Lithium Stickies?

[luthj]

Some other useful bits from the sites I posted above. This is my favorite graph. SOC reached vs absorb voltage.




This is also important. Lithium batteries can develop a memory effect after a partial charge discharge cycle(s).

For a memory effect to appear, an incomplete charge cycle followed by a discharge must have taken place earlier (memory-writing cycle). In this case, an abnormal increase in voltage can be observed afterwards as the charging process approaches the point where charging had stopped earlier; this creates a bump in the charging curve. Partial charging of all common types of lithium cells (with the notable exception of lithium titanate oxide Li4Ti5O12) leaves the cell with divided lithium-rich and lithium-poor phases which persist during and after discharge. In order to erase the cell memory of the previous interrupted cycle(s), a full charge must be performed (memory-releasing cycle) and this requires overcoming the bump caused by past partial cycles.
The memory effect was found to strengthen with the number of incomplete charge cycles performed before the erase cycle. It was also strengthened when a partial charge was followed by a shallow discharge, rather than a deep discharge.

These latter aspects have proved to be of key significance when considering the longer term performance of LiFePO4 batteries in house bank applications, because incomplete charge cycles are common when relying on renewable energy sources and shallow discharge cycles are also frequently experienced. These have the potential to render battery banks near unusable after as little as 2-3 years in regular service in the absence of memory-releasing cycles. Ineffective memory-releasing cycles are very common in DIY installations where the charging process is not properly controlled and/or configured incorrectly by fear of overcharging or due to widespread mythologies.

This basically means, that after multiple partial charge/discharge cycles, the battery would need charged to a higher voltage to recover the capacity. Using a simple voltage termination on absorb would result in only a partial charge. This can be overcome with occasional "equalize" charges to slightly high voltages. Another option is to use a voltage AND current based absorb termination. This combined with a slightly high voltage would allow for the extended absorb time required to de-stratify the lithium concentration in the cell. The graph above shows a single memory cycle. After 10-20 of these partial cycles, the voltage bump becomes more noticeable.

Finally,

While we showed earlier that voltages as low as 3.4V/cell were able to fully charge and even overcharge a LFP cell, this must now also be considered in the context of memory effects altering the charging curve of the cells. My experience so far has been that any termination voltage below at least 3.5V/cell should be considered as inadequate if the installation experiences incomplete charge cycles. Any charging system that is unable to provide an adequate absorption down to at least C/20 or less when required should also be considered as unfit for purpose, because it will fail to deliver charge cycles capable of erasing the cell memory.

Charging to at least 14V for a 4S pack is the minimum for partial cycled packs. Even with that it may be necessary to conduct a recovery charge with 14.2V or so until return current drops below the full indicator for the pack. In severe cases this needs to be followed with a deep discharge and recharge at the same voltage setpoint. As mentioned above minimum current should be C/20 to ensure complete transfer of the lithium ions.

 

[luthj]

Charge termination considerations, copied from
http://nordkyndesign.com/practical-characteristics-of-lithium-iron-phosphate-battery-cells/

Charge Termination
Since absorption voltage can’t practically be used to limit charging, it becomes a matter of determining when to stop. Charge termination ideally needs to occur before the battery is completely full, because most of the stress on the battery happens when it runs out of lithium to transfer, or when it can’t transfer lithium ions fast enough, such as when the charge rate is very high and the voltage is allowed to rise excessively.

The tell-tale sign of a fully charged (or overcharged) battery is that it is no longer able of absorbing any significant current

Voltage-Based Termination
If charging at very low currents, such as 0.05C, where internal resistance doesn’t meaningfully skew the voltage reading, termination can be implemented based on a voltage threshold on the basis that the current is then known to be low. A small solar system charging a sizable bank can fall in this category. In this case, charging must stop when the target voltage is reached and not resume until the voltage has dropped to a level indicating that the battery can and needs to be recharged again.

At higher currents, this strategy would err on the safe side by leaving an undercharged battery, but it is unsatisfactory, because charge absorption is still essential with lithium cells in order to erase the memory from previous partial cycles and make a good use of the capacity installed.

