[grosbeak]

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!

[Max]

Are you balance charging? It sounds like it is trying to balance charge the cells. If one cell is over, it trickle charges both cells at a low rate, while at the same time putting a load on the high cell to "burn off" the higher voltage(while the other one creeps up)
This could explain why you timed out and yet put very little current back in.
Next time you are charging, sneak a peek at the current that is being put back into the cells. You'll see it goes to a trickle once the packs near the peak. Sometimes the trickle balance part of the charge takes several hours. That would explain your timeout feature being tripped. I turn off my timeout feature when I have a pack that needs TLC via balance charging.

The best solution for an off balance pack is to cycle it a few times, and the best way to do that is to fly it to 50% or more of its capacity, then balance charge it back up.

In the cases where its happened to me, it all balanced out nicely after about 4 or 5 flights and balance recharges.

[grosbeak]

Thanks for the reply, Max. Responses, observations and questions:

Originally Posted by Max

Are you balance charging? It sounds like it is trying to balance charge the cells.

Yes, I am balance charging.

If one cell is over, it trickle charges both cells at a low rate, while at the same time putting a load on the high cell to "burn off" the higher voltage(while the other one creeps up)
This could explain why you timed out and yet put very little current back in.
Next time you are charging, sneak a peek at the current that is being put back into the cells.

IIRC it started at 1.1A and within 30 minutes it was down to 0.1A.

You'll see it goes to a trickle once the packs near the peak. Sometimes the trickle balance part of the charge takes several hours. That would explain your timeout feature being tripped. I turn off my timeout feature when I have a pack that needs TLC via balance charging.

The best solution for an off balance pack is to cycle it a few times, and the best way to do that is to fly it to 50% or more of its capacity, then balance charge it back up.

In the cases where its happened to me, it all balanced out nicely after about 4 or 5 flights and balance recharges.

Would it not be equally effective to cycle the pack with the charger?

[Dale Hunter]

John, if nothing else, simply leave the receiver turned on overnight. It will drain enough out of your pack that both cells will require enough energy to establish a ballanced pack. Running such a large capacity pack often gets topped off so often that it never gets down below 80% of capacity.
Yes your charger should be able to cycle your pack. Try that if you prefer.
Hope this helps.
-Dale-



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[grosbeak]

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.

[grosbeak]

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!

[Max]

John- Turn off the safety timeout , it is limiting your balancing ability.

Charging a LiFe in Balance mode has its own built in safety since the charger monitors the cells more closely than other charge modes.

If you change the charge mode to something else, then turn it back on.

I have had a couple LiFe packs that came with the balance plug wires put in the wrong order. Check the voltage at each post across the plug.

[grosbeak]

Max, thanks for the tip. I overcame the timeout another way; I increased the charge rate from 2.0A (0.52C) to 3.8A (1C)

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!

[Dale Hunter]

John, if it is convenient to take it back to Great Hobbies. Best see what they say.
Only my opinion, as they are the ones that will be replacing/refunding it.

-Dale-



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