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Old Jun 30, 2005, 08:18 AM
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Cheap, Easy LiPo Pack Balancing while Charging

Charlie Wang (Thunder Power) has anayzed a 3-cell LiPo pack imbalance problem as follows
( http://www.rcgroups.com/forums/showp...&postcount=331 ):

“Lets take a 3 cell (LiPo) pack with "pure"capacity imbalance problem(rated at 1000mAh)
Cell 1:1000mAh
Cell 2:1000mAh
Cell 3: 930mAh
If you discharge the pack to 90%, you will see large imbalance.
If you charge the pack back to 55%, you will see imbalance improve
If you fully charge the pack, the imbalance become minimum…
…But, once you discharge this pack deeper than 95%, the condition became more complex
After some deep discharge cycles and heavy duty applications, the pure capacity imbalance pack may no longer "be balanced" by slow charging.
More often, imbalance pack also contain other imbalance factors, such as IR variances and selfdischarge variances.”
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Old Jun 30, 2005, 08:22 AM
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The problem can be solved with simple Power Pole connectors.

Figure 1. shows a 3-cell pack with connectors and a single cell used to make up the pack. Goop is used to insulate the tabs, provide stress relief, reinforce the tab seals and allow visual observation of the tab-to-wire soldered interconnect. Each cell is prepared individually. This allows for easy replacement of cells in the pack. Three prepared cells are then strapped together with ordinary vinyl electrician’s tape.

Figure 2. shows the pack with its cells connected in series for flying.

Figure 3. shows the pack with its cells connected in parallel for charging. The charger is always at 4.2 volts, the single-cell setting.

That’s all there is to it. No specialized balancing circuitry is required and the cells are brought to equal voltage with each charge. In other words, the pack is automatically balanced with each charge; quite large cell-to-cell variances in capacity, IR and self-discharge rate (microshorts) are safely accomodated.

The health of the pack is easily maintained, because each cell is wide open to visual monitoring, electrical testing and cell replacement, when indicated.

There’s nothing new in any of this. It’s repeated here, because newbies to LiPos may not know about it and because recent discussions of LiPo charging apparatus have been dominated by means for electronic balancing during series charging, even for small, 2S and 3S packs, which comprise most of the market.

- RD
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Old Jun 30, 2005, 08:33 AM
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Please correct me if I am wrong, but shouldn't the cells have a very close voltage to each other to be connected in parallel, like 0.01v, to prevent the higher voltage cell from discharging into the lower voltage cell at a high amp rate?
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Old Jun 30, 2005, 09:21 AM
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Quote:
Originally Posted by TMetalMan
... shouldn't the cells have a very close voltage to each other to be connected in parallel, like 0.01v, to prevent the higher voltage cell from discharging into the lower voltage cell at a high amp rate?
No, not very close:

http://www.rcgroups.com/forums/showp...6&postcount=57

- RD
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Old Jun 30, 2005, 09:35 AM
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So I do stand corrected. That is a great idea! Didn't luc do something similar?
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Old Jun 30, 2005, 09:37 AM
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I agree parallel charging and then series discharging would elimanate the imbalance issues and there are a few who take that approach.

Thread here shows alternate connectors .

There are a few problems with this arragement for many however.

Many LiPoly cells are not available as single cells and even the ones which are are often higher than the same cells pre assembled in a pack.

Many Electri Flyers are not DIY types and do not wish to learn have to assembley their own packs. Many do not even want to try to add leads / connectors for checking or balancing Series wired packs.

If you are using larger packs such as 5S 3P or larger there are just to many connectors to be bothered with. Each connector and ssociated length of wire adds resistence to the pack and provides another potentical failure point.

Last on the problem list is the fact that there are no small ,portable, 4.2volt very high amp out put chargers available.
To charge a 3 ,1500 mAh cells in paralle at 1C requires 4.2 volts at 4.5 Amps. . To charge a 5S 3P 6000 pack at would require a 4.2 volts , 30 Ah power source / charger.

Even luc who hase been an advocate of parallel charging ,series dicharging has now stated that with the Thunder Power Smart Balance that he is going to using series charing and using the TP-205 to maintain cell balance.

LLinks to paralle charging/series discharging, Suzanne,Baldwin and TP Balancers.


Charles
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Old Jun 30, 2005, 10:05 AM
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Hello Charles,

Do you happen to know the difference in charge times between charging at 1C (30 amps) and 0.17C (5 amps) as illustrated in your 5S 3P illustration?