Time-Based Termination
Schemes involving a timed absorption period perform an approximate charge termination only. If the battery requires bulk charging and the duration of the absorption period has been determined wisely, a good charge cycle may result. If the battery is already full when charging begins, it will invariably suffer throughout the undesirable absorption phase; using a lower absorption voltage limits the stress placed on the cells, but fails to properly address the issue, increases the overall charging time and open the door to long-term capacity problems resulting from memory effects.

Nearly all so-called “smart” alternator controllers typically implement a time-based absorption strategy to provide a charge termination that is anything but smart… any charge termination is still a lot better than none however.

Absorption times with lithium iron phosphate batteries should typically not exceed 30-35 minutes in most situations, and much less if the battery is being charged at low current. If a time-based termination is going to be implemented, then the absorption time should be determined experimentally by monitoring the current taper.

Optimal Charge Termination
In all instances where significant charging currents are present, achieving proper termination requires monitoring both current and voltage to make an informed decision.

The voltage must be up at the absorption setpoint while the current is down at the charge termination limit; this indicates that the ability of the battery to absorb further charge is near its end. The final state of charge achieved depends on the combination of maximum voltage and minimum current, but changing the termination current is the only reliable way of altering the state of charge obtained and the voltage must always be sufficient to ensure memory effects from previous partial cycles can be overcome.

Charging equipment intended for lead-acid batteries is hardly ever able to perform a proper charge termination, because overcharging lead-acid cells (with the exception of gel-cells) is acceptable to some extent, there are no real safety considerations arising and batteries are relatively inexpensive. The functionality required is not present and the addition of the word “lithium” in the product brochure typically does exactly nothing to remedy to this situation. While battery voltage is always available, battery current is either not measured or the information is not exploited by the equipment. For this reason, the only place for realistically determining charge termination in a lithium battery system is at the BMS and the BMS should supervise the charging process.

 

[luthj]

This really drives home the necessity of a current based charge termination. Or a VERY short and conservative absorb timer/voltage. If a timer is used, it must be determined experimentally by monitoring return current. For example a 100AH bank charging from a 13.8-14.0V source of at least C/4 rate. A return current of 5%C or 5A, would be a starting point if you want more capacity. Though 7%C would be more conservative. If using a timer approach, measure a few charge cycles to determine the time between the setpoint being reached, and the bank current tapering to the "full" level. This is likely to be on the order of 5-25 minutes.

 

[john61ct]

At low enough current rates can overcharge at 3.45Vpc or even less.

CC-only, Stop At V charging is fine, simpler and safer.

Memory effect is negligible and easily avoided.

At high rates can safely - avoid reducing cycle lifetimes - go even to 3.60V and still be a couple % below Full.

Remember getting to Full is not something to turn into a goal in normal day to day cycling

that's an artifact from "Lead thinking"!

Rather, Full SoC is something generally to avoid, potentially very damaging - reducing cycle lifetime

Go there only when that is truly needed - usually never - and for a brief time only.

Doing so, and sitting there for hours, just to satisfy a poorly designed BMS is IMO stupid.

 

[luthj]

Looking at the data cited in the site I listed above, a year of cycling about 50% of the pack, and charging only with a lower voltage cutoff can result in significant capacity stuck within memory. If you are regularly charging up to the mid 90s SOC, this will be minimal, but only getting to the 60-70s SOC it will be much larger. This is something to consider for setups that run a charging deficit for much of the time. Over time the little voltage bump in the curve gets higher, and eventually triggers the charger to drop out earlier and earlier. This can eventually cause the charger to drop out as early as 60% SOC. which only exacerbates the partial cycling and memory effect.

I am not saying you need to get to 100% or even 95% every cycle. But not having sufficient voltage and/or duration to get to the high 90s every 10-20 cycles can produce significant memory effect over time. This is especially true with low current charge sources which may taper to only C/50 near the end of the day/cycle.


Regardless in a daily cycled pack high SOC wear is very unlikely. Even charging to 100% every other cycle would only have a small reducing in life, especially since camper loads will pull it down to the mid 90% within a few minutes.

Generally this type of effect can easily be detected by comparing resting voltage with the readout from the AH counting SOC meter. The bank will drop out of the upper voltage plateau much more quickly, and enter the edge of the lower knee earlier.


