1400 Ah Lithium from cells

[CatNewBee]

I am not aware of a serious memory effect on LFP batteries when not charged to 100% - this was mainly the case with NiCd cells, also Lead Acid batteries want SOC 100% to prevent buildup of large sulfide crystalls that reduce the active surface and degrades the battery over time.

The opposite is the case with LFP, they degrade faster when kept at high SOC and in high temp environments. They are happy when stored at 40% and charged only up to 90..95%. There is nothing wrong to give them a shot from time to time to balance but it is not necessary to do it on each cycle, especially with solar they would burn if you let them go daily to 14.4V or higher.

If you wnat to do this, you can set bulk/absorption to 13.8V, float to 13.5 and configure recond for 1 h to 14.2V every 4 weeks or so. So you have a stress-less system and have your kick-up every 4 weeks - or even only on request manually.

[OceanSeaSpray]

There is a memory effect, it was documented in Nature Materials in 2013 by Sasaki, Ukyo and Novak and it gets worse the more cycles you do. My experience with a number of installations and a few years of operation is that 14.0V is needed to maintain cell health on the long run.
We haven't had to do anything about balance yet, but it is not as good as it was initially.


Float at 13.5V and you will wreck your cells over time. This is above the voltage of fully charged cells. Lithium cells must not be "floated", charging must be terminated and they keep charging at 13.5V.


Cells are happy when stored at the lowest practical SOC, not 40% or some arbitrary number, even though it is better than full, there is also research highlighting this.


All this has been discussed at long length in the other thread.

[john61ct]

Thanks Bruce.

If I prefer to keep to average 3.45Vpc in normal daily use while cycling, which of these two charging patterns would you recommend to avoid creating that infamous memory bump?

The goal being ultimate longevity, fully using AH capacity not a consideration. And no Absorb time, just hitting target V and stopping.

A. 3.45Vpc most cycles, then every so often (6 cycles?) go higher, say 3.60V, but applying a load immediately after. Or

B. Stick to 3.45V as an average, but constantly vary the stop point, say between 3.35 and 3.55V

?

Also, sticking to manual per-cell top balancing when needed (no automated balancing in use), what is a good highest voltage to use given the above preferences?

Your input's much appreciated!

(And anyone else that feels qualified is welcome to give feedback as well of course)

[CatNewBee]

Float is not about storage but about daily use in conjunction with solar. The alternative is cycling and cycling also has a negative effect on lifespan.

What voltage you would consider safe for solar controller set up used on board as power supply for all devices without negative effects on the LFP battery life?

Cycling by switching off solar during the day is not an option, it decreases the life of the battery too and does not use the incoming energy.

I am not talking about storage / winterizing and float charge, it is about daily use with less / shallow cycling to have a) always enough energy and b) extend the battery life expectation.

Also set absorption to 14.0V because of memory effect and cycling daily to this level - what a solar controller would do - is probably negative too.

If I read all your warnings and posts -the safest option for LFP is to just store it empty in the bilge and never touch it.
Anything else will have a negative effect on the life span.

I guess, you refer to this article: https://phys.org/news/2013-04-memory...batteries.html

This memory effect is not permanent and not deteriorates the battery, it is temporary and reversible (by idle time or deep cycle). So it does not damage the cells on the long run as stated in the article in contrast to NiCd / NiMH / FLA / GEL / AGM.

[john61ct]

Your overall system design should allow you to isolate your LFP bank from the charging buss without shutting down the other power sources feeding your loads.

The way I use "cycling" just means as opposed to "storage".

Float is only relevant to storage, and even then, only wrt lead banks.

When LFP is not actively being cycled, as you point out, it should be kept at a relatively low SoC. In other words, only charge the bank when you know you'll soon be drawing it back down again.

Keeping the bank Full is dangerous thinking, leftover from lead mentality.

[john61ct]

No one said the memory effect is "damaging" to a LFP bank.

The article Bruce references is here
https://www.nature.com/articles/nmat3623

It is a relatively subtle effect, and over time, if not prevented and compensated for, can reduce **capacity**. Yes the effect can become permanent if the exact same setpoint/profile is repeated long enough.

