LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

[Delfin]

Originally Posted by donradcliffe

Having spent some time recently with a drop-in 200 amp-hr Life, I would agree with John. My experience was with a depleted 12v 4S 1P drop in, which came to me well onto the bottom shoulder with cell voltages ranging from 2.92 to 3.05. I started by bottom balancing the lower voltage cells, then switched to charging the whole pack.

At this point the light came on. On the shoulders, the voltage vs SOC is much steeper. so imbalanced cells are more apparent. Once you get into the normal operating range, the voltage vs SOC is very flat. As long as you stay inside that range, you should see very little difference between cell voltages unless you have a really weak cell.

As explained by EVM and CNB, it is true that above 95% SoC differences in cells are easier to spot. The reason some argue for avoiding the shoulders is not because it necessarily helps to keep cells in balance, but that imbalances as lower SoC aren't harmful to the cells until they get extreme. My point was also that the BMS isn't going to balance the cells except at shunting voltage, so if a cell never sees shunting voltage, then the pack won't be balanced if it needs to be. Does that matter if the bank is initially balanced? As I recall Mainesail thought not, or at least noted that his bank stayed in balance without pushing the charging routine into shunting voltage range.

I also believe I saw him write that he now pushes his charge routine to fully charge the pack periodically. Based on the experience of those users who say they see memory effects and a reduction in capacity as a result of operating in a narrow range of SoC and "avoiding the shoulders", such a routine of fully charging the batteries may well be advised.

It is also worth noting that manufacturers recommend charging to the 3.65 vpc level routinely. In the past, John has suggested that manufacturers' instructions should be ignored under the theory that they want you to buy their product, then destroy it by following their directions of usage, which will cause you to immediately buy more of their product. That sounds pretty absurd to me, so I tend to take to heart operating instructions, and in this case that means ignoring advice from people with little to no practical experience in the subject matter.

[anthem00]

Originally Posted by Delfin

It is also worth noting that manufacturers recommend charging to the 3.65 vpc level routinely. In the past, John has suggested that manufacturers' instructions should be ignored under the theory that they want you to buy their product, then destroy it by following their directions of usage, which will cause you to immediately buy more of their product. That sounds pretty absurd to me, so I tend to take to heart operating instructions, and in this case that means ignoring advice from people with little to no practical experience in the subject matter.

I think you need to read this literally and not adapt it to convenience. I believe that charging to 3.65V is fine. . And probably beneficial. What that doesnt mean is charging to 3.65 x 4 - which equals 14.6 Because if one battery is off by a 1v , that means the other 3 are over by 0.33 and getting 3.98V - or more extreme - if one cell is sitting/stuck at 2v, then the other three are getting 4.2V. Thats the issue. But since we dont want to individually charge to that level - we end up trying to shortcut or take the easy way out and write some sort of voltage charge level that we think can handle charging the 4 combined cells. But there is no "safe" voltage level if the possibility of a cell goes out of balance (either minorly or significantly).with the rest of the batteries in series.

Its also one of the reasons why people have issue with BMS units. The BMS units are saving you a macro voltage incident. They are a safeguard for the individual cell - and therefore the entire pack. Everyone is trying to compartmentalize those voltages in to 13.9 or 14.2 or no higher than 14.4 or whatever. But the reality is that isnt it., What you need to protect against is ANY individual cell going over 3.65 (or whatever that limit is). Not the combined four units (whcih could be 3 or 2 whatever). So its not 13.8 or 14.4 or even 12. Its that each individual cell has a lower and higher value that should not be exceeded for longevity.

[Delfin]

Originally Posted by anthem00

What you need to protect against is ANY individual cell going over 3.65 (or whatever that limit is). Not the combined four units (whcih could be 3 or 2 whatever). So its not 13.8 or 14.4 or even 12. Its that each individual cell has a lower and higher value that should not be exceeded for longevity.

Isn't that what the BMS does? - shunt current at an individual cell when the voltage of the cell hits the target, allowing the rest of the cells to "catch-up", resulting in a balanced pack.

