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Old May 07, 2007, 01:13 PM
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How much UNbalance is acceptable on new packs?

I wonder because today I received to 4s 1800mAh packs and one of them was perfect and the other was not.

Before charging them I measured the individual cell voltages.

pack 1 had 3.801 3.801 3.799 3.799 (which I consider perfect)
pack 2 had 3.808 3.766 3.801 3.809 (still have because I've not charged it yet)

Is that something to worry about or is it within the normal margins?

Thank you all for your help
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Old May 07, 2007, 01:22 PM
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My opinion is that there's nothing you should worry about. Charge/balance first, then you're good to go.
I had a pack 300mV out of balance when it arrived. None of the cells was lower than 3.4V though. I've charged/balanced it first. It's still holding well after 60+ flights.

Cirip
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Old May 07, 2007, 02:09 PM
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check your new pack anyway for DISCHARGE CAPACITY as a reference
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Old May 07, 2007, 06:34 PM
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hall is correct about, always doing a discharge after charging to see the Mah capacity. If you have a charger that shows that with a display. That way you will have a hard figure and later on down the road, you can always do another discharge to see if you have lost any capacity. Stop discharge at 3.2 volts per cell or 12.8 volts for a 4 cell pack.

That second pack, 2nd cell looks a little low.

Challenger413
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Old May 07, 2007, 09:02 PM
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I agree with the above posts. A 0.04V (second pack) spread is nearing my limit but not return type bad.
Charge and again check the balance. It might be right on the money. If not, balance and enjoy.
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Old May 08, 2007, 03:25 PM
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Thanks for the responses..

So I have now charged the second pack too, it was balancing all the way through the charge not like the other pack.

Then I discharged it at 2C (3.6A) to 12.8V and measured individual cell voltages from time to time during the discharge.

I got 1706mAh out of it and just before it was finished cell 3 had only 3.0V during the load.

Now after it has been sitting a while it has these resting voltages:
3.506 3.613 3.326 3.421
That is 0.287V difference between cell 2 and three.

I'm surprised that cell 2 was not the low cell in the end, but I must say that this looks like a really messed up pack to me.

I'm discharging the other pack now, which I've done 2 very light cycles on with my trex.

I'm curious to see whether the other pack acts about the same or not.
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Old May 08, 2007, 04:12 PM
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If a pack is bad, it will be BAD. I had a brand new 5S TP show up with 4 cells at 3.8 volts and one cell at 2.5 volts. I was replaced for free, including shipping back (Light Flight RC).
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Old May 08, 2007, 04:14 PM
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Ok, discharged the other pack the same way.

Got 1638mAh and the following voltages:
3.410 3.487 3.449 3.386 (0.101V difference)

No cell was down to 3.0 but maybe 3.1 in the end of the discharge.

Will be charging these at 0.5C next charge, I guess I will use both packs and see what happens in the future.

I'm not going to be happy if pack 2 fails early on me though.
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Old May 08, 2007, 04:19 PM
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It's possible it will come back into balance after charging.
Cell #2 may have the highest capacity - that's why it didn't "fill up all the way" during charge and why it didn't get as empty during discharge.
I'd charge it for an hour and recheck the balance. If they are much closer, I'd finish the charge and check again.
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Old May 10, 2007, 02:19 PM
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I have decided to run with both batteries. And they perform well, but I have not been pushing them too hard yet, only charging 1050 mAh after 5 mins flight.

The second pack is a bit more out of balance and the balancer works a bit more during charge, but as long as I don't discharge it too much I hope I will be ok.
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Old May 10, 2007, 02:57 PM
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The less taken out of the pack, the better the balance will stay. FMA has shown the negative effects of deep discharging on balance.

See pages 15-16 "What is Cell Unbalance"
http://www.fmadirect.com/support_docs/item_1197.pdf

What is the mah of the pack?
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Old May 10, 2007, 03:21 PM
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It's 1800, so I'll try to keep it to 1450mAh used or less.
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