Batteries & Chargers Discuss Li-P, Li-Ion, NiMh, Nicad battery technology and the chargers that juice 'em up!

Parallel Batteries?

Old 07-21-2008, 09:46 PM
  #26  
rcers
Super Contributor
 
rcers's Avatar
 
Join Date: Jul 2005
Location: Trophy Club TX
Posts: 6,314
Default

I too have great luck with Parallel setups. I really like them as I can use the "smaller" 1800-2100 packs in smaller airplanes and use those same packs in larger planes with parallel connections.

So you get a great deal "more for your money" IMHO since you can use the packs more frequently. I simply do not fly my larger ships as often as the smaller ones.

Works great - been doing it for a couple of years with nary an issue.

Mike
rcers is offline  
Old 07-21-2008, 11:45 PM
  #27  
Bub Steve
Super Contributor
 
Bub Steve's Avatar
 
Join Date: Aug 2006
Location: Shadeville Fl,
Posts: 7,189
Default

Originally Posted by Lieutenant Loughead View Post
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).
""""Each set of three cells were created at the same time, and are chemicaly equivalent to eachother."""" Thats the SECRET!!! I been do'in um like that for a long time but I always use two that I bought together at the same time,, bub steve
Attached Thumbnails Click image for larger version

Name:	petty an red 197.jpg
Views:	89
Size:	143.2 KB
ID:	73360  
Bub Steve is offline  
Old 07-22-2008, 12:01 AM
  #28  
coro
Member
 
Join Date: Oct 2005
Location: Slovakia
Posts: 73
Default

I am just curious how "killing" parallel harness looks like.
Is it possible that resistance of connection of one pack, is much worse (longer path for example) than second pack? In such case the better connection causes that most of the load is routed to one of the packs only..

You should stop thinking about harmful voltage if packs are connected in parallel, as they have exactly equal voltage (so would die both or none if voltage is the reason).
coro is offline  
Old 07-22-2008, 01:58 AM
  #29  
darylm44
Member
 
darylm44's Avatar
 
Join Date: Feb 2007
Location: Wasilla, AK
Posts: 643
Default

I have a 25 size E-flite Cub and run 2 3s 2100 in parallel. I discharge to about 50% and balance charge each battery separately with a Cellpro 4. I have used this set up for over a year now and never had a problem.
darylm44 is offline  
Old 07-23-2008, 04:28 PM
  #30  
mred
Super Contributor
 
mred's Avatar
 
Join Date: Jan 2008
Location: Glenwood, GA
Posts: 1,025
Default

Originally Posted by Rodneh View Post
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.
I have a question about this. You say that the packs don't need to be the same capacity. So if I have a 2000mAh pack and a 4000mAh pack and I am drawing 20 amps total from my motor, each pack is putting out 10 amps right?? Now then, if you have one pack that is twice the capacity of the other, will not the small pack run down faster then the other pack does
Ed
mred is offline  
Old 07-23-2008, 04:38 PM
  #31  
dditch66
Member
 
Join Date: Jun 2008
Posts: 161
Default

I think It should draw more current from the larger battery.
If it were the case that it drew 10 amps from each, then the 2000mAh pack would drop in voltage faster. If the 2000mAh pack voltage were lower than the 4000mAh pack then it would start drawing more from the 4000mAh pack. That can't be the case. So no way it would pull 10A from each.

Since they are in parallel, we know the voltages MUST be equal. Also the sum of the current draw from each pack MUST be equal to the total current draw (Kirchoff's Law). THUS, you'd be pulling likely 2/3 of the current draw from the 4000 battery and 1/3 from the 2000 pack. No battery pack actual mAh capacity is EXACTLY what they state on the label so you'd have some tolerance error in that estimated calculation I give.
dditch66 is offline  
Old 07-23-2008, 07:56 PM
  #32  
clockworks
Member
 
