LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

[mcarling]

Originally Posted by Lagoon4us

Small basic BMS units are available for under 500 bucks that give you a fuel gauge that resets, in the scheme of things worthwhile without overcomplicating the installation. It resets as the charger reaches full charge.

Sure, but the question is how does it determine that the charger has reached full charge? Some of us are asserting that looking for the change in the voltage as a function of SoC i.e. looking for the change in the slope of the curve is the better method. Others seem to be arguing that simply looking at voltage in isolation to determine that the charger has reached full charge is the better method.

[goboatingnow]

Originally Posted by mcarling

I concede that using dV/dSOC to recalibrate is not trivially easy and that the resets might be less frequent than we might like. I have also not seen any implementations yet but, in principle, this is something that the SoC meter manufacturers can do when they decide the LiFePO4 market has become important enough and something that I expect they will do this year or next year. Is this method perfect? No, of course not. Is it good enough? I think so. Is there a better alternative? I don't see one.

Im not sure I get your drift. As Ive said for Li is most cases voltage is a accurate reflection of charge, unless you get into high capacity low discharge areas.

Attempting to characterise the rate of change of voltage and relating that to SOC ( ie relating it to near 100% SOC) is fine in theory but as yet we have no real data on real life correlation or the variability so we tend to play safe and set quite conservative voltage set points. I don't see that changing in the near future.

Most comsumer applications feed back absolute nonsense as "fuel gauges" in Li technology. All they really indicate is a "charge me now" desire. thats the state of the art at present.

dave

[Lagoon4us]

The green section in the graph i posted is the 'user' determined area of charge on charge off, isn't this the gist of the debate? It is settable to give long life, the coloumb counter switches or blocks any further charge relegating it to maintenance.
Doesn't the reset give you accuracy to the fuel gauge?

[goboatingnow]

Originally Posted by Lagoon4us

The green section in the graph i posted is the 'user' determined area of charge on charge off, isn't this the gist of the debate? It is settable to give long life, the coloumb counter switches or blocks any further charge relegating it to maintenance.
Doesn't the reset give you accuracy to the fuel gauge?

The key here is the use of a voltage set point to stop charging, the amp counting is secondary.

All I was saying is you cannot use amp counting to determine charge cutoff, its simply not accurate enough.

why is there maintenance charging thats not really good for Li.


after that you have a fuel gauge that may or may not represent real life. you will know relative SOC with some degree of accuracy, but absolute SOC will remain elusive
Dave

[Lagoon4us]

Agree however the GSL MPPT built soley for Lithiums settings are as follows:-
1) HIGH CAPACITY 14.8 >
2) DEFAULT/FACTORY 14.6 > SATURATION COMES IN AT 13.9V FOR SETTINGS 1,2,3
3) LONG LIFE 14.3 >
4) MAINTENCE 14.1.

[goboatingnow]

a cutoff at 3.65, thats cutting things very fine. Thats very close to the theorectical max , i use the maintenance cutoff or at very least just the long life setting
whats saturation , is it the end of the CC period?
Charging Li is easy , its just different

[Maine Sail]

Originally Posted by goboatingnow

LI cells do not exhibit such current fall offs as you see in LA. The current ( depending on the cutoff voltage) may remain almost at max for the duration of the charge. Hence the issue of attempting fast charge near the 100% point. The battery simply never refuses current, unlike a LA which has a charge acceptance curve.

Terry has indicated his acceptable voltage range and I would agree, though with my custom setups on smaller batteries I can push a bit closer to the 100%. But with large capacity cells this is entering the "danger zone".

With Li, the battery doesn't indicate reliably a full position using current. Hence what you say cant really be applied. All you can do is to some extent arbitrarily select a cutoff point and use that voltage ( in reality you don't care at that point how much current is actually flowing into the batteries, the charge cycle terminates, end of story.

Dave

Dave,

I plan to charge to 13.8V or 14.0V, have not decided yet, and I'm still experimenting. In both cases at either 13.8V or 14.0V the current falls to near zero as the battery gets to the target limited voltage. I have now repeated this on the bench about 15-20 times. I know it's not a lot but it is very repeatable.

I plan to use this point, probably somewhere around 1-2A, and the target voltage to do an "auto" reset of my amp hour counter. I will also manually synch it when the sources can no longer get much current into the bank at a CV.. My counter will be programmed for less than the banks full capacity to account for the upper 10% or so of charge range I am not pushing into..

If I raise the voltage from the CV limit of 13.8V or 14.0V the current spikes back into bulk as the bank is not technically full. I suspect it can be "considered" full for Ah counting at a specific target voltage and low current range. I am only using Ah counting as a "fuel gauge"....

[goboatingnow]

Originally Posted by Maine Sail

Dave,

I plan to charge to 13.8V or 14.0V, have not decided yet, and I'm still experimenting. In both cases at either 13.8V or 14.0V the current falls to near zero as the battery gets to the target limited voltage. I have now repeated this on the bench about 15-20 times. I know it's not a lot but it is very repeatable.

