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Old Nov 19, 2003, 04:35 AM
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Parallel charging packs of different capacity

A 3S Kokam 3270 pack was charged in parallel with a 3S Thunder Power 2100. The residual voltages at outset were: Kokam 9.83 and Thunder Power 10.50.

The charger was a Lab. Power supply, with the voltage set at 12.65. The current was set by monitoring the initial current drawn by each cell as the charger current was increased, until one of the cells was drawing its C. The Kokam reached C first, @ 3.27 amps. The Thunder Power was drawing 1.83 amps. (total of 5.00 amps out of the charger). Current was measured using a Sears clamp-on digital ammeter, Voltage was measured using a Radio Shack DVM.

After 1.5 hours, current out of the charger had dropped to 0.40 amps and the charge was terminated. Residual voltages were: Kokam, 12.64 and Thunder Power 12.63.

There does not appear to be any reason why 3 or 4 or more packs of various C could not be charged in parallel. The problem of possible pack imbalance in each of the packs remains, however. It is recommended that a safety module be used on each pack.
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Old Nov 19, 2003, 05:20 AM
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Agreed if you are able to monitor the current flow into the individual packs. The voltage of the packs must be checked *before* you connect them in parallel: if the difference is too big the pack with the higher voltage will dump its charge resulting in a big current flowing.

Bottom line for me: it's ok if you know what your doing and if you abide by the lipo charge rules ...

Thanks for the testing.
Danny
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Old Nov 19, 2003, 06:29 AM
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Hi, Danny -

Actual monitoring of the individual pack currents throughout the charge isn't necessary, because the current remains stable for awhile and then tapers down as the packs approach terminal voltage.

The charger current has merely to be selected at the outset so as not to substantially exceed C for any of the packs.

Re current dumping from a high residual voltage pack to a low one, note that a difference of .7 volts at the outset of this particular test was no problem.

That's probably a representative difference in residual voltages to be found in flown packs.

Also, since current flowing into a pack or cell at a little above C is not particularly troublesome, dumping is not likely to be a problem unless the difference in residual voltages is quite large.

- RD
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Old Nov 19, 2003, 10:03 AM
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Interesting. I wondered how that would work. Theoretically I thought it should, but you proved the concept.
I agree that it shouldn't be done without safety pcb's. also.

Still, I think the safest,surest way to charge lithiums, is single cell charging, with connector adaptors to make the single cells,series or parrallel as you need them.

Sure, you would have needed 6 chargers @ 2 amps each, to do those packs, but if you are a DIYer, that's $60 worth of components.
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Old Nov 19, 2003, 11:18 AM
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uglystick,

I thoroughly agree with you!

We need a charger set at 4.2 volts only. It seems to me such a charger would be relatively simple, foolproof and inexpensive, even if variable amperage between 50 ma and, say, 4 amps were added.

- RD
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Old Nov 23, 2003, 06:16 AM
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Uglystick,

Hereís a charger thatíll do (See post No. 3).

http://www.rcgroups.com/forums/showt...hreadid=171296

Itís a Multi-lipo-in-series automatic cell count charger, but itís hard to see how it could misread single-cell voltage.

It's a bit more than your $60 figure, but it goes to eight amps.
Bigger LiPo cells are starting to come in from China - 8 amps might well be here by spring.

- RD
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Old May 19, 2005, 09:30 AM
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I was hoping to bring this thread back up so that someone can eleborate on the eletrical theory as to why charging different packs in parallel works. Also, some additional questions: Is it important to ensure the two packs are in nearly the same state - half discharged, near fully charged, etc? Do constant current/constant voltage charges operate properly when parallel charging is done? My small brain can't figure out how one pack couldn't hit the 4.2 V limit without the other doing so and causing the first pack to overcharged.

~Dan
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Old May 19, 2005, 09:40 AM
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Quote:
Originally Posted by Ninjak2k
I was hoping to bring this thread back up so that someone can eleborate on the eletrical theory as to why charging different packs in parallel works. Also, some additional questions: Is it important to ensure the two packs are in nearly the same state - half discharged, near fully charged, etc? Do constant current/constant voltage charges operate properly when parallel charging is done?

