LiFePO4 battery capacity decreasing

[CatNewBee]

Originally Posted by Delfin

Cat, isn't the low voltage target that is attainable a function of the BMS used? On mine, the BMS will isolate the battery when it reaches 3.0 volts.

Well, then it is your cut-off point. What BMS do you have and what are the settings?

The Super-B have an internal one AFAIK, no idea if you can configure it

[Delfin]

Originally Posted by CatNewBee

Well, then it is your cut-off point. What BMS do you have and what are the settings?

The Super-B have an internal one AFAIK, no idea if you can configure it

It's a Lithionics. No external control settings on this version.

[john61ct]

Originally Posted by Epicurean

I think I can be fully charged, yet that doesn't tell me my capacity. I think I shouldn't be fully charged but still want to have a large capacity.

Yes, "fully charged" just means

adding more charging energy no longer significantly raises the energy stored.

Usually defined (somewhat arbitrarily) as a combination of a top (Absorb) voltage and some stopping point of current measured as amps trail off.

The charge current rate (relative to bank Ah capacity) will enormously affect the timings of the profile transition points.

As mentioned, the only way to know your bank's State of Health (SoH, present Ah ** capacity **), is to do well-controlled and precisely timed constant-current load test.

And, the only way to know its SoC between 100% and 0%, is with a quality coulomb-counting SoC meter like Victron BMV, Xantrex LinkPro or Balmar SG200.

Voltage readings have nothing to do with either SoC or SoH.

[john61ct]

Originally Posted by Epicurean

The state of charge is done by the BMS so no idea what happens there.

Yes that is a destructively high voltage if you are trying for maximum longevity.

To do testing and maintenance procedures like load testing or balancing, best to break down the bank to its lowest-level components, ideally the 3.3V nominal cells.

There are many ways to balance cells aside from the "live" balancing performed (often at stupidly high voltages and too-low current rates) by your BMS.

Also, unless your BMS contains sophisticated coulomb-counting SoC metering circuitry, which I highly doubt,

you should completely ignore its guesstimate of capacity (SoH, Ah capacity). With a proper SoC meter you could at least calibrate it, see just how far it drifts.

Do you even have the ability to reset / tell it when you have reach your defined 100% SoC point?

[john61ct]

Originally Posted by Delfin

Cat, isn't the low voltage target that is attainable a function of the BMS used? On mine, the BMS will isolate the battery when it reaches 3.0 volts.

Yes 3.0Vpc (12.0V for a 4S bank) is a standard cut-off point, and good definition of 0% SoC for LFP.

Actual energy available going any lower than that is an insignificant fraction, and going even that low regularly is **very** destructive to longevity.

Should only be done for periodic testing / maintenance protocols, say a few times a year at most, and of course risks instantly turning your very expensive bank to scrap if not done properly, starting recharging immediately upon hitting that point.

[john61ct]

Originally Posted by Epicurean

The Super B Manual

If you could link to the data sheets and that manual for the specific model you are using that may help us clarify things for you.

BMS separately from the cells if at all possible.

[CatNewBee]

Originally Posted by john61ct

If you could link to the data sheets and that manual for the specific model you are using that may help us clarify things for you.

BMS separately from the cells if at all possible.

http://www.super-b.com/mobile

According data, stop charge 14.6V, deep discharge protection 8V, internal BMS

[john61ct]

Crazy, talk about last-ditch protection!

So I would def wrap that in additional LVD / OVD functionality, stop daily-use-cycle charging at 3.4-3.5Vpc and not let it go below 3V.

I wonder who actually makes their cells? Seem to recall a Dutch outfit investing big in that capability, but fab probably still located in China like the rest.

[Epicurean]

I found this (https://cdn.enfsolar.com/Product/pdf...ec6f398a8a.pdf) online.
And here's the manual: (http://www.super-b.com/content/files...160E-ZC_V2.pdf)

I spoke with the local "expert" who said that I should look to see if the cells equalized but the manual says "The Li-Ion battery is automatically balanced when the charger goes to float mode (AGM/GEL
battery charger algorithm) and no current is drawn from the battery."
\
AndBattery equalization
"During the batteries lifespan, the cells within the battery may be unbalanced due to high
discharge currents and short float charge periods. This may result in a loss of capacity and
overcharged cells. Cells may be equalized by means of the following procedure:
1. Apply a constant voltage of 14.2 and a current of < 2A to manually equalize the Li-Ion battery"

I confess there are so many voltages mentioned I am confused.

