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Interpreting LiPo Bounce Back Voltage

Old 06-06-2008, 03:39 AM
  #26  
Larry3215
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Ok, I think I see where the misunderstanding has been.

What you are calling 'bounce back' voltage is more commonly referred to as the resting voltage of the pack or cell.

I was assuming because you called it bounce back voltage that you were referencing the voltage that the cells bounced back FROM.

Since you're not, then that makes things much simpler.

You're reading far more into the resting voltages than is justified IMO

Originally Posted by Swift428 View Post
It's more important to know what LVC to use--depending on your flying pattern and battery condition (e.g 9.6) than knowing the individual cells voltage under load just before LVC.

I do at least 1 bench static condition charge/discharge cycle of a new LiPo. The bounce back voltage after 2 minutes, 15 minutes, 30 minutes and 1 hour is more than sufficient evidence as to whether:

1 - The LiPo is crappy having major BB cell voltage differences
No. As I said before - the lower the cells are discharged - even on packs that start out perfectly "in balance" - the greater the differences you will see between cell voltages. Thats true even on good, hi quality packs that are very well 'balanced'.

There is a difference between 'balanced' packs and 'matched' packs.

Balance is simply the voltage difference between the cells.

Its only important to know on fully or near fully charged packs. Thats because very small imbalances can result in over charging a cell with disastrous results.

'Matched' packs refers to packs with cells closely matched for capacity. Some vendors have implied that they also match the cells for Ir, but I haver my doubts on that.

Its the small capacity differences that cause the voltage differences you are seeing at deep discharge levels.

2 - If the cells are within 0.05v of each other during BB
Again - deeply discharged cells often show much larger differences than that and, in my opinion, its not anything to worry about.

3 - Whether or not to use an Ultra Balancer during BB.
No. You dont ever want to balance a discharged pack because its normal for packs to show differences when they are deeply discharged.

You can actually UNbalance a pack by doing that.

All the better balancers are designed to to take that into account.

The TP balancers for example will not do any balancing when the cells are below 3.2 volts. Between 3.2 and 4.1 the imbalance has to be greater than .03 volts. Only when the cells are above 4.1 volts does it start to do any fine balancing down to .oo5 volts.

The Hyperion balancers and its clones are similar in the way they work.

4 - If the cells are greater than 0.05v of each other during BB

5 - The amount of BB voltage after 2 min., 15 min., 30 min., & 1 hr.

6 - If the LVC was high enough to allow a BB to 11v after 1 hour.
The last one is the only one that really matters. I prefer my packs to 'bounce back' to a resting voltage of at least 3.7/cell, absolute minimum, within 1 or 2 minutes at most. Any deeper discharge harms the packs in my experience.

You might try a search on user name 'everydayflyer' both here and on RCGroups for more references to large imbalances at low charge states.

Charles has done a lot of testing on that issue and commented on it many times.

Its really a non-issue at low cell voltages.

If a pack starts off well balanced, then you discharge it and re-charge it and its out of balance after being re-charged - then you have a problem.

____________ .

We don't need to make this more difficult than is necessary [/quote]
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Old 06-06-2008, 04:32 AM
  #27  
Swift428
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Originally Posted by Larry3215 View Post
What you are calling 'bounce back' voltage is more commonly referred to as the resting voltage of the pack or cell.

I was assuming because you called it bounce back voltage that you were referencing the voltage that the cells bounced back FROM.
- Your DoubleTalk Isn't Helping

The rising voltage after the LVC (e.g. 9.6v to 11.1v) is referred to as bounce back voltage.

GIVE IT A REST
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Old 06-06-2008, 05:55 AM
  #28  
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Ok, I give up
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