There might be a BMS incorporated into a drop-in that doesn't do this, or one built by someone who doesn't care about balancing, but I guess I haven't run across that. My BMS shunts current from individual cells at 3.65v. I periodically charge to that level, per the manufacturer's recommendations, because I think memory effects are real, having experienced the echo of them when I was following a more conservative approach to charging.

Just being a user, I can't claim any expertise beyond personal experience, but the debate over charging regimes seems to be determined by whether you want a few more cycles (theoretically) with diminishing capacity by never fully charging the bank, or a few less cycles while maintaining original capacity the longest by periodically fully charging the bank. Since you'll get thousands of cycles either way, I choose the latter.

[john61ct]

Originally Posted by Delfin

the BMS isn't going to balance the cells except at shunting voltage, so if a cell never sees shunting voltage, then the pack won't be balanced

There are many many ways to keep cells balanced. All I'm saying is that if you are "stuck" with a BMS that does so very ineffectively, relative to your and the cells' needs, then other methods should be considered.

I absolutely do recommend going to higher SoC/voltage charge profiles, if so-called memory effects are observed.

This is not required for normal usage cycling however, should IMO be a maintenance protocol like equalizing FLA.

As should re-balancing, automated routines disabled if at all possible.

3.65Vpc charge termination, along with other specs laid out by cell manufacturer are **absolute maximum** ratings, the limits to which the cells can be subjected for short times, without causing immediate irreparable damage. These are "stress ratings" only, and not recommended for normal functional operating conditions. As with all electronics, regularly operating at these maximum rating conditions for extended periods will likely reduce longevity and reliability.

This has nothing to do with your Straw Man argument claiming some conspiracy to shorten lifetimes.

Please stop putting words in my mouth.

[anthem00]

Originally Posted by Delfin

Isn't that what the BMS does? - shunt current at an individual cell when the voltage of the cell hits the target, allowing the rest of the cells to "catch-up", resulting in a balanced pack.

There might be a BMS incorporated into a drop-in that doesn't do this, or one built by someone who doesn't care about balancing, but I guess I haven't run across that. My BMS shunts current from individual cells at 3.65v. I periodically charge to that level, per the manufacturer's recommendations, because I think memory effects are real, having experienced the echo of them when I was following a more conservative approach to charging.

Just being a user, I can't claim any expertise beyond personal experience, but the debate over charging regimes seems to be determined by whether you want a few more cycles (theoretically) with diminishing capacity by never fully charging the bank, or a few less cycles while maintaining original capacity the longest by periodically fully charging the bank. Since you'll get thousands of cycles either way, I choose the latter.

it is what a BMS is supposed to do. But there are people who are running them without and saying they have certain voltages set (like 13.8) so they dont need one. They are mis-applying the concept of 3.,65 to what is convenient for them which is 3.65 x 4 or something along those lines. People want to read what they want to read. The manufacturers are sayign 3.65 at the cell level. That doesnt mean you get to take the cell level and multiply it by the number of cells.

Same with the charging spec. Charging to 3.65 is fine. But not charging to 3.65 x 4. .. . Because of the imbalances that may or may not be there.

[donradcliffe]

I agree these systems are complex--too complex for the average user.

The LiFe battery I'm working on now came out of a Roadtrek RV. Roadtrek tried to develop a LiFe system for their RV's, and made the mistake of warranting it, and that drove them into bankruptcy.

The 200 ahr drop in that I am working on now (Their Rev H) has a BMS with a parasitic load of 3 amps FOR EACH BATTERY, and that was down from their original Rev which had 5 amps. Theses batteries would quickly die with no load, and the whole RV would go dark in 2-3 days. They had multiple charge sources (inverter/charger, solar, and big alternator, which were not set up with the proper charge voltages, and the HVC would again make the RV go dark.

[john61ct]

Originally Posted by Delfin

Isn't that what the BMS does? - shunt current at an individual cell when the voltage of the cell hits the target, allowing the rest of the cells to "catch-up", resulting in a balanced pack.

There are several different balancing methodologies even within the cheap circuitry included within "drop-ins".

Expensive BMSs differ even more.

