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

[deckofficer]

I'm discharging at 0.02C and just waiting for the knee to show up.

[goboatingnow]

Originally Posted by ebaugh

The discharging curve for my cells...show 10% more capacity at 3C discharge than .5C. But the difference is nearly nothing between 1C and .5C, so I think there may be a point of diminishing returns at fractions of C discharge rates.

Edit:... More capacity at .5C than 3C. Sorry...

That's interesting are they Winston cells, maybe Li doesn't follow Peukerts equation at all. Which might make sense as it was developed to model LA anyway.

I didn't get discharge curves versus capacity for mine ( early thunder sky )

Dave

[ebaugh]

If you are still at 3.3V, you may have a long wait. I've never tested it, but the knee is supposed to be around 3.1V on my cells.

[deckofficer]

How much longer could it be? This battery is rated 1280 w-hr and I'm past 1430 w-hr. A lot better than the first cycle at 980 w-hr, but I thought I timed it right so my constant checking of cell voltages didn't interview with my dinner time.

[ebaugh]

Originally Posted by goboatingnow

That's interesting are they Winston cells, maybe Li doesn't follow Peukerts equation at all. Which might make sense as it was developed to model LA anyway.

I didn't get discharge curves versus capacity for mine ( early thunder sky )

Dave

See:

http://elitepowersolutions.com/docs/...g%20Curves.pdf

They are GBS, LiFeMnPO4, but seem very similar to the older CALB cells specs. I've never understood the differences in the Thundersky/WInston/Sinopoly specs based on Yttrium.....

[ebaugh]

Originally Posted by deckofficer

How much longer could it be? This battery is rated 1280 w-hr and I'm past 1430 w-hr. A lot better than the first cycle at 980 w-hr, but I thought I timed it right so my constant checking of cell voltages didn't interview with my dinner time.

I don't know...but the results will be interesting after you double check them!

[deckofficer]

Here is my load.

MOV03052 - YouTube

[diugo]

Originally Posted by goboatingnow

What's that got to do with capacity. ? , Peukerts exponent ,or more correctly his equation models the capacity of a LA battery against differing discharge currents, with an exponent of 1, the battery capacity remains the same irrespective of amount of the discharge rate.

Internal impedance is related to voltage drop , ie the slope of the discharge curve, Li has extremely low resistance , ie its almost a perfect voltage source , ie the voltage only droops a tiny amount under load

Peukert "discovered" that cells dissipate more energy as heat the higher the rate of current. Not too surprisingly, internal resistance is the whole crux of his analysis. Ever heard of I2R? The Peukert exponent is derived from the cell's internal resistance.

As far as "voltage droops a tiny amount under load", that simply would not be the case with a Peukert of unity. Back to the real world, my four-cell battery drooped 0.5 volts under a 60A (0.2C) load. That's hardly a "tiny amount", and correlates well with an internal resistance around 2 millohms per cell, which is quite high IMHO.

That said, the voltage drop from 13.1 to 12.6V is much better than the AGMs' typical drop from 12.7 to 11.9V.

And two more reasons to stay happy: I'm at the lower end of the charge curve, and internal resistance is higher there. Also, the cells have not yet been fully cycled even once, after being on the shelf for two yeard, so there is likely "rust" (oxidation compounds) on the electrodes that will be cleaned off upon full recharge.

[diugo]

Originally Posted by deckofficer

How much longer could it be? This battery is rated 1280 w-hr and I'm past 1430 w-hr. A lot better than the first cycle at 980 w-hr, but I thought I timed it right so my constant checking of cell voltages didn't interview with my dinner time.

Bob, the 1280Wh capacity is at the "standard rate" of 0.5C. So with any Peukert exponent greater than 1, you'll see a huge increase at much smaller currents.

For example, the Trojan T-105 was 225Ah at ten amps but something like 400Ah at one amp, IIRC.

[Maine Sail]

Quick question? My four 400Ah cells are wired in series for a 13.2V bank. I just tested the internal resistance with my battery analyzer and it shows .81 mOhms. Winston rates each cell at .4 mOhm..? Thoughts....

Winston Cell Resistance Specs

[deckofficer]

Finally a change in voltage, all 4 cells at 3.30, down from 3.31 @ 1455 w-hr.

[bill good]

MAINE SAIL
You did not say you were testing each cell!! not the 4 in series.

Bill

[Maine Sail]

Originally Posted by diugo

Bob, the 1280Wh capacity is at the "standard rate" of 0.5C. So with any Peukert exponent greater than 1, you'll see a huge increase at much smaller currents.

For example, the Trojan T-105 was 225Ah at ten amps but something like 400Ah at one amp, IIRC.

T105 has Peukert of 1.24. It should provide 225Ah's at an 11.25A load for 20 hours..

With a 1A load for 20 hours it should give you 402Ah's, based on a Peukert of 1.24...

[Maine Sail]

Originally Posted by bill good

MAIN SAIL
You did not say you were testing each cell!! not the 4 in series.

Bill

No, my analyzer will only test the pack at 12V.... Need to test the cells too..

[NahanniV]

Originally Posted by Maine Sail

Quick question? My four 400Ah cells are wired in series for a 13.2V bank. I just tested the internal resistance with my battery analyzer and it shows .81 mOhms. Winston rates each cell at .4 mOhm..? Thoughts....

Winston Cell Resistance Specs

What type of a battery analyzer do you have ?

I'm also not sure from you post if your measurement is for all the cells in series ? Or a single cell ?

I think the rated value for the cells may be an impedance value, measured by passing an AC current through the cell; That test would give a lower value than the resistive load method outlined by diugo. I don't pretend to understand it all: How to Measure Internal Resistance – Battery University

I think the test outlined by diugo could give a good baseline to compare future tests against.

ALSO, if you missed my previous post, I'd be interested to know what the manufacturing codes on your cells are.

Thanks,
JM