I plan to use this point, probably somewhere around 1-2A, and the target voltage to do an "auto" reset of my amp hour counter. I will also manually synch it when the sources can no longer get much current into the bank at a CV.. My counter will be programmed for less than the banks full capacity to account for the upper 10% or so of charge range I am not pushing into..

If I raise the voltage from the CV limit of 13.8V or 14.0V the current spikes back into bulk as the bank is not technically full. I suspect it can be "considered" full for Ah counting at a specific target voltage and low current range. I am only using Ah counting as a "fuel gauge"....

Yes absolutely thats the way its done, just pick a point and cal it "full". 3.5v being a very safe number. the current falls to zero because of the battery voltage, it of course doesnt mean teh battery is charged , but who cares, its charged enough.

what the amp counter says is then just a handy guide.

What determines when your charger restarts charging have you determined that.

Dave

[mbianka]

Very interesting discussion. Sounds like the charging plan I use for 48 volt AGM electric propulsion battery bank may work for Lith Ion. I have two chargers I use:

1) Zivan NG-1 48 volt charger

2) Dual Pro Quad (4-12 volt)

When I need to charge my 48 volt bank I start off using my Zivan. It pauses to read the battery pack voltage while charging. At some point it goes from max current (16 amps) to around 4 amps. This shows up on my XBM battery monitor as being around 94% charge.

I then switch over to the Dual Pro 4 which is basically four individual chargers in one box which then charges each individual 12 volt battery in the 48 volt string independently. Each battery can reach full charge interdependently of the other three with no hand holding or intervention from me.
THE BIANKA LOG BLOG: BATTERY INVESTIGATION PART FIVE: Some Good News!

Because a 12 volt Lith Ion battery (bank) is essentially a series string like my 48 volt AGM propulsion bank. It would seem this would be the way to charge a Lith Ion bank too. This way each $ Lith Ion battery in the bank would always be kept in balance. Especially since they are packed so close together and some batteries may be warmer than others during charging. Of course the chargers to do this would be setup for the Lith Ion charge (voltage?) profile as opposed to my AGM's. IMO this would not be too complex to setup in a 12 volt Lith Ion house bank. But, for a 48 volt EP bank like mine it would mean a lot more chargers ($) and space to fit them. So I'm not jumping to Lith Ion's anytime soon. But, find the discussion here very informative.

[goboatingnow]

When I need to charge my 48 volt bank I start off using my Zivan. It pauses to read the battery pack voltage while charging. At some point it goes from max current (16 amps) to around 4 amps. This shows up on my XBM battery monitor as being around 94% charge.

why does it " pause" to read the voltage , for how long!, confused

[diugo]

Originally Posted by goboatingnow

I dont know your technical background, but you have a incorrect understanding of Peukerts law and its application.

Firslty Peukerts Law ONLY applies to the discharge of a battery, It does not model or attempt to model the charge cycle.

Secondly Peukerts Law, is only a "model" its not a direct battery effect. Peukerts was seeking a mathematical relationship to model and hence predict the loss of Wh capacity as a result of different discharge currents. Its merely a model and in real batteies can and do depart from the model,or in exotic chemistries like Li, have actual discharge profiles that differ from the Peukerts model. Ive yet to see an extensive study of the accuracy of model as it is applied to high capacity Li.

Note that the first order characteristic impedance of a battery is a complex value, it is comprised of static and dynamic elements (including real and reactive elements), simplistic applications can lead to errors of application. Higher order impedance models are quite complex and the subject of many Doctoral Thesis. ( even to this day for LA),

Blah, blah, blah. It's really depressing when so-called designers bury their heads in the sand and simply choose to ignore "the first order characteristic impedance of a battery", just because we have stumbled across an "exotic" chemistry like Li.

I keep repeating Ohm's Law, because I Squared R is the core of the derivation of the Peukert effect. It's really not rocket science. The simplest, most basic fact is, even the heaven-sent LFPs have internal resistance. It is the fundamental reason we see sag under load. That same resistance is in play during both charge and discharge. To say that Peukert applies only during discharge is just ludicrous.

I noticed that you directly quoted the Wikipedia article on the Peukert effect a few posts back, in a poor attempt to present yourself as authoritative in the subject---and to try to discredit MY apparent lack of technical background.

Well, I give up. I am moving on to greener pastures, and leaving the amp-hour fumbling to you geniuses. For the record, one of my degrees is a BSEE from the University of Michigan.

[mbianka]

Originally Posted by goboatingnow

why does it " pause" to read the voltage , for how long!, confused

Not sure of the time but, I expect it to be in the millisecond range. Never put a scope on it. It's probably done to make sure the battery (bank) is not over charged and is also used in some algorithm in the charger to detect state of charge too. This is fine for an individual battery but, in a series string like my EP bank it can not detect an overcharged battery in the string from an undercharged battery. So that's why I switch to the individual charging before it reaches full charge. Has worked well for the past five years.

