Undercharged lithium become unbalanced?

[BobandSharon]

Hi all. As we prepare to build our off grid retirement cabin, I’ve spent a lot of time researching all the latest components. We lived in Montana, off grid from 1992 to 2006. We had 2kw of panels, 40 - T105 batteries, a C60 pwm controller, a 12v 100a AC battery charger and a Chinese 12v - 8kw inverter. We ran our small 2kw generator about 100hrs each winter. The system was simple and bulletproof, we were never left in the dark, not once.
Our new system will be around 8kw of panels, Victron charge controllers, 2 - Victron 5kw inverter/chargers and 4-6 SOK or EG4 48v batteries (OR 24 - 12v 100ah AGM batteries s/p to 48v). The batteries are the question; I’ve read numerous posts here all related to lithium batteries becoming unbalanced during periods of low or no charging, such as weeks of cloudy weather. Our place is in North Idaho, and there’s a lot of cloudy winter weeks. The batteries would have to live and operate at 30-80% charge for days without problems. My initial understanding was that lithium could be discharged to near 0% over time with no issues. But what about extended periods at low partial charge?
I understand that lithium will charge faster and fuller than LA batteries, but will I end up having to run my generator more often with lithium just to maintain balance? I’d like to run the genset once the batteries reached say, 20% state of charge, and not have to worry about them becoming unbalanced in the mean time.
Are there any lithium batteries available that automatically maintain balance throughout their state of charge?
Thanks in advance!
Bob

 

[BentleyJ]

I’d like to run the genset once the batteries reached say, 20% state of charge, and not have to worry about them becoming unbalanced in the mean time.

LFP chemistry doesn't have to be fully cycled every time. LFP chemisty doesn't have "memory" like NiCad batteries. Micro-cycling is fine and there is some evidence that it may increase lifetime.

Are there any lithium batteries available that automatically maintain balance throughout their state of charge?

NO, please refer to the attached chart. It shows how the voltage is very stable over a wide range of charge conditions. Therefore cell balancing is only effective at the lower and upper end of the curve (knee) when voltage changes are much larger for small changes in SoC. Basic rule is to set a balancer to start only when the first cell reaches 3.4V and to be off below this value.

You will need to bring the battery up to full charge from time to time and let it float for several hours so the balancer and do its job.

 

[hwy17]

Are there any lithium batteries available that automatically maintain balance throughout their state of charge?

I've heard that NMC can do this, but I'm not sure about that. And most people avoid NMC due to fire risk.

You just need your batteries to get to full charge and stay there a while to balance. In the summer, solar should be fully charging them most days. In the winter if you're not hitting full charge with solar, you would want to use a generator to fully charge and balance them maybe every 2-4 weeks.

 

[BobandSharon]

Thank you. I appreciate the replies, and the answers make sense. A few extra hours of idle time on the inverter generator should be fine.
I don’t want to build a system that’s going to be tempermental. I’m looking for long term reliability, but with modern components.

 

[RCinFLA]

The batteries are the question; I’ve read numerous posts here all related to lithium batteries becoming unbalanced during periods of low or no charging, such as weeks of cloudy weather.

In several months, yes. Not significant in a few weeks.

LFP cells have between 0.2% total capacity/month to about 6%/mo. worse case for self leakage rate. Leakage is greater when ambient temperature is higher and when cells get older
.
All cells will unlikely have the same leakage rate. Over several months this self-leakage difference can added up to significant imbalance.

High discharge or charging currents accentuate mis-matching between cells causing greater rate of misbalancing in their SoC.

Are there any lithium batteries available that automatically maintain balance throughout their state of charge?

No, all batteries have some self-discharge rate that will often have variance between cells in battery causing imbalance over some period of time. All lithium-ion batteries use similar electrolyte which is the primary source of leakage current.

LFP battery BMS's require a cell voltage above about 3.4vdc to initiate a cell balance so if you do not run a full charge periodically you will not get much balancing. The amount of time held about 3.4v depends on BMS balance current and how misbalanced the cells.

At moderate charging current of about 0.2 C(A), there will likely be a BMS overvoltage cell trigger that shuts down charging prematurely when balancing get worse than about 1% SoC difference between cells.

BMS's with resistor bleed balancing may only do 50-150 mA of balancing. 1% SoC imbalance on a 100 AH battery is 1 AH, divided by 100 mA bleed will take ten hours held above 3.4v per cell to get fully balanced again. You can get active balancers with several amps of balancing current.

Besides BMS balancing, if you have a battery monitor with SoC % readout, you also have to bring batteries to full charge periodically every few months to reset the SoC Columb counter to 100% full charge to wash out cumulative errors that will build over time due to current measurement inaccuracy.

 

[Kornbread]

Most bms use a passive balance method where power from the highest cell is simply 'burnt' off in heat. IIRC, this is the method for both batteries mentioned. After several weeks of partial winter charge, where the batteries were never able to achieve and maintain a high voltage for any lenght of time, the bank of eg4 I had became significantly unbalanced.

 

[hwy17]

I’m looking for long term reliability, but with modern components.

I would say the key above all is understanding the mechanism of cell balancing and having the tools with your BMS to be able directly observe and confirm when balancing is happening and when it is done. Most decent quality batteries like EG4 will include this software.

