.

NiMH was a favorite in the early HEVs because of this self-balance phenomenon with near 100% efficiency in the typical 45-65% SoC operating range. If cells drifted out of balance, the higher SoC cells would take on less charge, and the lower SoC cells would catch up. A friend of mine did an experiment by deliberately charging the 28 Prius 6S 7.2V modules to 15.4% (7), 30.8% (7), 46.2% (7), and 61.5% (7). After a 150 mile drive, the pack was disassembled, and the stored capacity was measured. The capacity total SoC difference dropped from 46.1% to under 30% and each group of 7 modules' SoC changed uniformly.

Lithium does not exhibit this phenomenon on a scale significant enough to restore balance once imbalanced.

 

[CrazedFox1313]

Apologies everyone.
& Thank you for the input.
Here is the information requested.

The batteries are used
LG INR18650MH1,
3200-mAh,
10A,
Nominal 3.7V - full charge 4.2V

The design of the pack will be 280 cells in 7s 40P format.
The battery pack is estimated to be at 3211.6 Wh / 86.8 Ah.

The UPS is a Belkin pure-av ap30800fc10,
slightly modified to run cooler & external battery hookup. ( been doing its job for about 15 years.)
The UPS battery shutdown voltage is 22.8V for those who were wanting to know.

I am going to get a BMS for protection reasons.
But unless I externally charge the pack,
it won’t see any more than 80% of its intended charge from the UPS, due to its set float voltage.
—
—
Good folks here have been informative to the fact that everything should be fine with a good BMS.

So now, what BMS would be a good fit for this Lithium-ion 7S 40P battery ?

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[sunshine_eggo]

it won’t see any more than 80% of its intended charge from the UPS, due to its set float voltage.

This is actually a good thing. 3.7V chemistry degrades when stored at full charge. If 80% isn't enough for you, add cells until it is.

So now, what BMS would be a good fit for this Lithium-ion 7S 40P battery ?

Any 7S BMS rated for whatever output current you need. Preferably a smart BMS that gives you access to cell level data for convenience/troubleshooting. JBD is a good place to start. Avoid DALY. JK is popular for their active balancing, but they seem to have some firmware bugs as of late.

 

[CrazedFox1313]

This is actually a good thing. 3.7V chemistry degrades when stored at full charge. If 80% isn't enough for you, add cells until it is.



Any 7S BMS rated for whatever output current you need. Preferably a smart BMS that gives you access to cell level data for convenience/troubleshooting. JBD is a good place to start. Avoid DALY. JK is popular for their active balancing, but they seem to have some firmware bugs as of late.

.
Thanks.

Where is a good place to find JBD ones ?

& what happened to DALY ?
They had such a good reputation in the past.

 

[sunshine_eggo]

.
Thanks.

Where is a good place to find JBD ones ?

I bought a 14S from here, and got it within a few days:

JBD Bluetooth Smart BMS | Jiabaida Bluetooth bms |Sriko batteries

JBD Smart BMS Control & Monitor Your Lithium Battery in USA Find Your Perfect JBD Smart BMS Match. Manage & Protect Your Lithium Battery Shop Now!

srikobatteries.com

& what happened to DALY ?
They had such a good reputation in the past.

IMHO, the # of reports of DALY failures on this forum is notably higher than the others. Multiple users running multiple parallel batteries with identical DALY BMS will have multiple failures over time. My gut says this was associated with a time period where they had defects, and one guy on here got down to the hardware level and found a defect acknowledged by DALY. I can't find the thread.

DALY also trailed on their "smart" BMS features, and I'm still not sure if one can disable "balance only during charge." I KNOW you can on a JBD.

 

[CrazedFox1313]

I bought a 14S from here, and got it within a few days:

JBD Bluetooth Smart BMS | Jiabaida Bluetooth bms |Sriko batteries

JBD Smart BMS Control & Monitor Your Lithium Battery in USA Find Your Perfect JBD Smart BMS Match. Manage & Protect Your Lithium Battery Shop Now!

srikobatteries.com

IMHO, the # of reports of DALY failures on this forum is notably higher than the others. Multiple users running multiple parallel batteries with identical DALY BMS will have multiple failures over time. My gut says this was associated with a time period where they had defects, and one guy on here got down to the hardware level and found a defect acknowledged by DALY. I can't find the thread.

DALY also trailed on their "smart" BMS features, and I'm still not sure if one can disable "balance only during charge." I KNOW you can on a JBD.

.
Thanks.
I wonder when DALY went bad, used to be a dependable name in BMS modules.