[Maine Sail]

Originally Posted by mbianka

Because a 12 volt Lith Ion battery (bank) is essentially a series string like my 48 volt AGM propulsion bank. It would seem this would be the way to charge a Lith Ion bank too. This way each $ Lith Ion battery in the bank would always be kept in balance.

I don't know of any suitable chargers for the marine environment designed to charge 3.2V LiFePO4 cells... At home I use a 30V X 30A bench top power supply.. Top balancing is pretty easy and it seems from my very limited experience that once the initial balance is done the pack stays in-check pretty well. Mine have converged at about a 0.004A difference when compared to one another with a Calibrated Fluke 179.. Close enough for horse shoes as they say.

I have also purposely stayed out of the limits of my BMS so no cell level shunting has occurred and no "red-lights" have illuminated at the cell level.. I am now at 20 cycles all to varying depths and recharging to either 13.8V or 14.0V...

I will likely re-balance once per year in parallel as I did initially. What I would love is a marine "power supply" for use on a boat that allows control of voltage and current independently.

Originally Posted by mbianka

Especially since they are packed so close together and some batteries may be warmer than others during charging. Of course the chargers to do this would be setup for the Lith Ion charge (voltage?) profile as opposed to my AGM's. IMO this would not be too complex to setup in a 12 volt Lith Ion house bank. But, for a 48 volt EP bank like mine it would mean a lot more chargers ($) and space to fit them. So I'm not jumping to Lith Ion's anytime soon. But, find the discussion here very informative.

I suspect that any charger built to UL 1236, with the marine supplement, would be as costly as the cells. I do hope charge device manufacturers come up to speed on this. The best we can do for now is use devices that allow full custom programming. I have a Sterling charger that allows custom programming, my solar controller will be re-programmed by Genasun and my Bamar reg will get its own custom program.

I know companies like Balmar are working on it but according to Rick, "Everyone want's something different." Where to go, design wise, is not as simple as one would assume....

With Balmar for example you need to custom program the FLOAT voltage first, then work backwards to absorption then to "bulk"... You can't drop absorption or bulk voltages to any lower than .1 above the next stage. You can use float and absorption to turn off the reg, and stop charging, by setting it below the normal resting voltage. I am experimenting with it now to see how I want to set it up.

These are all little learning's that when working with lead acid you'd never know were even possible. I never knew you could drop the absorption or bulk voltages on the Balmar lower by working backwards and starting at float and I have custom programmed these for years but never needed to drop bulk or absorption low like we do with LiFePO4........

[goboatingnow]

Blah, blah, blah. It's really depressing when so-called designers bury their heads in the sand and simply choose to ignore "the first order characteristic impedance of a battery", just because we have stumbled across an "exotic" chemistry like Li.

I keep repeating Ohm's Law, because I Squared R is the core of the derivation of the Peukert effect. It's really not rocket science. The simplest, most basic fact is, even the heaven-sent LFPs have internal resistance. It is the fundamental reason we see sag under load. That same resistance is in play during both charge and discharge. To say that Peukert applies only during discharge is just ludicrous.

I noticed that you directly quoted the Wikipedia article on the Peukert effect a few posts back, in a poor attempt to present yourself as authoritative in the subject---and to try to discredit MY apparent lack of technical background.

Well, I give up. I am moving on to greener pastures, and leaving the amp-hour fumbling to you geniuses. For the record, one of my degrees is a BSEE from the University of Michigan.

I quoted wiki , because it there, would you like multiple sources.

perhaps you could read this article ( there are others ) , ( I spent a long time designing battery chargers).

"
A critical review of using the Peukert equation for determining the remaining capacity of lead-acid and lithium-ion batteries

In many applications it is essential to predict the remaining capacity of a battery reliably, accurately and simply. Several existing techniques for predicting the remaining capacity of a lead-acid battery discharged with a variable current are based on variants of Peukert's empirical equation, which relates the available capacity to a constant discharge current. This paper presents a critical review of these techniques in the light of experimental tests that were carried out on two lead-acid commercial batteries. The relevance of these Peukert's equation based techniques to lithium-ion batteries is also discussed in the light of tests carried on a lithium-ion power battery. The basic conclusion of the paper is that Peukert's equation cannot be used to predict the state of charge of a battery accurately unless it is discharged at a constant current and constant temperature.
"

Thats ones also quoted in Wiki as its quite well known.


Peukerts law attempts to model the diffusion process, but omits several other factors, it works well for constant discharge current and constant temperature. Its doesnt work well in disrupted cycles with varying current and temperature.

SImply stating that all batteries can be modelled from a first order Thevenin is disingenuous. yes a battery is in theory a perfect voltage source in series with a resistance, however its only an approximation and doesnt take into account other dynamic situations.

dave

[Badsanta]

I thank you both for tring to help me understant this. Still over my head. I hope to understand this buy the time I get the boater bucks. Many thanks for the input