I don't think its impact on longevity is well understood.

[CatNewBee]

Well, i agree, no one knows what will be the best. Isolating battery from charging and using charger on the system to feed the house is a theoretical approach. The battery must remain connected to the house all the time - you never know when someone pushes a button on a winch or uses some more power than the solar can provide, and if the charger provides power - the battery will be charged - there is no bypass to the battery.

The only option is to disable the charge source at all - or configure its output to a lower voltage - that said to set the "float" voltage to an acceptable level - that is where the solar controller remains all the time once he has finished charging. You can also reduce absorption Voltage to that level, so the controller never charges the battery to a high SOC. Complete shut off means cycling and reduces life span too. The decision is only what is worse, cycling or high SOC - the memory effect is not a major problem because reversible and easy to handle - it just needs time to disappear - and time it has when the battery remains full with no charging current while the solar controller feeds the house.

Thanks for the link, it is the same article I referred to, just a different publisher. However, no new findings since then.

[john61ct]

I have requested a full-text version of the paywalled article from the authors, if anyone finds an online source please post it.

Best so far: https://wk.baidu.com/view/ff10661a844769eae009eddf

[CatNewBee]

https://phys.org/news/2013-04-memory...batteries.html I am not sure if this is just an excerpt or the full article, but more complete than your first link.

[nebster]

I can confirm the memory effect "bump;" I see it in my charge curves after only 10 or 20 partials to the same SOC. I wouldn't have noticed it until OSS brought it up in the other thread.

That said, I've decided that it's too secondary/small of a factor to worry about. So far, I've had no trouble pushing the cells right past the bump if I really want to. I'd rather have ease of settings and stay lower on the SOC curve than add complexity with altered charging protocols. I'd be interested in learning from anyone who has data from a large number of charge cycles that shows meaningful capacity loss.

Perhaps this is one of those things where simply oversizing the pack from the start a little bit is the most reliable mitigation.

[john61ct]

Originally Posted by CatNewBee

Isolating battery from charging and using charger on the system to feed the house is a theoretical approach.

Not at all, many ways to achieve that.

I dunno about your setup, but no way my solar is relevant to the kind of load a winch pulls, and no one is pushing those buttons without me knowing it.

Keep the LFP bank connected to House loads all the time, of course. Cycling is not something to be avoided, just means 'using the bank'.

If you're worried about your solar production capacity going to waste, use it for a separate load dump or to top up a cheap lead bank that won't be harmed.

Or reduce your Float voltage to 3.25 or 3.27Vpc, may still hurt LFP longevity, but less anyway.

Lots of ways to skin a cat.


> The decision is only what is worse, cycling or high SOC

Avoiding the top-end stress is what gives you thousands more cycles so you don't need to worry about actually using that expensive bank.

> memory effect is not a major problem because reversible and easy to handle - it just needs time to disappear

I would like to see links to what gives you that impression. I don't think it's true.


> and time it has when the battery remains full with no charging current while the solar controller feeds the house

I don't understand this last at all. The LFP bank should never be sitting at Full, that is more harmful than everything else we're discussing here.

I thought you said your House loads would always be connected to your LFP bank?

[OceanSeaSpray]

Float is nothing else than a term often wrongly used in the documentation of charge controllers. Float used to be a state with a lower voltage limit where charging would resume up to an upper target, and then charging was turned off again and the voltage "floated" back down, like 13.0V ON / 13.8V OFF. This followed after absorption.

Modern controllers seldom float, they regulate. We are just talking about a "holding" voltage. If this voltage is higher than the resting voltage of the cells, they will keep charging. If it is higher than the resting voltage a full cell, i.e. ~3.35V, it will cause overcharging and lithium plating.
Floating would be just as harmful if charging restarted on a full battery.

All this has nothing to do with storage. Configuring the "float" level below the level of the full cells, i.e. 13.35V is just a way to artificially create a charge termination with a controller that is not really adequate for charging lithium batteries.
Unfortunately, some controllers don't stay in this state indefinitely and they will recharge anyway after some time, so they slam the battery to 100% SOC and then abuse it if it is not being cycled enough.

