Tail current charge table

[100 Proof]

Does anyone know of a reference chart or graph or table or equation that shows tail current in % C to indicate full charge at different bank charging voltages? What would the tail current in %C be to get to 90% at various bank voltages? For example, for a 12V bank I understand that .05C at 14.6V = full charge. What is it at other voltages down to 13.85?

I’ve searched the forum, but have so far been unable to locate such a reference. Possibly it doesn’t exist?

Thanks!

100 Proof

 

[TAS CPA]

This what you're looking for?

 

[Diysolar123]

FYI, They exist but it does not really work well with lifepo4 batteries due to the very flat discharge curve until you are almost fully charged or almost discharged.
There are tables but 20mv can mean a huge difference in capacity so its not as informative as you may like...

 

[100 Proof]

This what you're looking for?

No. That's voltage vs SOC. I'm looking for current at specific voltages to indicated SOC when charging.

 

[John Frum]

Does anyone know of a reference chart or graph or table or equation that shows tail current in % C to indicate full charge at different bank charging voltages? What would the tail current in %C be to get to 90% at various bank voltages? For example, for a 12V bank I understand that .05C at 14.6V = full charge. What is it at other voltages down to 13.85?

I’ve searched the forum, but have so far been unable to locate such a reference. Possibly it doesn’t exist?

Thanks!

100 Proof

I don't think what you are looking for exists.
The data sheets usually give you a tail current for a specified charge voltage and charge current at a standard temperature.

 

[100 Proof]

FYI, They exist but it does not really work well with lifepo4 batteries due to the very flat discharge curve until you are almost fully charged or almost discharged.
There are tables but 20mv can mean a huge difference in capacity so its not as informative as you may like...

Yeah. I figure they would only have value in the knee, perhaps for the last 10% - 15% at best.

 

[chrisski]

I’ve not seen that, but I want to know if your tail current is the same as I use it.

Setting my Victron battery monitor, I set a tail current that when the bulk phase is done, and charging starts you set a certain amperage so when charging drops at or below this amperage, the SOC jumps to 100%. That took some adjustment for me. Originally the SOC went from 85% to 100% because I had too many amps programmmed in, it dropped this to about 2% and then my SOC got as high as 99% and then change to 100%.

I think you’re looking for after bulk charge is done where amperage drops off, and what this amperage would be for Lithiums.

I think because of the flat charge curve as mentioned, it would be different for each battery pack. Probably much easier expressed as a % versus actual amps.

I can see how tail current would be useful in determining charge state. I will have my lithium’s built in two months, but perhaps tail currents would be more useful than a voltage In determining how much more amperage a battery can take in.

 

[John Frum]

Yeah. I figure they would only have value in the knee, perhaps for the last 10% - 15% at best.

I'm of the opinion that if you are charging cc/cv you want to see the battery pop into the knee to know its full and then stop.
That means you need to charge at a high enough rate that the battery doesn't get to full before it pops into the high knee.

My own setup charges at less that .1c so I have to be careful not to overcharge my battery.
I use a raspi connected to the bms to keep a lid on things.

 

[John Frum]

Somewhere between .2c and .5c generally speaking.
I get my battery plenty full without ever going to constant voltage.

 

[thegoogler]

I'm of the opinion that if you are charging cc/cv you want to see the battery pop into the knee to know its full and then stop.
That means you need to charge at a high enough rate that the battery doesn't get to full before it pops into the high knee.

My own setup charges at less that .1c so I have to be careful not to overcharge my battery.
I use a raspi connected to the bms to keep a lid on things.

In my experience, even at very low C rates (i.e. C/20) once the battery gets around 90% the # of Amps the battery will accept drops off. On my 10A bench charger (which I used to charge my 12V 280A pack) it pushed 8.5A for a full day before the amperage dropped off as it switched over to CV. This is also the point where the cell voltages, which were all within 0.02mV of each other at first, started to diverge. My point is the battery will pop into the high knee no matter what your C rate is.