Join Date: Dec 2007
Location: Cornwall, UK
Posts: 286
Default

Wouldn't the current drawn from each pack depend on the relative internal resistance of the packs, rather than the capacity of the packs? During light use (gliding/cruising) the packs would self-balance, but what would happen at WOT?
I'd also be worried about exceeding the "C" rating of the smaller pack.
clockworks is offline  
Old 07-23-2008, 08:06 PM
  #33  
dditch66
Member
 
Join Date: Jun 2008
Posts: 161
Default

Originally Posted by clockworks View Post
Wouldn't the current drawn from each pack depend on the relative internal resistance of the packs, rather than the capacity of the packs? During light use (gliding/cruising) the packs would self-balance, but what would happen at WOT?
I'd also be worried about exceeding the "C" rating of the smaller pack.
You know that would have to be correct. The more current you draw, the more the voltage drops. lower C rated packs typically drop more voltage I believe, then it would pull more from the higher C rated pack to compensate. Yet another reason why you could get away w/ using 2 batteries that are not the same C rating nor mAH.
This would make for an interesting experiment if you put a current probe on to each pack. You'd have to measure the current draw of each pack at 1/4,1/2, 3/4 and full throttle.
dditch66 is offline  
Old 07-23-2008, 10:47 PM
  #34  
mred
Super Contributor
 
mred's Avatar
 
Join Date: Jan 2008
Location: Glenwood, GA
Posts: 1,025
Default

Originally Posted by dditch66 View Post
I think It should draw more current from the larger battery.
If it were the case that it drew 10 amps from each, then the 2000mAh pack would drop in voltage faster. If the 2000mAh pack voltage were lower than the 4000mAh pack then it would start drawing more from the 4000mAh pack. That can't be the case. So no way it would pull 10A from each.

Since they are in parallel, we know the voltages MUST be equal. Also the sum of the current draw from each pack MUST be equal to the total current draw (Kirchoff's Law). THUS, you'd be pulling likely 2/3 of the current draw from the 4000 battery and 1/3 from the 2000 pack. No battery pack actual mAh capacity is EXACTLY what they state on the label so you'd have some tolerance error in that estimated calculation I give.
I would have answered sooner, but supper got in the way. Sorry bout that.

I don't think you took Ohms law into consideration. On two paths of current the current will divide between the two paths according to the resistance of the two paths. In other words, if both packs are the same internal resistance the current will divide equally between the two.

I just ran some test so I can give you some numbers so you won't think I am full of what ever. I had a 4000mAh 3 cell and a 2200mAh 3 cell and the total amp draw was 24.65 amps. The amp draw from the 4000mAh battery was 13.3 and the 2200mAh battery was 10.2 amps. The difference in amp draw was the difference in the internal resistance of the two batteries because they were not from the same manufacture. All being equal, both of the paths would have the same amp reading and the smaller battery would run flat before the bigger battery would. That is why you never put batteries in parallel unless they are the same capacity. You also don't want to put them in series for the same reason. You must put all batteries of the same capacity together no matter what you are doing. You don't have to put the same number of cells together for a series connection, but they have to be the same capacity.
Ed
mred is offline  
Old 07-24-2008, 03:22 AM
  #35  
dditch66
Member
 
Join Date: Jun 2008
Posts: 161
Default

Mrad,

I'm very familiar w/ Ohms law applying it to a load but I've never looked at it from the battery side.

Did you run that test only at full charge?

Your running those packs at like 5C or so. What were the C ratings of those packs?

Also, I predict the current draw ratio from one pack to the other pack would change as you :
A) discharge the batteries
B) draw different loads

If the smaller battery is going to go flat sooner, how will that happen if the battery voltages are in parrallel and the voltages would be the same during the discharge?
dditch66 is offline  
Old 07-24-2008, 01:42 PM
  #36  
mred
Super Contributor
 
mred's Avatar
 
Join Date: Jan 2008
Location: Glenwood, GA
Posts: 1,025
Default

Yes the batteries were at full load and I just took them off the chargers, so that both had fresh full charges. I think what you are doing is forgetting that batteries are not DC power supplies. With a power supply, you will have good voltage all the time and a constant current, but with a battery, you have a limited source of power supply.

