LFP memory effects thread

[john61ct]

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[Cpt Pat]

Reading through some of the recent posts:

"If all you have is a hammer, all problems look like nails."

If all you have is a voltmeter, charging LFPs looks only like an exercise in managing voltages.

That outlook works with lead-acid batteries. For LFPs, you have to measure acceptance current as well, and consider all the non-linear interrelationships between the two measurements. And if the available charge current continually changes, you're into a realm of impossible complexity. But if you stay in the linear region of the voltage versus SOC curve (stay off the knees and inside the 20-80% SOC region) and replace the amp-hours taken out with about 101% of that value put back in (regardless of the rate), you'll be back into a rational area of practical and manageable operation.

Recharge occasionally to 100% SOC for a memory release cycle.

[tanglewood]

I see your point about coulomb counting to deal with variable charge currents.


But I think there is another way that would be equally good, and probably a number of others too.


As you noted, full charge is reached at different I/V points. As acceptance current changes, so does the corresponding full voltage point. Armed with the IV curve that represents full charge, you could adjust the stop charge voltage based on the acceptance current. That way you would stop charging whenever you cross the "finish line", and it wouldn't matter where you cross it. It could be at a low I and low V point, or a high I and high V point.

[Cpt Pat]

Originally Posted by tanglewood

I see your point about coulomb counting to deal with variable charge currents.


But I think there is another way that would be equally good, and probably a number of others too.


As you noted, full charge is reached at different I/V points. As acceptance current changes, so does the corresponding full voltage point. Armed with the IV curve that represents full charge, you could adjust the stop charge voltage based on the acceptance current. That way you would stop charging whenever you cross the "finish line", and it wouldn't matter where you cross it. It could be at a low I and low V point, or a high I and high V point.

I'll take the liberty of calling that an "algorithmic" approach. Yes, that would work. You would have to recalibrate the algorithm to accommodate battery aging (hopefully not too often) and cell temperature (which would be a major variable with LFPs), but it should work as well or better than coulomb counting. It's only downside that I see is complexity.

I'm glad you describe the I versus V factors so well and succinctly. What really bothers me, and I what believe causes needless conflict in discussions, is quoting cell voltages without specifying the charge currents used to reach those voltages. One must have both numbers to make any sense at all out of charging methods.

It would be very helpful if the battery manufactures provided us with I/V and ambient temperatures numbers to form that algorithm, instead of stating just one cell voltage at one charging rate to achieve 100% SOC. If someone developed an algorithmic charger, maybe the manufacturers would fall in line or lose market share.

As it stands, they are marketing mostly to the electric vehicle market where the vehicle makers have full control to "tune" the charger to the batteries. We users of PVs for charging are on our own. And the manufacturers of shorepowered charges and PV charge controllers seem to just slap the word "lithium" on their products without consideration of the needs of LFP batteries. Nearly all use voltage-only charge state detection.

[john61ct]

Originally Posted by Cpt Pat

It would be very helpful if the battery manufactures provided us with I/V and ambient temperatures numbers to form that algorithm, instead of stating just one cell voltage at one charging rate to achieve 100% SOC.

Yes would be nice.

But I believe in a context where

.2C is considered a pretty slow charge rate

then the Absorb V setpoint would not need to vary based on whether the rate is 0.1C or 0.4C

as long as stop-charge is based on endAmps.

Your current rate is too low to follow that usual practice.

CC-only stop-charge profiles are usually at a **lower** SoC point than adding CV time,

but in that very-low C-rate scenario, even that is in danger of over charging.

You've chosen to rely on a coulomb-counting stop-charge method to avoid that, but as you point out that is not the only possible solution to the very-low C-rate problem.

Another might be creating a SoC vs top-voltage table, based on say a 3min rest **for that bank** at your usual / average current rate.

As 100% is approached the charge pause could be lengthened.

Of course does not work IRL with concurrent loads.

[Q Xopa]

Originally Posted by john61ct

Yes would be nice.

But I believe in a context where

.2C is considered a pretty slow charge rate

then the Absorb V setpoint would not need to vary based on whether the rate is 0.1C or 0.4C

as long as stop-charge is based on endAmps.