Aside from the initial bench charge, my setup uses a few solar panels, but even still it maxes out at barely C/20 right now (I've seen 17.1A peak), yet even with the Li controller set to 14.4V absorption the BMS has never detected an overcharge of any cells, and after ~2 months of daily use my cells certainly *seem* just fine.

(FWIW I've been leaving the MPPT controller at 13.6V normally just to try to limit the controller from charging up to 100%, though because the absorption stage runs for 3 hours that doesn't really work as desired. But I did run it one day at 14.4V to rapidly charge the battery after a few days of use with limited recharging)

 

[John Frum]

In my experience, even at very low C rates (i.e. C/20) once the battery gets around 90% the # of Amps the battery will accept drops off.

During the constant voltage part of the charge the current flow is determined by the potential difference between the charge source and the battery.
Tail current just means terminating the charge before the current flow approaches 0.

 

[thegoogler]

During the constant voltage part of the charge the current flow is determined by the potential difference between the charge source and the battery.
Tail current just means terminating the charge before the current flow approaches 0.

Yep that's what I'd expect. I guess I'm confused by this statement

"That means you need to charge at a high enough rate that the battery doesn't get to full before it pops into the high knee."

What I was saying is that even at a low C rate I've still seen the switch-over happen as it approaches the knee. I know the battery manufacturers say when you hit 0.05C you should (can?) stop charging, but in my totally anecdotal experience I've not seen an issue with charging at 0.05C so long as you are not charging at >14.6V and have a good BMS. In short, so long as you're charging at 14.6V or less you can't be full (100%) without getting into the knee, and low C rates don't prevent you from getting into the voltage knee.

Note: If your charger is pushing 15V or more (i.e. assume your charge is pushing 30A because you set it to 18V) in order to cheat and feed more Amps faster during the CC stage then yes I do believe at a low C rate you might be able to get "full" before it kicks over the CV stage

Now that said I can also see why a battery manufacturer would recommend that you stop at 0.05C. In my experience my cells stayed within 0.02mV of each other up to 3.45V/cell. After that they quickly started to diverge the first time I top-balanced. 3.45V/cell is ~99.5% SoC so it's really "full enough", and it's past the 90% knee which should happen around 3.35V. If you're building an EV with high C rates for charge and discharge then you can likely get away without using a full BMS by simply stopping your charge when you get into the knee and the rate drops off. That drop to C/20 is an indication that the battery is 90-95% charged and that you should stop (soon). By stopping at that point you also keep the individual cell voltages from diverging too much so again a full BMS isn't strictly necessary for those EV manufacturers

The above is all anecdotal experience and lots of reading of forum and vendor docs, so take it or leave it, I suppose.

 

[Steve_S]

End-Amps / Tail-Current tables don't exist for LFP that I am aware of but likely would not be hard to generate. I'm not sure how useful it would be in a practical sense as it would likely cause more confusion.

A general guideline used to work out End Amps is calculated @ 0.05C from the Highest AH Battery Pack in a Bank.
IE: 100AH X 0.05 = 5A, 200AH X 0.05 = 10A or 280AH X 0.05 = 14A.

As I use a Midnite Classic-200 SCC with a WizBangJr, to monitor the end-amps and act accordingly. I use Float which levels everything up nicely to my preset voltage limits.

 

[John Frum]

"That means you need to charge at a high enough rate that the battery doesn't get to full before it pops into the high knee."

What I was saying is that even at a low C rate I've still seen the switch-over happen as it approaches the knee. I know the battery manufacturers say when you hit 0.05C you should (can?) stop charging, but in my totally anecdotal experience I've not seen an issue with charging at 0.05C so long as you are not charging at >14.6V and have a good BMS. In short, so long as you're charging at 14.6V or less you can't be full (100%) without getting into the knee, and low C rates don't prevent you from getting into the voltage knee.

That is encouraging.
I hope I'm wrong.
Will test more.

 

[thegoogler]

0.05C might be commonly stated, but I don't think you have to use that value. At least from the experts here using LFP:

MPPT with BMV - Which Tail Current will it use for Float? 4% or 1 Amp? - Victron Community

community.victronenergy.com

Tail current with LiFePo - Victron Community

community.victronenergy.com

In that case the Victron MPPT controller is using <0.01C as the tail current by default (1A).