In this case, the batteries are the resistors and they also power the load. Now if both were the same internal resistance, they would both have the same current flow across them and would run the system until one or the other gave out. When that happens the high battery would try and feed the low battery along with the load and sooner rather then later you are going to have a major problem with the smaller battery. I could run the system to destruction to prove this, but I am not in the mood to burn up a good battery. I'm also to cheap, but if you don't believe me, feel free to do the test any time you feel like it. I think you will find that your small battery will not be good for much after you finish the test. Take care now.

Ed
mred is offline  
Old 07-24-2008, 03:17 PM
  #37  
slipstick
Super Contributor
 
Join Date: Oct 2005
Location: N.Staffs, UK
Posts: 2,350
Default

Nice theory Ed, but unfortunately not in accord with reality. What you're forgetting is that the internal resistance of a pack is not a fixed value (i.e. it's nothing like a resistor) but varies with the charge state. With a single battery that's precisely why the voltage sags as the capacity is used up. What that typically means in a parallel combination is that the resistance of the weaker battery increases faster that that of the stronger battery as its charge level goes down. Therefore the current provided by the weaker battery also reduces.

Since the two batteries are discharging together to the same voltage it is never the case that "the high battery would try to feed the low battery as well as the load". What actually happens is that the battery with the greater capacity remaining feeds a greater proportion of the current demand until such time as the total current capability of the two batteries is less than the current needed. And as if by magic both batteries will have run out of charge at the same time .

BTW because this subject keeps coming up and some of the answers don't seem very logical at first sight I have done the tests (including mismatched packs, IIRC 2100mAh 20C + 500mAh 15C in parallel). I have never seen a negative current flow (i.e. one battery charging the other instead of both discharging into the load) and I haven't yet damaged a battery either.

Perhaps I'm just lucky .

Steve
slipstick is offline  
Old 07-24-2008, 03:36 PM
  #38  
dditch66
Member
 
Join Date: Jun 2008
Posts: 161
Default

Slipstick beat me to the punch. I think you said it better than I was able to.

I have some batteries I am preparing for a test and I'll report my findings. I plan to measure the current flow at 4 stages in the discharge. I'll measure the cells of each along w/ the total voltage and report the current draw at each stage. I'll even record the current draw while I lower the throttle to prove if the packs are trying to charge each other or not (what I think your concern maybe is Mred)

Batteries have a discharge curve. Each technology is different but they all I believe do it (at least Lead Acid, alkalline, NiCd. NiMh, LiPo do). They can be charged to a max potential but as you discharge them, the voltage goes down. Eventually at a given load, the voltage will drop like a rock and trouble starts. That's why good ESC's stop that from happening. With the packs in parallel, the voltage of each pack WILL be the same at all times.

Here is a table of "generic" battery capacity for a LiPo Vs. Cell potential that someone posted here once:

4.2 100%
4.17 98%
4.13 95%
4.1 92%
4.07 89%
4.03 86%
4.00 82%
3.96 78%
3.93 74%
3.9 69%
3.87 64%
3.83 59%
3.8 53%
3.77 47%
3.73 36%
3.7 22%
3.66 12%

If this chart is true, it's a pretty linear drop of voltage vs. mAhr draw until you get to about 50% then it about doubles.
dditch66 is offline  
Old 07-25-2008, 02:28 AM
  #39  
dditch66
Member
 
Join Date: Jun 2008
Posts: 161
Default

I was able to run an experiment tonight.
I'm not completely satisfied w/ how I ran it. I wish I had a volt meter always connected to monitor the voltage and 2 current probes but alas, I didn't have the equipment, but here is a description of what I did:

When measuring current, I let the motor run for at least 10 seconds before making a measurement.