Your current rate is too low to follow that usual practice.

CC-only stop-charge profiles are usually at a **lower** SoC point than adding CV time,

but in that very-low C-rate scenario, even that is in danger of over charging.

You've chosen to rely on a coulomb-counting stop-charge method to avoid that, but as you point out that is not the only possible solution to the very-low C-rate problem.

Another might be creating a SoC vs top-voltage table, based on say a 3min rest **for that bank** at your usual / average current rate.

As 100% is approached the charge pause could be lengthened.

Of course does not work IRL with concurrent loads.

I dont have a LFP system yet. I am here trying to decide which setup to get.

All of the ideas being implimented and proposed sound reasonsble.

However as well as they may, or may not work, how are they practically implimented?

'End Amps', I know some people are fans of this, which is fine. But unless we have the right Magnum chargers how do we do this without sitting and watching when to stop charging.

This sounds like a low charge rate issue, from Solar scenerio. So if we have our Magnum charger and its a very cloudy day or dusk reducing our charge to trigger our 'end amps' stop charging rate.

So it looks to me we could end up being under charged, which sounds to me (if I understand the jist of this thread) like what causes this 'memory effect'.

Or if we dont have this 'end amps', as most of us dont, there is conversely the risk of overcharging at very low charge rates, like a cloudy day.

IV 'algorithmic' proposed method. Again sounds reasonable however how can we get this to automatically end charging when we are in town provisioning?

I dont know of any chargers we can program this way.

CP, maybe I missed it, but 'Coloumb counting'. Also sounds reasonable. You do this manually monitoring?

Maybe we are 'over thinking' this issue, which is not really so much of an issue?

I dont pretend to know the answers. Just asking.

[newhaul]

Originally Posted by Q Xopa

I dont have a LFP system yet. I am here trying to decide which setup to get.

All of the ideas being implimented and proposed sound reasonsble.

However as well as they may, or may not work, how are they practically implimented?

'End Amps', I know some people are fans of this, which is fine. But unless we have the right Magnum chargers how do we do this without sitting and watching when to stop charging.

This sounds like a low charge rate issue, from Solar scenerio. So if we have our Magnum charger and its a very cloudy day or dusk reducing our charge to trigger our 'end amps' stop charging rate.

So it looks to me we could end up being under charged, which sounds to me (if I understand the jist of this thread) like what causes this 'memory effect'.

Or if we dont have this 'end amps', as most of us dont, there is conversely the risk of overcharging at very low charge rates, like a cloudy day.

IV 'algorithmic' proposed method. Again sounds reasonable however how can we get this to automatically end charging when we are in town provisioning?

I dont know of any chargers we can program this way.

CP, maybe I missed it, but 'Coloumb counting'. Also sounds reasonable. You do this manually monitoring?

Maybe we are 'over thinking' this issue, which is not really so much of an issue?

I dont pretend to know the answers. Just asking.

yes consistently undercaharging can apparently lead to this memory effect but it seems to so far be the general consensus that as long as you achieve a full 100% end amps charge every 25 50 cycles you are avoiding that from happening but I am only 2 years in and I run lots of solar so I have not experienced the " memory " effect to this point.

[john61ct]

Well knowing where the industry needs to improve, is better than thinking everything's all set for all cases and screwing up our expensive banks.

Originally Posted by Q Xopa

End Amps', I know some people are fans of this, which is fine. But unless we have the right Magnum chargers how do we do this without sitting and watching when to stop charging.

Not every charge needs a perfect end point.

Some scenarios, manually stop-charging is required, or accept lifetime may be reduced a bit.

Or just stick with lead, plenty of options.

> So it looks to me we could end up being under charged, which sounds to me (if I understand the jist of this thread) like what causes this 'memory effect'.

> Maybe we are 'over thinking' this issue, which is not really so much of an issue?

IMO easy to overestimate its prevalence and importance. Just don't go for months and years stopping at the exact same spot, mix it up and once ot twice a month get to a higher definition of 100%.

