Testing LiFePo4 cells

[Just]

Hello All,

How is it possible to test/sort the LiFePo4 cells (which of them are good and which bad)? What are techniques?

Thank you!

[MAGICPIE3FOCUSPOWER]

Charging, discharging and measure the capacity and internal resistance.
So you need a charger with those features.

[Kirk]

Get a charger like the iCharger 208B, that has modes for checking individual cell voltage, cell resistance etc. RTFM

[Just]

Should I test the cells according to their resistance? What resistance should be considered as normal? What's one as a bad resistance/cell?

Is the resistance a single criteria of decision?

As for the cell capacity, between what voltages should it be measured? Should it be measured between 2.0V and 3.4V for LiFePo4 cells (ever for discharging or charging)?

Thank you!

[truly_bent]

Think about this...

if you were to measure the resistance of each and every cell, don't you think a pattern would emerge from whatever resistance values you happened to note?

Eh?

[MAGICPIE3FOCUSPOWER]

Also about the capacity measurement, it depend what kind of charger you have.
Some charger has the features to set your own min. and max voltage for charging and discharging.

[Just]

"Some charger has the features to set your own min. and max voltage for charging and discharging" - I have exactly this charger - it also shows charge/discharge amount in Amps per hour.

The questions is so that between what voltages does the nominal capacity should be measured? My guess it should be measured between nominal voltages (2.0V and 3.3V), but I'm not sure. Probably it should be measured between 0V and 3.3V (amount of current required to charge the battery from 0V 3.3V)

Thank you 

[MAGICPIE3FOCUSPOWER]

Just set the  min and max voltages according your battery specifications.
Or just use the default settings of th charger.

[Cornelius]

NEVER Discharge Lithium cells to 0V!

If you have done so, and managed to get them charged up again, you're just lucky.
The lower discharge voltage for LiFePo4 cells are normally 2.5V, and never below 2.0V. If a cell are below 2.5V, one should normally trickle-charge the cell until it reaches 2.5V, then apply full charge.

And fyi;
Most LiFePo4 cells have an end-of-charge voltage at 3.65V max; preferably 3.60V (You'll lose 2-3% capacity by skipping those last 0.05V, but the cells will last much longer.).

So to test the capacity of your cells, charge them up to 3.60/3.65V, and discharge them down to 2.50V with a known load, then you have your capacity.

For those of us that are lucky enough to have the iCharger (I got the 1010b+), there's a separate discharge-charge cycle program there.
Also, to get the internal resistance for your cells, press and hold the stop button for 2 seconds until it beeps, and then press the left arrow button.

Edit:
I guess the specified internal resistance are measured when the cell are fully charged.

My experience says that a cell that are worn usually shows a greater difference in the IR when discharged versus charged, compared to a good and healthy cell.
I do not know the spec. IR for your cells, but I have some 3.3V/10Ah cylindric cells here that have an IR of around 6-7mOhm when fully charged (3.60V), and around 20mOhm when discharged (2.50V). These are good cells that I get in excess of 11Ah out of when keeping them inside the limits.

[Just]

Thanks for your comment, Cornelius!

Why is not possible to check the cell's resistance just by a regular DMM (Digital MultiMeter)?

Could the cell's resistance be changed when the cell full charged/discharged or should it stay constant? Does it depend on its capacity?

What resistance should I measure for the LiFePo4 cells?

Thank you!

[Lollandster]

Thanks for your comment, Cornelius!

Why is not possible to check the cell's resistance just by a regular DMM (Digital MultiMeter)?

Could the cell's resistance be changed when the cell full charged/discharged or should it stay constant? Does it depend on its capacity?

What resistance should I measure for the LiFePo4 cells?

Thank you!

I don't know if you can reliably measure internal resistance on a LiFePO4 battery with a normal DMM due to accuracy problems. If you want to try you'll just have to measure the discharge current of the battery while connected to a known resistance and use the formula R_int=(U-I*R_load)/I or R_int=(U/I)-R_load Where R_int=Internal resistance U=Battery voltage before connecting the resistor I=Currrent while connected to the resistance and R_load=Resistance of the resistor used to discharge.

I don't know LiFePO4 very well but I would guess the internal resistance would be reflected by the amount of current you can drag out of it. So for a 10AH 3.3V 10C battery it should be less than R=U/I=3.3v/(10Ah*10C)=0.033ohm or 33mOhm

[Cornelius]

Why is not possible to check the cell's resistance just by a regular DMM (Digital MultiMeter)?

Because the way the dmm measures resistance are by sending a known current through the circuit you want to measure; if there's another current (a battery) present, the dmm gets confused. Besides, a dmm's resistance scale are too coarse to measure milliOhms.

One can use a dmm, but it's a bit cumbersome though, and very inaccurate. It's easier to have 2 dmm's; one to measure Volt, and another to measure Current(Ampere), and one needs a load.

The theory are that one connects the load to the battery in series with the amp-meter to get the current, and then measures the voltage drop over the load. Then, one can calculate the battery's internal resistance;

  VoltageDrop / Ampere = Ohm

Could the cell's resistance be changed when the cell full charged/discharged or should it stay constant? Does it depend on its capacity?

It usually changes. A discharged battery have a little higher resistance than a fully charged one. (See my previous post... )

What resistance should I measure for the LiFePo4 cells?

Who knows?!
You have to find the Spec-sheet for your specific cells; it should be specified there. But I guess between 7mOhm and 15mOhm for a single good cell, capable of delivering atleast 20C.

As the cells wear out, the IR rises, and one doesn't get the previous Ah out of them, and also, they cannot deliver so much continous current as they used to... - but I guess you already experienced that...

[Just]

Thank you all! Now it's much clearer for me how to deal with the cells and how to test them! Good Cooperation - thank you !

[Morgen 3Eman]

As an old guy who has seen some funny things happen, I must caution:

CAUTION:  You cannot measure the the internal resistance of a battery by setting your meter to Ohms function and putting the test leads across the battery.

Of course, all the funny things happened to other people, never to me.

TTFN,
Dennis

[Just]

Thank you for the CAUTION!

It seems better to buy a good Internal Resistance Battery Meter than playing tricks with a DMM or even two... Honestly it was my bad idea of using it for such measurements . Now I'm just laughing thinking about it ...

BTW, can somebody recommend a good cheap  Internal Resistance Battery Meter?

Thank you!