Experiment 1:
2100mAh 20C pack Deans Plug
1500mAh 25C pack JST plug to Deans plug converter.
Deans Y splitter

Both packs balance charged and were 12.56V using my meter.
Initial current draw:
2100/20C : 22A
1500/25C : 13.8A

After 2 minutes of run time at WOT
2100/20C : 21A
1500/25C : 12.5A

2 more minutes of run time at WOT
2100/20C : 16.8A
1500/25C : 14.7A

At this point, the ESC started to throttle limit to I could not make good measurements so I stopped.
At the end, while at idle, I measured the following:
2100/20C : -0.5A
1500/25C : +0.5A


Experiment 2
1800mAh 20C pack Deans Plug
1500mAh 10C pack JST plug to Deans plug converter.
Both balanced at 12.56V

Initial current draw
1800/20C : 23.3A
1500/10C : 13.6A

1 minute or run time
1800/20C : 23.0A
1500/10C : 12.7A
parallel voltage = 11.79V

1 more minute run time
1800/20C : 22.0A
1500/10C : 11.0A

1 more minute run time
1800/20C : 21.0A
1500/10C : 11.2A

1 more minute of run time
1800/20C : 19.0A
1500/10C : 13.0A

Idle current measurement:
1800/20C : -2.1A
1500/10C : +2.1A

At the end of this test, I measured the voltage of each pack:
1800/20C : 10.67V
1500/10C : 10.88V
(this is why the 1500/10C was charging the 1800/20C)


My conclusions:
We didn't think about how the battery pack voltage potential will decrease under load. 2 Packs that are different will drop different voltage amounts based on the current draw. The fact that the packs once pulled at the end of the test were different voltages shows this effect.

When the motor is idle, one pack DOES begin to charge the other pack!
Would this effect have diminished if I ran the motor not at WOT the whole time? Maybe.

I'm wondering if the extra wire for the JST to Deans adaptor dropped enough voltage to mess w/ the current draw and battery loads?
I'm hoping to try this again using a combination of more similar batteries (2100 parallel w/ the 1800 for example)
dditch66 is offline  
Old 07-25-2008, 11:02 AM
  #40  
slipstick
Super Contributor
 
Join Date: Oct 2005
Location: N.Staffs, UK
Posts: 2,350
Default

Yes sorry, I should have said "I have never seen a negative current flow UNDER LOAD".

Interesting to see some more testing. BTW when you say your measurements were "at idle" do you mean completely stopped or running at a low (idle) speed ? You're showing a net current flow to the load of exactly zero and I've never seen a motor that can idle slowly with no current at all so I'm guessing completely stopped . Of course with no load at all you're showing that the effective recovery voltage of the 2 packs can be different, which is certainly true. But fortunately at that point there's no real current loading so no practical problem.

Steve
slipstick is offline  
Old 07-25-2008, 11:49 AM
  #41  
ministeve2003
Class Clown
 
ministeve2003's Avatar
 
Join Date: Sep 2007
Location: Montclair, California
Posts: 4,472
Default

every time I've put two packs togeather I've burned out an esc, but thats prob because I pull to many amps....lol
SK
ministeve2003 is offline  
Old 07-25-2008, 01:29 PM
  #42  
dditch66
Member
 
Join Date: Jun 2008
Posts: 161
Default

When I say Idle, I mean stopped, yes.

BTW, the current draw when I last ran just the 2100/20C by itself I recall was 26A.
With the parallel batteries, they were able to offer more current than before. Maybe that's why you burnt up the ESC ministeve?

I really like my dualsky ESC. It is rated for 60A continuous and will gently throttle back the motor as the pack gets down to its last 1/3 of capacity. I have a cheaper one that just dies for like 10 seconds (as I dive down to the runway to land).
dditch66 is offline  
Old 07-26-2008, 05:29 AM
  #43  
john339
Member
 
Join Date: Mar 2008
Posts: 65
Default

Interesting reading.
I can add that I use 4 3s2100 batteries in my chopper.
Two sets of two in parallel and then the two sets in series to get my 22 volts.
I bought these batteries at the same time and have only used them in this configuration since new.
After each days flying I balance charge all 4 and then hook em up again in the two sets of two and I am ready for another day.
I have a flying buddy that tried the same thing.
He forgot and didn't balance charge all four and ended up with 2 balanced and charged and two that were discharged. He hooked them up at the park and took off.
He had a fire start and it ruined all four batteries and his chopper.
The opinion was the discharged batteries sucked up the amps and so did his motor and too much heat caused the fire.
We really don't know what caused it but that seems plausible to me.
john339 is offline  
Old 07-26-2008, 07:27 PM
  #44  
Rodneh
Member
 