Eric

[OceanSeaSpray]

The memory effect becomes catastrophic if it is allowed to compound. It causes early charge termination, early charge termination means the memory can't be erased and the charge starts terminating earlier and earlier until the capacity is gone.

A gentlemen once stepped into the other thread to explain how his fleet of 100+ RVs fitted with LFP banks had to return to base for "maintenance" every 3 years or so, with the cells having to be re-balanced and force-charged to 3.7V several times and held there to regain the lost capacity, i.e. erase the memory. Reason? Inadequate charging: 13.8V, insufficient absorption etc.
It was a fiasco of an epic magnitude and it didn't take long before that post got somehow pulled, which is unfortunate.

The whole "low charging voltage" nonsense started because people thought they could control charging by altering the voltage. It doesn't work. Anything from 3.4V up will eventually overcharge and cause damage, anything less than 3.4V is not enough to get a full charge. The only answer is charge termination and no amount of discussion about The Voltage can change that.

If charging happens at extremely low C-rate, the absorption time tends towards zero. Still, if there is a memory from previous cycles, erasing it requires holding enough voltage while the current tapers off. My experience is that 3.5V/cell is the minimum sustainable for long-term operation without issues or intervention.
What stresses the cells is overcharging. As long as they easily absorb the current, they are happy. Operating them at high SOC causes premature aging, so discharge them and don't charge unless needed.

The challenge with LAs is keeping them full. The challenges with LFP are not charging them unless it is needed and recharging them properly without overcharging when needed. Use a switch if you have to. It is not great, but it beats a stupid controller trying to fill them up all the time.

[nebster]

Well, it sounds plausible, but we don't know about the mysterious 100+ RVs gentleman, since he's not here to ask more questions. We also don't hear about this mysterious "catastrophic" knee-shortening in other installs, and I am a skeptic until the data is present.

(If, in fact, the "force charging" every three years is necessary, by the way, I am not sure if that constitutes a catastrophe.)

Let me revisit this by posing a hypothetical pair of charging approaches:

Approach 1: charge at 3.55V (or higher) using a *theoretically* perfect, drift-free coulumb counter, stopping charge at 80% SOC -- or pick any SOC, really -- every time, using a current that is low enough that we stay in CC.

Approach 2: charge at a lower voltage, say, 3.40V, with a CC/CV regime, but with a taper and current- or time-based termination that also arrives at the same 80% SOC. (Or again, some other SOC meaningfully less than 100% and the same as above, for the sake of this thought experiment.)

Question: do both batteries end up with the same "memory hump" and knee-lengthening over time? Put another way, is it the act of always stopping at a certain SOC, or is it the "trickle-charging" at the end of a slow charge that does it? Do we know?

[CatNewBee]

Since all this ends up to speculations, rumors and not proofed failure estimations my conclusion of it all is - try and time will tell. No new findings without taking risks.

What I consider highly probable is:

a) high temp shortens battery life
b) overcharging too
c) deep discharge also (less deeper cycles lead to longer life)
d) low temps charge/discharge is also bad.


What say the specs ofthe manufacturer?

full at 3.65V.
initial charging to 4.0V
deep discharge below 2.8V
overcharge damage above 4.0V

Victron settings 14.2V / float 13.5V (3.55V / 3.375V)

sounds conservative enough for 3000-5000 happy cycles especially if not too deep, hot, cold.

The usage pattern is an irregular balanced mix of eventualy high charge/discharge currents (inverter, galley apliances), long shallow currents (nav gear, lighting, entertainment), mix of varios charge currents - solar, gen...) and finaly a bms and balancer that take care that the system does not go rogue.

So chances are (besides some abuse by 1/2C - that is not really an abuse - 1C permanent charge/discharge is in the specs ) something will be wrong and something right.

In average we will be happy with the battery for a long time. Nothing lasts forever so, one day we will replace it with a more advanced technology, chances are the vessel will be sold long before the battery dies.