 

[Ampster]

You just need your batteries to get to full charge

Most BMSs start balancing at 3.4 volts per cell and some BMSs allow that setting to be adjusted. Therefore you do not need to take the cells all the way to 3.65 per cell for them to balance. My DIY pack charged to 3.5 volts per cell and the balancer kept them in balance.

 

[hwy17]

Most BMSs start balancing at 3.4 volts per cell and some BMSs allow that setting to be adjusted. Therefore you do not need to take the cells all the way to 3.65 per cell. My DIY pack charged to 3.5 volts per cell and the balancer kept them in balance.

Well, full charge is a bit of a spectrum isn't it. I balance at 3.437 and consider 3.437 fully charged once it's absorbed.

I don't mean they need to be taken to 3.65, just that they need to be held at 3.4+ to allow a passive balancer to work.

 

[toms]

My LiFePO4 battery is over 12 years old now. It often goes all winter without being fully charged or activating balancing. I wouldn’t worry about it if I were you.

 

[Cmiller]

My LiFePO4 battery is over 12 years old now. It often goes all winter without being fully charged or activating balancing. I wouldn’t worry about it if I were you.

I have seen quite a few brands of lithium batteries with imbalance issues within the first 2-3 years of age! A lot of it comes down to a combination of cell quality and use case. By use case I am referring to off-grid vs grid tie with battery backup vs grid tie/ self consumption, etc. It's not a cut and dried thing. Another factor is how much solar or other charging one has. If, from March through October, someone's batteries get fully charged every day or 2 and their charge profile is set in a way that allows for top end balancing on every charge cycle, then they will possibly not have any issues. If the batteries are always full (and held at a continuous ~54.4V), and are just there for backup for a grid outage, then they will likely never have issues. On the other hand, if someone is off-grid and have long periods of time (even in spring and fall) where the batteries never get up to a ~55V+, then they will likely have imbalance issues, especially if they only have BMS passive balancing and no active balancing!

Your specific use and battery combination are obviously a good combination!

Are there any lithium batteries available that automatically maintain balance throughout their state of charge?

Because of how flat the voltage curve is between 20-80% in lithium batteries, you really can't do much balancing in that range. Where the imbalance shows up is at the top and the bottom! Once your SOC gets to 80%+ the highest cells will soon begin to have voltage rise due to being nearly full (if you have imbalances). On the flip side, at the bottom end, you will see voltage drop on lower cells when you have imbalances.

I would recommend having your gen auto start based off voltage, rather than SOC, as SOC can run off too easily in the winter months with extended low PV power time frames! Remember that your Victron battery monitor or Smartshunt (I am assuming you will have one of these) will inevitably run off little by little over time. (EDIT* Whenever the batteries get full, it will re-calibrate and be accurate again!) Personally at home, my gen starts at 50.4 volts. Anywhere in the 50.2-50.8V range will work fairly well, and you can fine tune from there to get it to where you want it. Oftentimes 50.6 will be around 15-20% SOC.

I would highly recommend going with a battery that has active balancing! Active balancing "shuttles" excess power from the highest cell to the lowest one, and it does it through the whole range of the battery instead of just during charging and at the very top end. As I mentioned, it won't be doing much between 20-80%, but it will be doing it anytime the difference between highest and lowest cells is more than XXV (We often set that to ~0.05V). Also, the active balancers often balance at a rate of between 1-4A. That is way better than the mA rate that a BMS's passive balancing will try to keep up at!

Personally, I have seen too many imbalance issues with only passive balancing, and I wouldn't even consider buying a battery without active balancing!

If you have active balancing in your batteries, you shouldn't have to worry about manually needing to balance during winter months, as imbalances happen relatively slowly, and summer months should keep after the balancing for the most part.

 

[BobandSharon]

Thank you all for helping me understand lifepo cell balancing a bit better. Active balancing seems like a feature all batteries should have, just to reduce charge time if nothing else.
I read that the SOK and EG4 batteries all use passive balancing ( including the EG4 Power Pro which recently caught my eye). Can an active balancer be added to the system?
Thanks again!
Bob

 

[Kornbread]

Thank you all for helping me understand lifepo cell balancing a bit better. Active balancing seems like a feature all batteries should have, just to reduce charge time if nothing else.
I read that the SOK and EG4 batteries all use passive balancing ( including the EG4 Power Pro which recently caught my eye). Can an active balancer be added to the system?
Thanks again!
Bob

Check out andy's off grid garage on youtube and the video about some of the active balancers he tried. He's got some good info. Don't know specifically if any of the add-on active balancers will work with the eg4 or sok server racks.

 

[Cmiller]

Can an active balancer be added to the system?

Active balancers can be added to any battery. However, just keep in mind that adding an active balancer will likely void the warranty. And also, how hard it is to add it will depend on how the cell connectors are fastened (welded vs bolted), and/or how the BMS wires are connected (soldered vs bolted). If there is no bolt connection there already, you will need to either solder the balancer wires, or drill and tap the cell connectors.

As @Kornbread mentioned, check out Andy's Garage for balancer info. I would look around online for other balancer resources as well. Personally, I have only ever worked with JK balancers. JK balancer work great, but there are likely other brands out there that work fine as well.