~Dan
This has been discussed at great lengths several times.
Yes CC/CV chargers work correctly when parallel charing packs of different capacities. CC/CV chargers all the only type normally recomended for Li Polys.

The voltages of the packs should be approximately the same ,I use a maximun difference of 0.1 volts myself. I start with the lowest voltage pack and add others as their voltage is reached. I start charge at 0.3C and increase as necessary to maintain this rate until all packs are added then increase to approximately 1C .

Charles
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Old May 19, 2005, 10:15 AM
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Charles, I've run across several of threads on parallel charging, but the theory behind it was never discussed. If you're aware of a post that does go into that, I'd really like to read it. Searches haven't found it yet. I found graphs showing it being done, but I want to know the why.

Thanks,
Dan
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Old May 19, 2005, 09:59 PM
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Quote:
Originally Posted by Ninjak2k
Charles, I've run across several of threads on parallel charging, but the theory behind it was never discussed. If you're aware of a post that does go into that, I'd really like to read it. Searches haven't found it yet. I found graphs showing it being done, but I want to know the why.

Thanks,
Dan
I am not great with the search function either. Not sure if you mean parallel charging single cells or cell in series. Wnen single cells are charged in parallel then all are at the exact same voltage at the end of charge so they are in perfect balance. When packs of series connected cells are charged in parallel it just saves a lot of charging time. I can charge 5 each 1200 mAh 3S batteries at one time on my Astro Flight 109 which means 5 batteries charged in about the same time as one.


Charles
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Old May 20, 2005, 05:54 AM
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Quote:
Originally Posted by Ninjak2k
... the eletrical theory ... how one pack couldn't hit the 4.2 V limit without the other doing so and causing the first pack to overcharged...
Electrical node theory: All leads connected to a single node (i.e., point) have to be at the same potential (i.e., voltage). All cells (packs) in parallel are connected to the same plus and minus nodes, ergo, no cell can "get ahead" or "fall behind" the voltage of any other.

While there is a voltage gradient within the cells that can vary from cell to cell, this gradient is substantially equalized during the tapering phase of a charge and after discharge termination before the cells are disconnected from each other.

- RD
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Old May 20, 2005, 07:32 AM
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RD Blakeslee : How can You measure current flowing to one and to other paralel conected packs? Do not tell me that You did insert current meter into circuit, as it changes everything by its voltage drop and reading is far from real value..?
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Old May 20, 2005, 07:40 AM
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From the very first post

Current was measured using a Sears clamp-on digital ammeter, Voltage was measured using a Radio Shack DVM.


__________________________


I will add that there are in line shunts which due to the fact that their resistence is only 0.001 ohm do not cause severly inferior data results.
No everyone tries to use a $10 DVM with an 10A inline function to taake such measurements.


Charles
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Old May 20, 2005, 08:05 AM
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Ok, I was blind.. As charge voltage is higher than discharge, it is ok to charge in paralel..
But in a moment of conecting two packs of diferent voltage, i am afraid of high current spike until two voltages adapts - You are sure that it is safe?
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Old May 20, 2005, 08:51 AM
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Quote:
Originally Posted by coro
Ok, I was blind.. As charge voltage is higher than discharge, it is ok to charge in paralel..
But in a moment of conecting two packs of diferent voltage, i am afraid of high current spike until two voltages adapts - You are sure that it is safe?
That is the reason that I only connect packs of approximately the same voltages in parallel. I use 0.2 as the maximum difference myself. Others use less and some use more. Suzanne(user name here) posted a formular to determine the resistor value(s) to use for packs of greatly different volts.
Much is made of this inrush current but I have not seen any real data to support that it presents any problems. Very often a LiPoly battery will have cells which are 0.2 volts out of balance and they get charged at the same rate for all cells. I believe it is best to start a charge slow and alow all of the cells to reach the same nominal voltage before increasing the charge rate. One feature which I really like about the Astro Flight 109 charger is the ability to vary the charge rate at will during the charge. Many other chargers such as the Triton start at a very low rate and only increase the charge rate in small steps which also lets cells normalize.



Charles
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