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› Question on lipo balance circuits
10-14-2005 09:13 PM  12 years agoPost 1
Riverman

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Peace River, Alberta

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Hello

I have a heli kit on order that includes a lithium polymer 11.1v 2200mah battery. I was initially impressed at the batteries capacity, but after getting a preview of the same battery that a coworker had already recieved, I am less than impressed.
From what I've read, it is actually 6cells running two parallel ciruits of 3 cells each. I was moritied to see it didn't come with a balance ciruit. I though it was a death sentence (for me or the battery?!) to charge a lipo through the "output" leads.
What I would like to do is open the battery up and turn it into two batteries of 1100mah each. If possible (and I can't see why not) I will wire up a simple balance connector for charging.
I've never opened up a lithium polymer battery before. Does anyone have any experience in modifying lipos?

Thanks, Brett

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10-17-2005 04:55 PM  12 years agoPost 2
tean

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Gloucestershire,UK

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Hello Riverman,
It is very common practice to connect cells in parallel to increase the available current and capacity. I'm not aware of any drawbacks to this, at least not in the model context, provided the cells are kept connected permenently and charged and discharged together. It is only when parallel connected batteries are seperated and charged/discharged that problems can occur. When they are re-connected, quite large currents could flow as the total charge redistributes itself. Parallel connected cells can be thought of as one larger cell.

The other, almost universal, practice is to connect cells in series to increase the total voltage. This can cause problems if one of the cells, or compound parallel cells, in the chain becomes fully charged before the rest. The charger, can only see the overall pack voltage and continues to pump current into the pack until the overall "full" voltage is reached. The prematurely full cell thus gets overcharged and could be damaged.
To counter this, all the cells in the chain must be at the same state of charge when the pack is assembled and matched cells should be used. Over many cycles, the charge levels between the cells in the chain can drift and periodic "rebalancing" is recommended. To achieve this, most packs have a small, multi-pin connector which allows access to the individual cells in the chain. Rebalancing can be by charging each cell to a known, usually full, state seperately, or by connecting a voltage sensitive shunt accross each cell and charging the whole pack as one.The shunts protect the cells from overcharge, and, if all the shunts are calibrated the same, all the cells should have the same voltage at the end of the charge. Caution should be used, in case the shunts prevent the charger from recognising the "fully charged" voltage.
How often this is needed depends on how well matched the cells are. I have some 10s TP prolites that have over 50 cycles on them and have never been balanced. All the cells are within 0.01V of each other. Some packs are not fitted with a balance connector. This is either because the manufacturer feels his cells are so well matched that they will never go out of balance, or he's just too mean to fit one.

Your battery sounds like 3s2p pack. That is, a chain of 3 "compound cells", each consisting of 2 parallel cells. A very common combination. I have one of these myself and it has been charged many, many times through the "output" leads with no problems.
I can't see what you acheive by splitting you pack in two. If your pack has no balance connector, you must weigh up the risk of killing it by opening it up to fit one, and the risk of killing it by letting it drift slowly out of balance. You will need a good, hot soldering iron to minimise the amount of heat that goes into the tags, and bear in mind that even a momentary short circuit is very damaging to lipos and you must not fully discharge them to avoid the danger of this. If it was mine, I would probably run it as it is for 30-40 flights and then consider fitting a connector. If I bug****ed it up then, at least I would have had some use out of it.

There is lots of good information at

http://www.batteryuniversity.com/
and
http://www.buchmann.ca/default.asp

Hope this helps
Andy

Logo14 Vbar, Logo500 CSM flybarless, Trex 600 CSM flybarless, 3DMP, Logo10, Lipoly, Mini T, .......

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10-18-2005 06:04 AM  12 years agoPost 3
Riverman

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Peace River, Alberta

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Hello Andy

Thanks for taking the time to respond. I really appreciate the info. I'm still new to this whole lithium polymer thing.

Brett

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10-18-2005 07:01 AM  12 years agoPost 4
Micro-Maniac

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Pasco,Washington Formerly: Captain Chaos

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TP balancers plug into the balance leads and automatically balance all the cells at once by discharging the higher voltage cells to match the lowest voltage cell. They can be plugged into newer TP chargers to communicate individual cell info to the charger telling it to shut off if a cell exceeds a certain voltage. Much easier and simpler than trying to manually charge/discharge each individual cell to a certain voltage. I wouldn't buy anything other than Prolites at this point. I love the TP line of products right now.

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