It looks like to really find out SoC I need to disconnect it from all loads. But that's difficult because we live aboard, on the hook. In a couple of weeks we'll put it on the hard and move off so I may hire him then.

[CatNewBee]

Actually it is not too hard to equalize the cells with the built in BMS.

The cheaper BMS circuits and cell balancer, that are also used in sealed enclosures work that way:

the goal is to do not burn any energy stored in the cells for balancing to prevent the risk of self discharge, therefore there is a threshold of 3.6V at cell level, that translates to 14.40V pack voltage of a balanced battery, below this the cell balancing modules are inactive.

When the cell voltage rises above 3.6V, the module swithes on and shot cuts the cell with a resistor or something similar, that limits the current to 1...2A. The other cells accept the full charge current, while the one cell with active balancers charges with 1...2A less because of the bypass.

Eventually the next cell goes over the threshold and turns on their bypass, then the third and finally the last one, At some point the Voltage rises to 3.65V, respective 13.6V and then stays there for a while (absorption) during this time the battery gets warm due to burning energy on the cell modules until all cells are equal. and the current drops, the cells do not accept more charge, because there is no more voltage difference between the cells and the charger. the charger switches to float, the cell modules keep balancing until the cells go below 3.6V (13.4V pack) and the battery is then 98% full and resting with no more current going in and out.

If you permanently stop charging below 14.4V, what many advocate for longevity, you simply do not allow the BMS do his work and the cells run out of balance Assuming you stop at 14.0V, you can have a perfectly balanced battery with all 4 cells at 3.5V or a battery with one cell at 3.6V, one at 3.55, one at 3.59 and one at 3.26V what means, 2 cells almost full at 95%, one at 80% and one at 50% SOC. When discharging the empty cell will likely trigger low voltage cell protection, while the othr still have 50% energy.

Bottomline is, let he BMS do his work and do not mess with the charging profile if you do not know what you are doing and what impact it may have when never allowing the battery to synch the cells.

Of course with a configurable BMS you can set the balancing start voltage to a lower value and also allow balancing only when a charge current over the balancing current is going in to make it safe to cut off earlier and stop charging at 90% SOC, while having a balanced battery.

[CatNewBee]

Oh man, 14.6V of course, not 13.6V cut off absorption Voltage...

and 14.4 instead of 13.4V to stop balancing when battery goes to float...

[Moontide]

Have not read all the posts, but everything I have seen says DO NOT use float stage with LiFePO4 batteries. Go to absorption then shut off the charger. Seems float stage can damage them.



Also, it is apparently better to leave them around 80% charged when "storing" them, not 100%.


Hopefully the battery manufacturer has some specific info. I would look at that, not the instructions for the SOC meter.


Bill

[CatNewBee]

Depends on your usage and float settings. Below 13.5V you are OK when using the batteries while cruising. They do not accept more amps, no cuurent is flowing, they just stay around 95%, while the energy coming in is used by the loads on board. In the evening the batteries are discharged to what ever your energy footprint is and re-charged the day after.

I partly agree on storage to disconnect them and keep them around 50% SOC, but when in the water I would allow float at 13.3 ... 13.4V and charging absorption to 13.8...14.0V, they will be around 80-90% full, not too good, but they can infinitely run the bilge pumps if necessary, so take it as an insurance fee. It is a trade in of several cycles less for a floating boat. A flooded boat would kill them probably on the long run too.

[john61ct]

Originally Posted by Moontide

Have not read all the posts, but

You should.

> everything I have seen says DO NOT use float stage with LiFePO4 batteries. Go to absorption then shut off the charger. Seems float stage can damage them.

Well damage is overstating. Not ideal for maximizing longevity is more accurate. And the degree depends on the voltage setpoint.

Not likely the cause of the problem here, if any.


> Also, it is apparently better to leave them around 80% charged when "storing" them, not 100%.

No, usual guideline is around / below 50%.

But that assumes not completely isolated, BMS can be a significant draw.

Lower the better, as long as no chance of getting down to the danger zone is IMO most accurate.

[Epicurean]

I wish to thank everyone who offered information. I'm not sure if this will help anyone, besides me, but I did solve, I believe, my problem. I raised the absorption voltage, the voltage at which the charge controller throttles back the charging current from 13.95 v to 14.0 v, and after two days the battery capacity went to 100%. Now I reset it back to 13.6 v. I guess in fear of damaging it at the high end I was too conservative. In their manual they claim I should be able to keep that at 14.3 -14.6 V. But, if I don't need to, I probably won't.