I'm using my Hitec X1 charger to balance charge the LiFePO4 batteries in my 50cc plane; 2.0A charging rate. The batteries are 3850 mAh for the RX and 2200 mAh for the Ignition.

The ignition charges fine - After a little flying on Wednesday it took 4m 15s to put 72 mAh back into the battery.
The rx, however, is what's weird. It charged for 120m 00s, at which point it exceeded the safety time limit. 317 mAh were put back in.

On a small Lithium battery tester the ignition reads:

#1 -3.44
#2 - 3.45
All - 6.82

The rx reads:

#1 - 3.57
#2 - 3.36
All - 6.86

The rx battery doesn't appear to be properly balanced.

Questions:

1. Why is this happening with the rx battery?
2. What should I do about it?

Cheers!

It is out of balance and the cure is to "balance" it. Why it's out is anyone's guess. Perhaps it never was in balance?

With it that far out of balance, it's going to take a long time to get it charged since the charger is going to keep drawing down that high cell. What I've done before is used a 2 pin connector like for regular Rx batteries and charge each cell individually through the balance port. That gives you a straight forward full charge for both and then will let you see if there is a battery issue. If you do that and still see a voltage disparity, you may have a bad cell.

It is not that far out of balance - just balance it.

That's how the CellPro chargers work, they only charge through the balance tab

After receiving a lot of opinions, I decided to try discharging the battery. Again, this is a 6.6V 2-cell 3850 mAh LiFePO4. The discharge rate was 1.0A and the cutoff was 4.0V (both default values).

I gathered data at random points throughout the discharge; the last data point at 111m 30s into the discharge showed the discharge rate at 1.0A, the removed capacity at 1732 mAh and the voltage at 4.66V

At some point around the 119th minute, an error appeared: BATTERY ERR CELL LOW VOL. I checked the cells with the charger and they read at 3.07V and 1.05V. Not encouraging.

I'm currently balance charging the battery; it is taking a charge. I will post the results (later) in the morning.

At this point I would replace the battery. Through your efforts you have discovered a battery with a bad cell and thus saved your airplane. Congrats!

Not so fast!

The charge rate I used was 2.0A (0.52C), which is the charger's default setting. After I started the charge it occurred to me to look up the specs of the battery; I was surprised to find the max charge and discharge rates at 4C and 20C, respectively.

I was also surprised - very pleasantly so - when I went down to the shop to check the results of the charge and found it had reached the safety timeout of 120m with cells at 3.59V and 3.59V!

You did discharge one cell to 1.05V which is well below it's safe discharge voltage. IMO at this point there is no way you can be 100% certain the cell is not damaged and will not discharge at a higher rate then the good cell. It's not worth the risk, replace the battery.

There had to be a catch - there's always a catch. I'm going to do another discharge and see what I see.

I charged the battery and the cells were balanced at 3.59 / 3.59. Then I I did another discharge - cells at 3.07 / 1.50. Looks like a damaged cell. Plus, I'm only seeing about 1740 mAh coming out and going in, and it's a 3850 mAh pack.

I'm guessing there's no point in repeating the charge / discharge cycle. If I'm wrong, set me straight!

No point indeed. It is just a small cell. I wonder why the capacity was reduced to about half.

It's a defective cell. If the battery is still under warranty send it in for replacement. The pack is failing both tests for cell quality- voltage and discharge capacity.

I came across this amazing A123 FAQ. Here's the kicker:

- - -

Q: My Charger can charge A123 and has a cycler, but when I set a 2v per cell cutoff, at the end of the discharge one cell is way below 2 volts! What's up with that?

We've seen this in the Hitec X4, it's also common across several other multichemistry charger/cyclers. What's happening is the cycler's discharge cutoff trigger is watching the total pack voltage and ignoring the individual cell voltage. This is a source programming error in my opinion, just as the charge routine is individual cell voltage controlled, it's no less important for lithium packs to terminate the discharge routine based on individual cell voltage. Here's the 'issue': A123's have a very,very sharp 'knee' in the discharge curve and actual capacity varies as much as 5-10% between cells. (an artifact of A123's 'nano' plating technology.. negative plate surface area varies a bit from cell to cell) This issue can set up an overdischarge condition on the lower capacity cell, so we've adjusted the discharge target up from 2v per cell to 2.5v to give a cycler that does not have individual terminal voltage control a bit of 'padding' as a safety margin.

- - -

My charger is a HiTec X1; essentially, one quarter of an X4. I think there's a very good chance that the same thing has happened to me. I'll be sure to change the discharge from the default 4.0V to 5.0V in the future.

I was going to try my LHS for a refund on the pack but it seems it's my fault for assuming that the discharge cycle balanced the cells.

Most all chargers work that way , and is a fault of the pack (capacity mismatch) and not the charger. I'm not aware of any charger that will balance on a discharge cycle. Perhaps some of the good ones may at least monitor the individual cells though?