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JR-Spektrum . Gyro Hobbies . E-flite

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e-Electric Batteries & Chargers > Li Poly Batterys: Series AND Parallel???
 
 
Marty
Senior Heliman
Location: Australia

I am running 2 cell Kokam 1020mAH batts. They are connected in series to achieve the required voltage for the system (ie. 3.7V x2 =7.4V).
My Wes Tech charger is rated to charge three cells at a time, maximum.
Can an extra cell be added IN PARALLEL to the existing series setup to keep the same voltage but up the available amperage for longer flight times? Or if you are connecting series and parallel do you always need the same no. of cells hooked up together ie. 2 series with 2 in parallel?
Three in this series/parallel setup would be nice and give me that little extra flight time and still hold the voltage at the current level of 7.2V which is inside the ratings of the ESC/Motor.
Thanks for your advice.
Cheers.
06-19-2003 Over year old.
 
 
d_wheel
Senior Heliman
Location: Deep in the heart of Texas.

Marty,

If I understand you correctly, you are asking if it is OK to add 1 cell in parallel to the two you already have. If that's the case, the answer is NO. According to the folks who use LiPol packs, it is ok to add 2 cells in parallel with the two you already have, for a total of 4 cells (2S2P - meaning 2 Series 2 Parallel). Your charger will be set for 2 cells, and if possible, use a 2 amp charge rate. Make sure both sets of cells are at the same charge state before attaching the charger for the initial charge. This can be done in several ways. The best way would be to charge both sets and then connect them in parallel when fully charged. I believe the 3 cell maximum your charger is referring to is the number of cells it can handle in series, not the actual number of cells total.

If you connect your extra cell in parallel with one of the existing cells, your other cell will be in danger of receiving a reverse charge. The single cell will run down first because it is only capable of delivering half the current of the parallel set. When this happens, the parallel cells will conduct current through the depleted cell which will in effect apply a reverse charge. Also, the pack probably would not charge correctly. The parallel set would most likely receive less than a full charge and/or the single cell could be over charged.

Later;

D.W.
06-19-2003 Over year old.
 
 
stevem
Senior Heliman
Location: Oklahoma

DW is correct. You would have to parallel exact number of cells. You can make another 2 cell series pack and connect + to + and - to - to acheive the 2S2P pack. Seperate the 2 cell packs for charging though. The jury is still out on parallel charging lipo cells and I wouold hate for you to ruin some cells finding out that the two packs were not well balanced.

Optionally, you can get higher capacity cells that probably won't weigh as much as 4 lower capacity cells.

The E-Tech cells, at 1200mah will probably put out more than the extra 180ma that the cells are rated because the e-techs will deliver more of their capacity under a load. Just another option.

Good luck with it and let us know how it goes!

Steve
06-19-2003 Over year old.
 
 
d_wheel
Senior Heliman
Location: Deep in the heart of Texas.

The jury is still out on parallel charging

Hello Steve,

Just when I was starting to think that LiPols might be the exception to the rule about not parallel charging batteries!!! Have you had or heard of someone having a bad experience with this type of charging? I have always been skeptical about doing so, but this is the first negative I have seen so would like to get more info on it if possible.

Later;

D.W.
06-19-2003 Over year old.
 
 
Marty
Senior Heliman
Location: Australia

Thanks for all the input gentlemen.
"I believe the 3 cell maximum your charger is referring to is the number of cells it can handle in series, not the actual number of cells total."
Do I understand correctly that for example if 2 single cells are connected in parallel, the charger only "sees" it as one cell because the voltage is still 3.7V, the only difference being that it will take more time/current to charge them??
And also, parallel charging of LiPols in ANY format can be damaging to individual cells, thus meaning that you should only charge in series and connect parallel later? The only reason why I ask this second question again is because I have seen commercially constucted 2s2p cells and therefore charging these would be seem damaging, thus how can they market these?
Thanks again.
06-19-2003 Over year old.
 
 
d_wheel
Senior Heliman
Location: Deep in the heart of Texas.

Hello again Marty,

I am not familiar with your charger so can't say for sure how yours is set up. On my chargers, I have to choose a few parameters before starting a charge. The two most important are voltage and current. One charger has jumpers so I select the number of cells in series and plug in the jumper accordingly. The other jumper is for current, which I set according to the current rating of my pack. My other charger requires that I actually set the voltage and current on the LCD screen, but it will handle more current so will come in handy when I get a large pack for my heli.

And yes, the charger sees two cells connected in parallel as one cell at a higher current rating.

As for charging cells in parallel, this has always been a huge NO-NO! However, I have read message after message after message that with LiPol packs, it is OK to do so. I hope this is the case because large parallel packs would take quite a while to charge if it were necessary to separate the individual packs..

Later;

D.W.
06-20-2003 Over year old.
 
 
donlynn
Key Veteran
Location: New Zealand

Gidday

I have micro E' heli mates who use 2s2p Li Po and have charged them in this configuration several times a flying session for over 6 months, I would guess you would be wise to do this from the begiining of the cells life rather than a pre used mix and match of different cycle styles.

Regards Don
06-20-2003 Over year old.
 
 
Marty
Senior Heliman
Location: Australia

Thanks Don, interesting. How's things in NZ? Helen Clark still kicking? Boy what a mission for a dentist
06-20-2003 Over year old.
 
 
Frebaln
Heliman
Location: Nor Cal

Trying to build up a lipo pack, but need some suggestions please. I have 4ea Hicell 3.2v 900mAh lipos. Im planning on running them in my GWS A-10, I have done seperates in my hummingbird and it worked fine so I thought about similar concept for the A-10. Wasn't sure which would be a better combo, I have a hobbico mkII charger.

Option 1, serial 2 cells @ 7.4v 900 mAh, GWS 100ESC-per motor

Option 2, parralel 4 cells @ 7.4v 1800mAh, GWS 300ESC-both motors

What would be the easiest way to build and charge, also how do I exactly build them up ???

Thanks in advance
10-28-2003 Over year old.
 
 
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e-Electric Batteries & Chargers > Li Poly Batterys: Series AND Parallel???
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