> Or if we dont have this 'end amps', as most of us dont, there is conversely the risk of overcharging at very low charge rates, like a cloudy day.

Most of us have more than one source of energy inputs, and there are multiple ways of preventing this scenario, relatively rare to start with, but overstating the magnitude of the potential problems is not helpful.

But few should believe that LFP is a must have, in fact the most knowledgable think it's not for regular owners, DIY should be discouraged, only good for those ready to spend thousands on a packaged system, say $500 per year?

Meantime, discussing the techie issues and their techie solutions is helpful for the community's awareness and even enjoyable for some.

[CatNewBee]

I like this discussions, especially from experts who have no LFP packs and have never charged or equalized a battery. The lesser practical experience, the stronger the opinions.

What I can tell is, there is no charge current after reaching the float voltage. You charge to your comfort zone, mine is 14.3V for the MPPT controller, stay there for few minutes and go to float at 13.5V.

This leads to a cut-off 3.575...3.6V cell voltage, enough for the balancing, estimated SOC around 95% of the total capacity.

Float at 13.5V means no charge current down to cell voltage 3.375V, BUT provision of energy to all gear that is on from solar, the battery stays full until we draw more power.

As cruisers our battery cycles most of the time between 70% and 90%, eventually down to 50% and up to 95, considered full charge every few days. If we have excess energy, we burn it for hot water, ce cubes, ice cream, washing machine, water maker, baking of bread, whatever.

We produce and consume about 8kWh per day. This would translate to 600Ah, in reality the battery contributes about 300Ah.in cycling, the other 300Ah are burned directly from the source (solar array). Our capacity choice of 1000Ah gives us a comfort window of 4...5 days bad weather with no need to run a generator.

All discussions about pampering the cells, disconnecting the battery for longevity, bumps on memory effects are academic. We have found our perfect energy source and storage, we look at the gauges of the BMS telling the SOC, Voltage and Current and on the BMV, that shows the Ah drawn.

For us it is only important to not draw constantly more than 4kW on 220V.

Easy reading, even for the admiral with no knowledge about LFP, inverters, chargers, balancers, whatsoever.

LFP just work unattended and reliable.

My observation is (because I do want to know what is going on) the cells do drift, and the balancers do constantly a good job to keep them in balance and healthy, it are always the same cells, that drift in the pack. I am happy with the set up so far.

Some battery statistics:
4 cells Winston 1000Ah.
Deepest discharge after initializing in the Lab 1280Ah.
Deepest discharge in operation 750Ah
Average discharge 550Ah
Charged/discharged energy 765kWh or 59.000Ah so far.

The BMV counted 21 total charge cycles and 38 synchronization in 16 months of operations, the boat was 8 month in the marina over the winter with LFP and solar running and 8 months of liveaboard cruising.

Easy living on the hook.

Extrapolating 5000 cycles it means 1900 months left for living on board or 158 years. I guess, I can sacrifice a few cycles, do not expect to sail when I am over 200 years old.

[Q Xopa]

Originally Posted by john61ct

Well knowing where the industry needs to improve, is better than thinking everything's all set for all cases and screwing up our expensive banks.

Not every charge needs a perfect end point.

Some scenarios, manually stop-charging is required, or accept lifetime may be reduced a bit.

Or just stick with lead, plenty of options.

> So it looks to me we could end up being under charged, which sounds to me (if I understand the jist of this thread) like what causes this 'memory effect'.

> Maybe we are 'over thinking' this issue, which is not really so much of an issue?

IMO easy to overestimate its prevalence and importance. Just don't go for months and years stopping at the exact same spot, mix it up and once ot twice a month get to a higher definition of 100%.

> Or if we dont have this 'end amps', as most of us dont, there is conversely the risk of overcharging at very low charge rates, like a cloudy day.

Most of us have more than one source of energy inputs, and there are multiple ways of preventing this scenario, relatively rare to start with, but overstating the magnitude of the potential problems is not helpful.

But few should believe that LFP is a must have, in fact the most knowledgable think it's not for regular owners, DIY should be discouraged, only good for those ready to spend thousands on a packaged system, say $500 per year?