It seems like it takes 1.5 - 2.0 hours to charge at 1C, but I have never tried charging at around 0.2C.

Tom
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Old Jun 30, 2005, 10:31 AM
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Quote:
Originally Posted by SilverFoxCPF
Hello Charles,

Do you happen to know the difference in charge times between charging at 1C (30 amps) and 0.17C (5 amps) as illustrated in your 5S 3P illustration?

It seems like it takes 1.5 - 2.0 hours to charge at 1C, but I have never tried charging at around 0.2C.

Tom
Not engraved in stone however.
Most cells with an open voltage of 3.7 when charged at 1C will be between 80 and 85% charged 4.1 volt (open) and 4.2volts under charging 60 minutes. This is the point where most chargers go into the CV stage. At this point the current is reduced and starts to taper. Depending on the IR of the cells,leads,connectors,and the chargers programing it may take between 20 to 60 minutes to complete the charge from and open voltage of 4.1 per cell to 4.20 volts per cell.
Cells / packs charged at lower than 1C will take longer to reach the end of CC / start of CV stage but will take less time in the CV stage. Cells charge from 3.7 to 4.1 volts per cell very linear to the C rate. 1C requiring 1Hour and 0.2C requiring 5 hours.
Most seem to think that it is a major inconvenience to monitor LiPolys being charged for two hours or less. I can only imagine how they would fell about taking 11 hours or longer to charge a couple of larger packs at 0.2C.
I have said for many months that parallel charging would never become popular with the masses and that series charging with balancers would become common.
Questions ,How many balancers have came to market in the past 6 month?
How many chargers / packs designed for parallel charging ,series discharging have came to market?

Consider this. You are flying a $3000 -$5000 electric air plane or heli with a 10S4P LiPoly pack. Would you prefer to have all of the cells tabs directly soldered together and only on set of leads to the ESC or would you rather have two more connectors for each cell (minu the two outputs) .
Case one all connections slodered . case two 38 addational connectors to be concerned with plus you strill have all of the solder joints puls crimped joints or twice as many slodered joints. If uou first build your 4P packs then you only have the the adational connectors required to series these groups however this is still more complicated than the first option.

Charles
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Old Jun 30, 2005, 10:52 AM
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A 3-cell, 2500mah LiPo pack will parallel charge @ 1C on an Astro 109 charger.

If a pack is charged in series until the voltage reaches 4.1 volts/cell or so, then switched to parallel mode, larger packs can be charge-balanced in a reasonable timeframe.

- RD
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Old Jun 30, 2005, 11:09 AM
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BTW, I'm really not interested any more in what "The Masses" will or won't do.

These disclosures are for those who are not necessarily followers of currently fashionable practice. They will balance the pros and cons, and make their own determinations.

This system will not be offered commercially because it would undo three years of momentum in the manufacture and marketing of SMALL PACKS with their complicated, mysterious (therefore highly profitable) electronic solutions to series-charging disorders.

- RD
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Old Jun 30, 2005, 11:39 AM
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Just as some are not interested in what the masses prefer in convince and what the manufactures prefer be it profit motivated or decreased liability driven both sides deserve equal air time.


Charles
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Old Jun 30, 2005, 12:06 PM
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Just as some would give an example that they do not use themselves...


fb
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Old Jun 30, 2005, 12:15 PM
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Quote:
Originally Posted by fly_boy99
Just as some would give an example that they do not use themselves...


fb
I use them a lot. They are much fun to use.

Charles
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Old Jun 30, 2005, 12:30 PM
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Let me refresh your memory:

Consider this. You are flying a $3000 -$5000 electric air plane or heli with a 10S4P LiPoly pack.


fb

Quote:
Originally Posted by everydayflyer
I use them a lot. They are much fun to use.

Charles
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Old Jun 30, 2005, 12:49 PM
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Quote:
Originally Posted by fly_boy99
Let me refresh your memory:

Consider this. You are flying a $3000 -$5000 electric air plane or heli with a 10S4P LiPoly pack.


fb
No I do not, but do use Sermos /APP connectors,have used Suzanne Li-Po Balancers and TP-205 Balancer have used 2S 1P,S2P,3S1P thru 3S5P ,4S 1P ,4S2P . Have have performed over 2000 charge cycles and over 300 of these were balancing/chargeing. All of these with LiPolys. I have also used up to 24 NiCad in series and up to three sets in parallel.I feel I have a fair working knowledge of the complexitites of the connections involved.


Charles
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