As mentioned, even 3.5VPC can overcharge if left beyond the absorb period/current. With current monitoring on charging, the memory effect is not likely to happen, as during the occasional topping charging, the absorb period will continue until the return current setpoint is met.

 

[john61ct]

I've always emphasized that all charge sources need to be user-custom adjustable in their setpoints.

It really is not hard to take care of a LFP bank for optimal longevity, likely decades - even just bare cells - with minimal tools, most important being knowledge and attention.

Blindly following vendor specs, or allowing some automated system to run things without testing & monitoring what's going on, is just silly if longevity is a priority.

 

[luthj]

I would also like to point out, that unlike lead acid sulfation, LFP memory effects can be completely removed with a conditioning charge, or in severe cases, a series of charge and discharge cycles. Try doing that with a sulfated lead acid bank...

 

[john61ct]

However overcharging or going too low,

especially sitting at the shoulders either end

leads to permanent unrecoverable damage (loss of lifetime cycles)

Note I am not talking about the piddling few thousand cycles vendors spec under EV or other abusive conditions.

I believe with proper care diligently avoiding the shoulders in normal cycling, calendar lifespan will become just as or more significant than cycle counting.

Ten years of daily usage with zero loss of capacity from mfg rated Ah is not even difficult, we just don't have enough data to know how long past that is possible.

 

[luthj]

Ten years of daily usage with zero loss of capacity from mfg rated Ah is not even difficult, we just don't have enough data to know how long past that is possible.

This is generally only possible for prismatic cells, because they are rated at 2-6Hr rates, and the MFGs pad the capacity by an extra few percent to reach the desired lifespan in higher rate applications. At the 20hr rate a prismatic will deliver 2-5% more capacity than at the 6hr rate. Add another extra 2-3%, and you may be 10% above nominal rating at 20Hr. Most cylindrical cells are not so conservatively rated, and thus will experience some capacity loss beyond there nominal rating every year.

There are reports of ~1000 cycles (98-20% SOC) with only a few percent reduction in capacity. So obviously its quite reasonable.

Which begs the question, how long will drop ins like BB last if you actually ignore there 14.6V absorb advice, at least without a very short timer? It may be that BB and other cylindrical based units need a bit higher voltage for their balancing scheme. Notably, with well matched cells balancing is a rare occurrence.

If Tesla can get 300k miles (over 1000 cycles) from a LNM pack running at high rates, obviously it is not unreasonable to expect many times that from LFP in fractional C applications. Notably Tesla considers replacement/degradation at around 80-85% of new capacity. Which is the criteria I use as well. Many folks over the years will claim 5-10K cycles on packs, but they have enough extra capacity, that dipping into the 70% or 60% of new capacity range is okay.

 

[john61ct]

Yes of course talking quality prismatics, silly to consider another form factor in this use case.

And I've seen zero capacity loss from actual commission-time benchmarking after 8 years, 2000+ cycles.

But following vendor Full-spec charging and letting them sit there def won't get you there.

Standard industry EoL is 80%, but with LFP I bet a few decades from now we'll see that can safely be pushed to maybe even 50%, say for off grid homeless shelters.

I don't know of any drop-ins I'd use myself, poor value IMO. With BB I believe there isn't even any access to the cells.

But for people OK with replacing maybe every few years, they are easy. . .

 

[jacobconroy]

Holy cow! Lots of good infos. I just came back from a camping trip and am tired tonight...but I'll read through all this (and the links) carefully tomorrow.

Glad this is taking off. Sure going to help me (and hopefully others) out.

 

[Hillbilly Heaven]

I am suffering from information overload.

 

[john61ct]

To learn if you're motivated:

Create a facility for organizing notes that suits your learning style.

Read things multiple times, alternating between skimming for the gist, and close parsing to build meaning wrt the details.

Build a glossary table of unknown terms

google using the jargon and keywords like 101 basics how-to

Ask specific questions after spending time on the above

 

[luthj]

...I'll read through all this (and the links) carefully tomorrow...

Read through it all in one day?!

Its gonna take you a little bit. Thankfully that while the material is a little complex, the distilled down rules/regulations are much simpler.

Rules of LFP for longevity.

