LFP memory effects thread

[tanglewood]

Originally Posted by rgleason

I am having difficulty with OCV, what is it?

Open Circuit Voltage, and in this case presumably resting voltage with little to no current.

[AGM]

Originally Posted by rgleason

AGM - Is one of the things you look for to determine if a battery is nearing 90-95%, that "dQ/dV"? In other words, when you increase voltage in a small jump, the spike in Current becomes less and less responsive. The spike is less and the drop from that spike becomes more gradual because the battery resistance has started to go up a little?
I am having difficulty with OCV, what is it?

Open Circuit Voltage, or Voc.
Yes, the incremental capacity dQ/dV (Ah/V) peaks at about 3.36Vpc during charging.
You could say the charge acceptance maxes out there at thousands of Ah per Volt.
Because Ah/V is the same as (A/V)*h, this means the electrical conductivity also peaks whilst the resistance (V/A) is at its lowest.
The IC drops off rapidly past 3.36Vpc which can be seen in my diagram.
After hours of constant voltage charging @ 13.75V (3.4375Vpc), IC has come down to about 0.3Ah/V (dQ is the area under the current peak).
A further drop off can be seen in the following voltage steps.
If you want to use the IC curve for reaching a certain SOC target like 95%, I don't think that's feasible. I'd rely on a simple charge counter for this.

According to some research, memory can lock away a significant amount of charge as the (partial state of charge) cycle count increases.
The majority of this loss has been reported to be recoverable by simply raising the charging voltage.
Their figures for a 11% loss point to a 5% recovery @ 3.70Vpc and another 1% @ 3.75Vpc.

[OceanSeaSpray]

Originally Posted by tanglewood

OK, very interesting.

You are defining/detecting this memory loss as when SOC as determined by charge voltage and return current is higher than SOC as determined by post-charge OCV. Correct?

Residual current. No, charge voltage and residual current don't determine SOC, it depends too much on the charging history and age of the battery. It is only a termination condition.

1/ Post-charge OCV -> SOC
2/ "Accumulated current" during charging / Nominal capacity -> SOC

Here, both appear in agreement, but my best efforts at recharging so far have only produced 76% SOC.

The important figure here is 1/, because it confirms that there is more upside left for charging (i.e. more charge carriers to be moved), even though the termination condition has been met.
If 1/ indicated 100%, then 2/ would reflect the present maximum capacity of the battery and the capacity deficit would be irreversible.

[tanglewood]

Originally Posted by OceanSeaSpray

Residual current. No, charge voltage and residual current don't determine SOC, it depends too much on the charging history and age of the battery. It is only a termination condition.s

And the difference between return and residual current is?

[OceanSeaSpray]

Originally Posted by tanglewood

And the difference between return and residual current is?

"Return" current is meaningless. That is all the difference.

[rgleason]

Tanglewood:
Would you mind defining "return" current and "residual" current so I can follow the question? Thank you.

[tanglewood]

Originally Posted by rgleason

Tanglewood:
Would you mind defining "return" current and "residual" current so I can follow the question? Thank you.

Sure, in common parlance, they are the same thing, and are the actual current "returning" to the battery while charging. It's also commonly called the "acceptance current" to indicate it's what the battery will "accept" at any given charge voltage. When talking about "return" and "residual" current, one is usually talking about the end of the charge cycle where the charge current has diminished down to some low level, and is often used as a trigger to ending the charge cycle.


Or perhaps OceanSeaSpray has a different dictionary that he'd like to share with us so we can follow what he is saying and not frustrate him so much.

[Delfin]

Originally Posted by OceanSeaSpray

Residual current. No, charge voltage and residual current don't determine SOC, it depends too much on the charging history and age of the battery. It is only a termination condition.

Sounds like you are conflating State of Charge with State of Health. When your batteries hit the capacity wall that you say by definition is temporary but has proven to be intractable, their SoC can be 100% of their diminished capacity while their SoH would be determined by "charging history and age of the battery," and could be the 70% or so you experience.

