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Parallel Batteries?

Old 07-10-2008, 03:46 AM
  #1  
JGMM_BC
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Default Parallel Batteries?

Can anyone explain how to parallel 2 batteries? I have 2 Lipo 3cells 2000mAh 11.1 V that I need to parallel.

Any pictures of such a setup?
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Old 07-10-2008, 04:04 AM
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firemanbill
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you just simply need a connector like this.
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Old 07-10-2008, 04:21 AM
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Angler-Hi
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Parallel simply doubles the mah...right? I always get them confused. Man, I gotta start writing this stuff down
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Old 07-10-2008, 04:24 AM
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Bub Steve
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Originally Posted by Angler-Hi View Post
Parallel simply doubles the mah...right? I always get them confused. Man, I gotta start writing this stuff down
Right you are,,, bub, steve
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Old 07-10-2008, 04:25 AM
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Thanks Steve, bub.
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Old 07-10-2008, 04:42 AM
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Here ya go John.

http://www.dollarrcparts.com/cam-1058b.html
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Old 07-10-2008, 12:55 PM
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JGMM_BC
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Default I see the light...

Thank guys, just what I was looking for.

So can I use a similar harness to join my ESCs?

John

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Old 07-10-2008, 03:05 PM
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Red Scholefield
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NO, this is asking for trouble.
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Old 07-10-2008, 03:21 PM
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Originally Posted by JGMM_BC View Post
Thank guys, just what I was looking for.

So can I use a similar harness to join my ESCs?
Depends what you mean by "join my ESCs". If you want to connect two ESCs to one battery then similar yes, but not the same. You'd need two female connectors and one male.

Steve
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Old 07-10-2008, 03:30 PM
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I highly discourage you from joining any battery packs in parallel or series...

I've done this very thing several times -- mostly for testing power systems on my thrust bench, but sometimes for flight. EVERY time, one of the two batteries has been damaged to a point that they will not hold voltage under a load.

I think what happens is this:
  • You have six cells you are connecting together.
  • Each set of three cells were created at the same time, and are chemicaly equivalent to eachother.
  • When you connect them together, and apply a load, one of the six cells is weaker than the others -- in my experience it is typically the center cell in the weaker pack.
  • That cell falls below 3.0 volts, during high load (full throttle) conditions. The cell slowly dies over time.
  • You end up with a 5-cell battery pack, or a 3-cell and a 2-cell battery pack. :o
Seriously -- you're better off buying a single 3s 4400 mAh battery pack, OR flying with a single 3s 2200 mAh battery pack, and keeping your flights short (less than 10 minutes).
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Old 07-10-2008, 04:19 PM
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I've been flying my P-38 with paralleled 3S 2100 packs for over a year now with no problems. It's not always possible to put in a higher mAh pack due to space contstraints. Lot's of people parallel and series packs with no issues as long as it's done correctly.
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Old 07-10-2008, 04:25 PM
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I've been flying 2 4s 4000 packs in My Waco for well over a year now connected in series.

I have had zero problems doing this.
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Old 07-10-2008, 04:31 PM
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LOL -- okay, so what am I doing wrong? I've killed several (5?) packs, doing this. :o
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Old 07-10-2008, 04:34 PM
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Originally Posted by Lieutenant Loughead View Post
LOL -- okay, so what am I doing wrong? I've killed several (5?) packs, doing this. :o

I'm not sure what you're doing, but what ever it is, I would quit.

OK, I'm sure that was really helpfull. Maybe you can post what your set up is that's toasting the batteries.
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Old 07-10-2008, 04:39 PM
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On either LiPo or Pb batteries, you should have no problems in paralleling them as long as the number of cells in each pack is the same. They do not even have to have the same capacity, i.e. you could safely parallel a 1200 mah pack with a 2400 mah pack as long as each has the same number of cells and are of the same type.
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Old 07-10-2008, 06:07 PM
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Red Scholefield
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Originally Posted by Lieutenant Loughead View Post
LOL -- okay, so what am I doing wrong? I've killed several (5?) packs, doing this. :o
Not Balancing? or using a balancing system that discharges each cell to balance vs. charging them individually to the correct level.

Too high a charge cut off? (faulty charger)

Cheapest battery possible?

Exceeding about 10C discharge (I don't care what hokey rating the vendor has put on them).
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Old 07-10-2008, 09:00 PM
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I'm with Red S. Ultimately, it boils down to:
* Not balancing
* Over-discharging (running too long)
* Over-amping
* Over-charging

If you set your timer to use 80% or less of the total capacity (I shoot for 70% as a safety margin), and that runtime is at least 5 minutes, you should be fine. Just remember to balance every so often.

I run a 6S2P setup in my trainer. Been going great for dozens of flights, few balance issues.
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Old 07-10-2008, 09:34 PM
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Originally Posted by redgiki View Post
Ultimately, it boils down to:
* Not balancing
* Over-discharging (running too long)
* Over-amping
* Over-charging
Guys -- I can assure you that it's none of the above.

* Not balancing --> I balance all my packs with an Astroflight Blinkie. Many of these failed batteries were used only once or twice, on the ground, in thrust bench testing -- they never even got into the air!

* Over-discharging (running too long) --> Again, this is not the case. Since I was using these packs on the ground, in a laboratory environment, I can tell you, without any doubt, that the packs were never over discharged. When I charge, I use my Triton, which tells me the mAh put back into the battery -- I record this in my flight log (if flown) and my thrust bench test spreadsheet (if used during a test).