1. Thou shall not overcharge. When return current drops, charging must be removed.

2. Thou shalt not float charge. If a float voltage is present, it should be below the 70% SOC level of the pack (suggested 13.2 for a 12v pack). Its okay to let the pack sit at lower SOC.

3. Though shalt not over discharge. This means having a Low Voltage disconnect (automatic).

4. Though shalt not charge at high rates at low temps. Very slow charging is okay, and discharging is okay. Depending on the usage case, a charge (or pack) low temp disconnect may be called for. This can be the same relay/contactor as the LVD/HVD.

5. Though shalt not exceed 14.6V or 3.65VPC in normal operation. Ideally lower in most cases. An alarm above this level is suggested. A High Voltage Disconnect at the packs max voltage (per MFG specs) MUST be installed. I would personally set the alarm a bit higher than my highest charge source (usually less than 14.2V, often lower).


There are other commandments for DIY packs, but that is a deep rabbit hole. I think the biggest takeway that should be drilled into everyone, is that charging above 14.2V has little or no value, and is detrimental. Regardless of what the MFG suggests!!! And in fact LFP 12V nominal packs will easily charge from 13.8-14.0V. Obviously the votlage drop in the wiring and the output of the charge source play a roll. To get the most from a larger charge source (above 0.4C), you may need an extra tenth of a volt or two.


Lead acid is about the same complexity really. Its just that charging equipment is designed around it, so its easier to give people recommendations with regards to existing components, such as CC/CV charge profiles with timers, etc. A significant bit of complexity is introduced by the needs for disconnects to prevent instant bricking a pack with over discharge.

 

[john61ct]

The summary I first linked to should (hopefully) include all that,

and the marinehowto post is manageable, but

yes the very long CF thread it leads to though very educational, is. . . very long.

I would pick a couple nits here, or add details, acknowledging I prioritize longevity more than most:

Thou shall not overcharge. When return current drops, charging must be removed.

Since healthy charge rates are down at 0.2 - 0.3C anyway, holding absorb is only needed if every % capacity Full is desired, and should have loads running or coming soon, don't sit at high SoC.

Staying away from Full is better.

If you believe memory effect is a factor, just vary your charge termination point, and go to "vendor spec Full" as a maintenance routine once in a while, like when checking for imbalances.

Balance methods that "require" sitting at too-high SoC should be avoided.

Though shalt not charge at high rates at low temps.

Best to avoid rates higher than .4C, at all ever.

Best to be very careful about charging at all if you think bank core is below freezing. Yes a .05 or 0.1C rate is safer, but just how cold is OK, even at low rates, needs close consultation with the mfg specs docs. And don't just blindly believe Winston on this issue, I'm very skeptical of his claims.

Though shalt not exceed 14.6V or 3.65VPC in normal operation. Ideally lower in most cases.

13.8V / 3.45Vpc for longevity, especially at healthy (low) charge rates. If faster charging is justified, maybe 14V / 3.5Vpc, but if holding Absorb / CV until endAmps anyway, nothing's really gained beyond a few minutes runtime.

At very low rates (like solar) it is easy to overcharge even at voltages lower than this.

Of course voltages must be measured precisely and at the bank, not trusting charge source readouts.

Lead acid is about the same complexity really. Its just that charging equipment is designed around it

The key difference is the (much) higher up-front investment if getting a quality system, in NA maybe 10x higher cost per Ah.

The cost per Ah per year MAY turn out to be lower, but such a long ROI gets very risky, when a single "event" can instantly and irrevocably turn the bank to scrap, or if the owner buys into the "drop-in" kool-aid or just blindly follows industry charging norms.

In fact I'll go so far as to say generally speaking, LFP does not make sense just economically,

other factors like the higher density, less V sag, faster charging really are what might justify it, for those like racing yachts where those are important and the price difference of a few grand is chump change.

In other markets, if only say triple the cost per usable Ah, that tilts things more in LFP's favor.

Of course plenty of people are happy to pay lots more for the social status factor, or just to have bleeding-edge tech to play with as a hobby.

Which are perfectly valid, to the point many say "longevity schmevity" and don't mind replacing even an expensive bank every 3-5 years.

Me? I want to leave it to my grandkids. . .