 

[thegoogler]

That is encouraging.
I hope I'm wrong.
Will test more.

I'll be interested to see your results.

I may be wrong and just lucky so far. I'm not an electrical engineer. I'm just speaking from the limited experience I've had so far. But yeah I have a large capacity battery pack (12V 280Ah) and only relatively low-amperage charging methods (10A bench charger or <20A solar panels) and thus far I've had no issues with my "conservative" settings.

 

[Substrate]

End-Amps / Tail-Current tables don't exist for LFP that I am aware of but likely would not be hard to generate. I'm not sure how useful it would be in a practical sense as it would likely cause more confusion.

@Steve_S

Sorry to be a bit necro-replying, but I found my tail-current vs voltage chart after all these years!

I channeled the spirits of wise EV'ers from years past, who knew about things like this, voltage-diffusion (where depending on C rates of charge, the terminal voltage leads / lags the chemical reaction.. but we won't go there for now)

Objective: Full charge to 100% and using tail-current at a specific CV voltage as your guide to stop.

Here using a typical 4S battery - nominal 12v LiFeP04 cell CV voltages in ( )

3.6v (14.4v) = end amps tail current C/20
3.55v (14.2v) = end amps tail current C/60
3.5v (14.0v) = end amp tail current C/100
3.45v (13.8v) = end amps tail current = zero

When guys didn't pay attention to this, what was found was degradation due to overcharge. This added up quickly, because they were charging in cycles faster than we do, but it did demonstrate the affect.

Unfortunately, this nugget from those in the know frequently got overlooked, (or they were using 2nd-hand cells with unknown prior history) and the user concentrated solely on voltage, and tried to drive cells to zero tail current to be considered fully charged at high(er) CV voltages. That's bad because for absolutely no real gain, they just put premature-aging overcharge hours on their bank every cycle.

 

[Substrate]

Note to lurkers about the last part of the table:

3.45v (13.8v) = end amps tail current = zero

This does NOT mean you can sit at 3.45v CV (or 13.8v for a 12v system) forever! It can still be overcharged at that low voltage. Ie, just because you finally reach zero amps tail current at this low voltage, does not mean you can go on a 4-week vacation sitting at 13.8v with no current flowing till the end of time.

What it did mean is that if you reached zero amps overnight while sleeping, you made sure you pulled the charge, but it wasn't so time critical as it would be if you overcharged at the higher voltages! Ie, it just gave you some hours of leeway without causing a lot of harm on a practical level. But you can't let it be like that forever.

 

[100 Proof]

@Steve_S

Sorry to be a bit necro-replying, but I found my tail-current vs voltage chart after all these years!

I channeled the spirits of wise EV'ers from years past, who knew about things like this, voltage-diffusion (where depending on C rates of charge, the terminal voltage leads / lags the chemical reaction.. but we won't go there for now)

Objective: Full charge to 100% and using tail-current at a specific CV voltage as your guide to stop.

Here using a typical 4S battery - nominal 12v LiFeP04 cell CV voltages in ( )

3.6v (14.4v) = end amps tail current C/20
3.55v (14.2v) = end amps tail current C/60
3.5v (14.0v) = end amp tail current C/100
3.45v (13.8v) = end amps tail current = zero

When guys didn't pay attention to this, what was found was degradation due to overcharge. This added up quickly, because they were charging in cycles faster than we do, but it did demonstrate the affect.

Unfortunately, this nugget from those in the know frequently got overlooked, (or they were using 2nd-hand cells with unknown prior history) and the user concentrated solely on voltage, and tried to drive cells to zero tail current to be considered fully charged at high(er) CV voltages. That's bad because for absolutely no real gain, they just put premature-aging overcharge hours on their bank every cycle.

Thanks! Took a minute, but I'm happy to finally see my original question answered!

 

[ericbakuladavis]

Eric Bretscher wrote that such a table can be generated with interpolation. On the page linked below, there's a heading "Charge Termination Condition" under which an explanation and a table are given:

Charging Marine Lithium Battery Banks | Nordkyn Design

nordkyndesign.com