* Over-amping --> Same as above. Part of the thrust bench testing is to record the maximum amps drawn during the test. The amp draw was well within the normal operating limits of the battery. In the case that I reached burst current thresholds, the system is at full throttle less than 5 seconds anyway.

* Over-charging --> I guess this is a possibility, but why do I have some batteries that are 4 years old (and still going strong)? All my batteries are charged on my Triton charger, or my Polycharge charger (with WattsUp meter installed in-line, so I can monitor the number of mAh put back into the battery). Since I have batteries that are good for 4 years, and all my batteries use the same charger, I don't think the charger is the issue.

The only thing these failed batteries have in common is that they were used in parallel with batteries that have SURVIVED a long time. That tells me the STRONG batteries caused the WEAK batteries to fall below 3.0 volts per cell, when installed in parallel.
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Old 07-10-2008, 11:52 PM
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Originally Posted by Lieutenant Loughead View Post
The only thing these failed batteries have in common is that they were used in parallel with batteries that have SURVIVED a long time. That tells me the STRONG batteries caused the WEAK batteries to fall below 3.0 volts per cell, when installed in parallel.
I'd be interested in your explanation of how two batteries connected in parallel can have different voltages. AFAIK if one of the batteries goes below 3V so does the other one. After all there's only two short fat pieces of wires between the two.....there can't be any voltage drop across that.

BTW I too have used a variety of different batteries in parallel and never had the slightest problems with any of them.

Steve
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Old 07-11-2008, 12:13 AM
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everydayflyer
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The only thing these failed batteries have in common is that they were used in parallel with batteries that have SURVIVED a long time.
Which means they no doubt have high IR. Packs with very high IR should not be paralled with packs with very low IR. In such a case the low IR packs will deliver much more of the amps. / mAh called for during high rate discharges.


Charles
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Old 07-11-2008, 02:24 AM
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Charles -- right on!

Something else I failed to mention earlier is that all batteries that I have connected in parallel were purchased at the same time, from the same vendor -- so I *assume* they were all from the same batch at the factory...

I have never put two LiPos in parallel, and had both batteries survive. I really don't know why.

Slipstick -- I'm talking about CELL voltage, not battery voltage. A single CELL can always drop below 3.0 volts, while the other cells are above 3.0 volts. If you put more cells in the system, you are increasing your chances of finding a "weak" cell. The pack voltage may still show 10.98 volts -- but that doesn't mean each cell is exactly 3.66 volts!

This is the whole reason we balance our cells!
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Old 07-11-2008, 08:31 AM
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Seems odd to me that one person has had 100% failure, while others have had no problems doing the same thing - must be imbalance between the cells in LL's case.

Since connecting individual cells in parallel is perfectly safe, I wonder if it would be a good idea (or even possible) to parallel the balance connectors as well as the main power cables? This would exactly duplicate how purpose-built parallel packs are wired, and should ensure that the cells discharge more evenly.
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Old 07-11-2008, 04:44 PM
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I agree -- it is very strange. Obviously I'm doing SOMETHING wrong... I just can't figure out what it is...

Tell me this -- in what order do you guys connect battery #1, battery #2, and the ESC?
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Old 07-11-2008, 07:47 PM
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Parallel:
I"m no expert on Lipos but from what I've read, Putting two packs in Parallel is ok so long as both packs are balance charged to the same voltage. I know LL is saying that his packs are balanced so I'm not necessarily speaking to him.
If you have 2 packs of different mAh it's still ok so long as each individual pack's C rating can handle the load. Then you know that your ok.
Even so, if you are normally pulling say 20A from a single pack and you put 2 packs in parallel, then your going to pull approximatly 10A from each pack (assuming similar mAhr ratings). In theory, if you have 2 10C packs in parallel, it makes one 20C pack but give yourself some margin here for safety. Each pack should heat up less also.
If you were to have 2 packs in Parallel and one was at a different charge state, that could cause the one of lower charge state to suck up alot of current and overcharge the lower battery once connected (don't think packs have built in diode protection do they?).
I have 1800 and 2100 3s packs I'm planning to parallel to get a longer flight time.
My balancer does a great job of getting me to 4.20 per cell every time.

For parallel:
Good manufacturers combine cells that are balanced to produce the same mAh. If you combine in series cells that don't produce the same mAh, you can have a cell or cells draw down to a lower voltage than the others and the ESC which measures total cell voltage won't see that a cell is too low and may damage it.
If you have say 2 3 cell packs in series, your ESC better be able to handle 6 cell packs or it won't protect the packs from a low discharge voltage.


BTW, I have an inexpensive Hong Kong 4s pack, its been great and each cell discharges after a normal flight to nearly the same voltage levels as each other, but if I slow discharge on my computer balanced charger, it will usually have 2 cells get lower than the others. I try not to drain the pack all the way down to the point the ESC starts to lower my throttle. So far so good. I think the cheap price I paid shows in this case.
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Old 07-11-2008, 09:12 PM
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Originally Posted by Lieutenant Loughead View Post
I agree -- it is very strange. Obviously I'm doing SOMETHING wrong... I just can't figure out what it is...

Tell me this -- in what order do you guys connect battery #1, battery #2, and the ESC?
I first connect the packs together by way of a Dean Y harness and so both packs connect to ESC at the same time. This shoul not make any difference as there is no load when ESC is connected ,well not enough load to matter.

Yes you can parallel the balancing leads as well so that cells remain balanced with their twins.
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