Just being able to adjust the start-balance voltage setpoint is a huge improvement, as is allowing a higher balancing current rate than the usual sub-1A.

And setting BMSs aside, a plethora of methods and other kit is available that get the job done much more safely and effectively.

That is why the safest default recommendation is to completely avoid using BMSs to do that job.

But most people don't want to take the time to understand the intertwined factors involved, nor any added costs, so just assume the mainstream way must be fine.

Which of course it usually is, for those not concerned about extending longevity.

[john61ct]

Originally Posted by donradcliffe

Roadtrek tried to develop a LiFe system for their RV's, and made the mistake of warranting it, and that drove them into bankruptcy.

The 200 ahr drop in that I am working on now (Their Rev H) has a BMS with a parasitic load of 3 amps FOR EACH BATTERY, and that was down from their original Rev which had 5 amps.

Yes, another huge issue is that bad BMSs are often the **cause** of early bank failure.

Which is why many owners just ensure they get protective functionality with simpler devices they fully understand, and take personal responsibility for understanding and monitoring their system's functioning.

And even good quality BMS are likely to fail, long before the cells wear out.

Assuming all is well with fully automated systems is foolhardy, with a bank worth thousands, multiple redundant failsafe layers of protection are well justified.

[evm1024]

Let's see, Winston for their LFP400AHA specifies a max voltage of 4.00 to avoid damage. Not 3.65 volts. Further, they specify an operating range of 3.8 to 2.8 volts.

No death and destruction at 3.65 vpc as was stated.

On another note - to have a string of cells and to have one cell 1.0 volts different than the others is - well gosh - a red herring.

[john61ct]

Mr. Winston Chung also claims his Yttrium doping allows for high-current charging at much lower temperatures than other vendors recommend.

You first 8-)

Originally Posted by evm1024

On another note - to have a string of cells and to have one cell 1.0 volts different than the others is - well gosh - a red herring.

https://i.imgur.com/SOKbxQy.jpg

Even 0.1V is a **much** higher imbalance than any owner should allow.

[anthem00]

Originally Posted by evm1024

Let's see, Winston for their LFP400AHA specifies a max voltage of 4.00 to avoid damage. Not 3.65 volts. Further, they specify an operating range of 3.8 to 2.8 volts.

No death and destruction at 3.65 vpc as was stated.

On another note - to have a string of cells and to have one cell 1.0 volts different than the others is - well gosh - a red herring. It appears that that poster has no direct experience as well. Perhaps I'm wrong on that and they will post their LiFePO4 experience base. We all know that some will not.

how will you know without a BMS system (or voltage controlled relay) that cuts out based on cell voltage and not high/low limit voltage ? Thats the entire concept of a BMS in these units - to protect the battery bank in case of a wayward cell that will affect the entire bank. Its done on the cell level and not the bank level. So if you're gone with no cell level management to disconnect the entire supply side, if one cell goes bad - it ruins the entire bank as you now send 14volts in to 3 cells instead of 4. Where are you to manage the charging problem ? Yeah liveaboards have a potential advantage here, but what if you're sleeping or on shore ?

As for my experience. yeah I've run winston 400 pack on my RV for years. I've also got a new bank for CALB SE200 cells to go in to my boat. I dont monitor every single cell voltage and log it as I'm here to use my cells. But I'm a tinkerer at heart. And I've actually played with building my own as well as using other BMS units like Orion JR, etc. Currently playing around with a Allen Bradley PLC (Micro 850 w/IF8 analog inputs being fed by CR magnetics voltage sensors per cell), controlling blueseas 700 bi-stable solenoids, as well as alternator regulator safe cutoffs as my alternators are 265amp balmar alternators. . . I have each cell voltage and can make whatever controls I need. Including the right 6-10 second delays for cutting the B+ field wire on the alternator regulator for safe disconnect. So, Yeah, I dont have massive data logs of whatever voltage and life spans, but I'm a pretty heavy user of lifepo4 battery systems, and I've rolled my own BMS unit as well.