Meantime, discussing the techie issues and their techie solutions is helpful for the community's awareness and even enjoyable for some.

Thank you once again for your insight.

Who is overstating this issue?

When you suggest sticking with Lead. I currently have lead, but of course it also has issues.

The title of this Thread refers to LFP. Im not sure why you are mentioning Lead?

**Usually** Shore/ Generators/ Alternator charging are not such a low C rate therefore not so applicable to this issue.

[tanglewood]

Q-Xopa brings up a really good point about mapping theoretical charge protocols to actual chargers and their capabilities. Theory vs practice.


With LA as well as LFP, people will go on endlessly about charging and proper care for max life. You would think we are handling eggs. But in reality, the vast majority of people just plug in batteries and go about their business using them, never giving them a second thought. And guess what? They work just fine and provide perfectly acceptable service life.


Now I can drone on as well as the next guy about all sorts of clever LFP charging protocols, but when it comes to building a system, you have to deal with the chargers in front of you. A few are so limited that you really shouldn't use them, but not always. For example, a fixed voltage alternator could actually work just fine for an LFP system if the voltage set point isn't outrageous. And even if it is, a relay controlled by the BMS to turn it off on full charge will make it completely usable. But the point is that we need to use what exists in the market to get a best fit with LFP.


As CatNewBee describes, this is actually not that hard, and LFP batteries are not fragile like eggs. I think we obsess over their handling because they are so darn expensive. But keep in mind that you can wreck a LA battery through gross over or under charge just as well as you can LFP. This difference is that you might get a few swings at wrecking LA, where it's typically one strike and you're out with LFP.


Anyway, most all 3-stage chargers have a time limit on absorption, and that seems plenty good enough in practice. Heck, tons of people use Victron Inverter chargers and mains chargers, and Victron is held up as one of the premium engineered system. Yet the multi/quattro line has a min absorb time of 1hr, and the Skylla chargers have a min absorb time of 2 hrs. Plus they have various re-bulk algorithms that will periodically top up your batteries while on extended mains power - exactly the opposite of what you want to do with LFP - and you can't turn it off.


But we not only survive, we flourish using LFP

[tanglewood]

Another interesting thing with respect to this memory effect, and how to clear it....


Recommendations are to do a periodic charge to a higher voltage to clear any memory bumps. 3.65 to 3.75 vpc seems commonly recommended. But if you try to reach those voltages, many BMSs will be screaming bloody murder with alarms, shutting down chargers, and even disconnecting the banks. So good luck implementing that approach......

[john61ct]

Yes, with some LFP products, the BMS is designed to protect the cells **from** their owner. No thanks.

To me, "BMS" means a collection of functionalities, ideally the protective ones implemented in redundant layers.

The devices you select should of course match your target care specifications, within the limitations of your ability, and what is available on the market, how high a priority you put on automation, etc.

And be **removable**, not just to be replaced when they fail, but easily bypassed when appropriate.

[OceanSeaSpray]

Originally Posted by AGM

Anyway, memory can be erased at 3.75Vpc @ C/3 termination.

C/3?? You must mean C/30. In all cases, definitely not once the memory effect has deeply set in. I have tested raising the voltage and there is no easy answer in this direction. The battery is too resistive.

[OceanSeaSpray]

Originally Posted by tanglewood

BTW, it's still a bit unclear to me whether the capacity loss reported by the one or two people in this thread is indeed a cumulative memory effect, or a loss of capacity due to some other effect.

It is quite clear for me. When I have charged up to 76%, the voltage is up to target and the current so far down that it is taking forever to continue, so classic termination has been reached. The OCV then settles down to something also indicating the same SOC.
The OCV is an indication of the distribution of the charge carriers in the cell and it says there are plenty more left to displace.

If the cell was suffering from permanent capacity loss due to loss of lithium, the OCV would indicate a high SOC following the end of charge. Furthermore, no matter what I do, the current won't taper down to zero. The cells keep charging gently at their own pace. It looks like it is a matter of continuing until they really reach the top... so far it hasn't been practical to do it and the memory effect still stands.