But your definition of these terms is likely different....

[Delfin]

I mentioned in an earlier post that my two year old bank used to charge to whatever bulk voltage setting I was using, but recently the bank voltage would only rise to .3 - .4 volts less than the bulk setting. I believe this is because consistent with the advice of some, I avoided ever charging above 3.55 vpc, but I don't know that for sure. What I have done is charge to 3.65 vpc every fifth cycle by setting bulk to 3.7 vpc for the last couple of weeks with an absorption time of 30 minutes. This seems to have reversed the issue I identified and now if bulk is set at 3.65, the bank rises to that value.

I have no idea if this was the onset of the dreaded memory effect, but it would seem to confirm Tanglewood's observation that an "equalization" of an LFP bank periodically is advised.

[evm1024]

Originally Posted by Delfin

I mentioned in an earlier post that my two year old bank used to charge to whatever bulk voltage setting I was using, but recently the bank voltage would only rise to .3 - .4 volts less than the bulk setting. I believe this is because consistent with the advice of some, I avoided ever charging above 3.55 vpc, but I don't know that for sure. What I have done is charge to 3.65 vpc every fifth cycle by setting bulk to 3.7 vpc for the last couple of weeks with an absorption time of 30 minutes. This seems to have reversed the issue I identified and now if bulk is set at 3.65, the bank rises to that value.

I have no idea if this was the onset of the dreaded memory effect, but it would seem to confirm Tanglewood's observation that an "equalization" of an LFP bank periodically is advised.

That is good info. I had planned on going to 3.7 vpc for a few cycles to see what would happen but have not had time. Thanks

[Delfin]

Originally Posted by evm1024

That is good info. I had planned on going to 3.7 vpc for a few cycles to see what would happen but have not had time. Thanks

Not sure if this is relevant info to add, but my BMS will start to shunt current from a cell at 3.65 v, so even if I am reading 3.7 on a voltmeter, the cell is only seeing 3.65 v, or so I assume.

[rgleason]

Originally Posted by tanglewood

Sure, in common parlance, they are the same thing, and are the actual current "returning" to the battery while charging. It's also commonly called the "acceptance current" to indicate it's what the battery will "accept" at any given charge voltage. When talking about "return" and "residual" current, one is usually talking about the end of the charge cycle where the charge current has diminished down to some low level, and is often used as a trigger to ending the charge cycle.


Or perhaps OceanSeaSpray has a different dictionary that he'd like to share with us so we can follow what he is saying and not frustrate him so much.

Thanks, I guess that is what I was calling "trailing current" having picked that up from John I think. Boy we could use a "Terminology" thread, where everybody could enter terms they use. John could even enter his use of esoteric acronyms too. Then we could just do a search or add a new term.


Thank you Delfin sharing, and your precise description of what you are doing and its positive impact on your batteries. It is very gratifying I am sure and very helpful to the rest of us (even though I don't have these batteries yet)

[Delfin]

We've been discussing charge current and a periodic high voltage charge for battery health and not developing memory, but I wonder whether periodic deep discharges are also advised. If we are cruising, there are many days when we take the bank down by only 40 - 50%. Perhaps mixing that up once a month with discharges to 15% SoC might be useful, and perhaps Mainesail's capacity tests are doing that for him.

[OceanSeaSpray]

Originally Posted by Delfin

Sounds like you are conflating State of Charge with State of Health. When your batteries hit the capacity wall that you say by definition is temporary but has proven to be intractable, their SoC can be 100% of their diminished capacity while their SoH would be determined by "charging history and age of the battery," and could be the 70% or so you experience.

SOC in this case is indeed an indication of SOH, but it doesn't mean that it can't be improved. It hasn't proven to be intractable yet, because there is a couple of approaches that I need to try. It has certainly resisted a few attempts, raising the charging voltage being one of them. That would probably work if the memory effect hadn't set in so deeply.

[rgleason]

What would happen if you did a controlled charge routine on each cell without the BMS I wonder? I suppose this is a lot of work.