[CatNewBee]

Cell differences of 1V are very unlikely, but differences at the top end before stop charging lets say at 14.2 or 14.4V can be between 50 and up to 100mV between the lowest and the highest cell even with a quite well balanced pack if you run some cycles without hitting the top, as happens on board here. We often end up with SOC between 70 and 90% at the end of the day, because we are cooking in the afternoon before the solar reaches the full charge. We use to hit the float only 3 to 4 times per month.

When the battery reaches the full state you can watch how the voltage difference increases with every Ah charged and you can see how the balancer takes the charge from the top cell and distributes it to the pack on the REC ABMS. You see it, because it switches balancing on for some time and then off to measure the result and estimate if the donor cell is still the one with the highest voltage, then selects the highest cell and continues. Balancing is supressed if the difference between highest and lowest cell is below the set threshold.

Simpler cell module BMS indikate by LED, if a balancer is active by blinking and when all 4 led are on, all cells have reached the desired range and are almost full.

There are many different systems around. What I can tell is, it is always the same cell that causes trouble, there are rarely 4 identical cells, except you preselect them from a large batch. And the drift will grow if you not balance, the weakest cell will overcharge notoriously and will fail.

[anthem00]

Originally Posted by CatNewBee

Cell differences of 1V are very unlikely, but differences at the top end before stop charging lets say at 14.2 or 14.4V can be between 50 and up to 100mV between the lowest and the highest cell even with a quite well balanced pack if you run some cycles without hitting the top, as happens on board here. We often end up with SOC between 70 and 90% at the end of the day, because we are cooking in the afternoon before the solar reaches the full charge. We use to hit the float only 3 to 4 times per month.

I agree 100% that cell differences of 1v is very unlikely. It was strictly a hypothetical number to illustrate the point that the 1v difference is picked up by the others. However on a bad cell, that variance is well over 1v (from all accounts 2v to the entire 3.3v).

[evm1024]

Originally Posted by anthem00

how will you know without a BMS system (or voltage controlled relay) that cuts out based on cell voltage and not high/low limit voltage ? Thats the entire concept of a BMS in these units - to protect the battery bank in case of a wayward cell that will affect the entire bank. Its done on the cell level and not the bank level. So if you're gone with no cell level management to disconnect the entire supply side, if one cell goes bad - it ruins the entire bank as you now send 14volts in to 3 cells instead of 4. Where are you to manage the charging problem ? Yeah liveaboards have a potential advantage here, but what if you're sleeping or on shore ?

As for my experience. yeah I've run winston 400 pack on my RV for years. I've also got a new bank for CALB SE200 cells to go in to my boat. I dont monitor every single cell voltage and log it as I'm here to use my cells. But I'm a tinkerer at heart. And I've actually played with building my own as well as using other BMS units like Orion JR, etc. Currently playing around with a Allen Bradley PLC (Micro 850 w/IF8 analog inputs being fed by CR magnetics voltage sensors per cell). I have each cell voltage and can make whatever controls I need. Including the right 6-10 second delays for cutting the B+ field wire on the alternator regulator for safe disconnect. So, Yeah, I dont have massive data logs of whatever voltage and life spans, but I'm a pretty heavy user of lifepo4 battery systems, and I've rolled my own BMS unit as well.

Great, thanks for sharing you experience base.

I've been running a Winston 700 AH bank since 2014 on my boat.

As noted in other threads I've got 2 weak cells that may have been seconds from the beginning or may have developed a capacity loss. I tend to suspect the former.

I've been trying to recover some of the lost capacity using differing charge/discharge profiles so I am measuring individual cell voltages quite often.

On my bank I am using cell boards that will generate a disconnect signal if any cell voltage is too high or too low.

I do not let the cell boards do any shunting. I let the pack voltage control the Victron inverter/charger.

[CatNewBee]

Originally Posted by anthem00

I agree 100% that cell differences of 1v is very unlikely. It was strictly a hypothetical number to illustrate the point that the 1v difference is picked up by the others. However on a bad cell, that variance is well over 1v (from all accounts 2v to the entire 3.3v).

I know, but in this case the bms would have engaged and cut the busses with an error message.