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Old Jan 06, 2004, 12:35 PM
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LiPo cell imbalance: How Long, How much?

There seems to be much concern about cell imbalance especially in series packs. Is there any data that shows how long (how many charges) it takes before you have to rebalance? How much imbalance can be tolerated?

Hank
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Old Jan 06, 2004, 12:42 PM
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You don't want much if you charge to full capacity every time. Maybe 0.1 volt difference max. Thing is, once they're balanced, they seem to stay this way for a good long time. Only LiPo pack I've had so far that has gone out suffered significant crash damage.

I've charged 3S packs of the old Sony Li-Ion 18650 cells dozens of times and saw no imbalance creep in at all.

Be warned about the crash damage thing, though. The LiPo pack broke loose and hit the back of the motor. Crushed the ends of some of the cells by 1/8". By a cursory check it appeared to be OK. Would take a charge, would discharge as if it were OK. But one of the damaged cells was leaking electrons internally, so that cell started to drop at about 1 volt per day. In a day it would have been sufficiently out of balance to present to opportunity for disaster if I had charged it in that state.
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Old Jan 07, 2004, 05:03 AM
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I'm afraid there seems to be no easy answer.

I have measured many packs, and some are still fine after fifty cycles, while others are way out of balance after only a dozen cycles.

One of the contributing factors is temperature profile across the pack, another is self discharge.

The worst packs I've seen had one cell with higher self discharge rate than the others.

Some people swear that the problem of pack imbalance doesn't even exist, others have lost quite a few packs already due to imbalance / resulting overcharge / ballooning.

So you have four options at this time:

1) ignore the problem and take the risk

2) add center taps, measure your cells frequently and charge individually, when required

3) put LiPo-Balancers in your packs or charging harness

4) use a special system of connectors and cables to charge all cells in parallel (practical for smaller packs only)
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Old Jan 07, 2004, 01:24 PM
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Thanks Suzanne.

It looks like in any case that the cell terminals have to be accessable whether they are center tapped or a balancing circuit is used. It would seem to me that the easiest and cheapest method would to be to use center taps and small sockets like Tanic is using. The center tap wiring and sockets would not have to be heavy since it is not going to carry much current to charge the individual cells.

BTW, I just got a Micro Mark catalog and they are advertizing a Li-ion powered Dremel tool. So Li technology is moving into the power tool business.

Hank
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Old Jan 08, 2004, 05:12 AM
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Hank,

if you use LiPo-Balancers, you do not need center taps.
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Old Jan 08, 2004, 07:44 AM
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LiPo-Balancers?

What are they and who sells them?
Dave
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Old Jan 08, 2004, 08:54 AM
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Suzanne's own product.
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Old Jan 08, 2004, 11:10 AM
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Re: LiPo-Balancers?

Quote:
Originally posted by debogus
What are they and who sells them?
Dave
Dave,

I have made the schematics and PCB layout available here for everybody to use:

http://www.rcgroups.com/forums/showt...hreadid=157641

If you have any questions on how to build them, just ask and I'll try to help.

Also, if you don't want to build your own, I still have some stock.

Suzanne
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Old Jan 08, 2004, 12:01 PM
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Now I'm confused. I thought you DID need to have center taps of series packs.....but in post 5 of this thread, Suzanne says:

"if you use LiPo-Balancers, you do not need center taps."

Help !

Jim
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Old Jan 08, 2004, 02:30 PM
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Jim: The question is when is a center tap not a center tap? How can you balance a pack without access to each cell's terminals? Maybe I just don't understand Suzanne's method.

Hank
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Old Jan 08, 2004, 02:52 PM
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Suzanne's gizmo does have a connection to each cell, but it need not be accessible from outside the pack. Is it a tap? Well, not if you consider external access as defining a tap. Semantics.

BTW I have done a pack with taps, and proceeded to short one against the ESC when I was stuffing the pack into the plane. Plane launched OK. 20 seconds later the ESC burned to a crisp.

At least I think that's what caused it. They were shrink-tube covered, but that must have been cut thru by pressure or vibration or both. Makes Suzanne's solution (i.e no exposed taps) look more attractive.
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