Join Date: Oct 2005
Location: Florida, USA
Posts: 753
Default

Slipstick in post 37 gave an excellent answer, 100% correct and in easy to understand words. Good job slipstick!!
Rodneh is offline  
Old 07-26-2008, 07:44 PM
  #45  
Moxus
Gremlin
 
Join Date: Jun 2008
Posts: 641
Default

lipo contruction:

one "cell" isnt really one cell.
inside what you guys call a "cell", is several laminated plates of "battery stuff" and aluminum foil wich acts as current rail. each plate is a cell of its own, within the battery cell. paralelling simply doubles the number of "cells within a cell", electrotechnically.
what you need to do to get a parallell pack that acts EXACTLY the same as a bigger pack is to parallell the balance plugs also.
if you paralell BOTH the main current output AND the balance plug, you wont have any problems at all with weak cells, any more than you have with each of the packs on its own. i suspect that leutenants problem was that he didnt use a balance plug, so the weaker middle cell of one pack was allowed to discharge too mcuh, but the capacity left on the stronger pack made it hard to tell that the weak pack was running dry.
this wont happend with a balance plug in parallell, as all cells will have exactly the same voltage as its twin cell in the other pack.
Moxus is offline  
Old 07-26-2008, 09:58 PM
  #46  
Red Scholefield
Batteries/Chargers
 
Red Scholefield's Avatar
 
Join Date: Jul 2005
Location: Newberry FL
Posts: 726
Default

Originally Posted by slipstick View Post
Nice theory Ed, but unfortunately not in accord with reality. What you're forgetting is that the internal resistance of a pack is not a fixed value (i.e. it's nothing like a resistor) but varies with the charge state. With a single battery that's precisely why the voltage sags as the capacity is used up. What that typically means in a parallel combination is that the resistance of the weaker battery increases faster that that of the stronger battery as its charge level goes down. Therefore the current provided by the weaker battery also reduces.

Since the two batteries are discharging together to the same voltage it is never the case that "the high battery would try to feed the low battery as well as the load". What actually happens is that the battery with the greater capacity remaining feeds a greater proportion of the current demand until such time as the total current capability of the two batteries is less than the current needed. And as if by magic both batteries will have run out of charge at the same time .

BTW because this subject keeps coming up and some of the answers don't seem very logical at first sight I have done the tests (including mismatched packs, IIRC 2100mAh 20C + 500mAh 15C in parallel). I have never seen a negative current flow (i.e. one battery charging the other instead of both discharging into the load) and I haven't yet damaged a battery either.

Perhaps I'm just lucky .

Steve
Great answer Steve, but it seems that there are still many out there that believe water flows uphill and one pack will charge another of an equal number of cells.
Red Scholefield is offline  
Old 10-01-2008, 07:14 AM
  #47  
jimweda
Member
 
Join Date: Jul 2007
Posts: 79
Default

Obviously you're doing something wrong since putting packs in series or parallel are very common and you're the first person I've ever heard that had problems aside from putting unlike packs together or making mistakes while charging. You'd better check your set up and see what's going wrong. I've done both without any issues. The only thing I like to keep a common factor is that I start out with fresh packs of the same in manufacturer, "S" count and mAh. I'm just a little more particular about how I treat my batteries and have about 10 that are over a year old and still hold 12.6 off the charger. You might want to stop scaring people who are new to using multiple packs until you figure out what you're doing wrong.

Jim

Originally Posted by Lieutenant Loughead View Post
LOL -- okay, so what am I doing wrong? I've killed several (5?) packs, doing this. :o
jimweda is offline  
Old 10-01-2008, 07:24 AM
  #48  
ministeve2003
Class Clown
 
ministeve2003's Avatar
 
Join Date: Sep 2007
Location: Montclair, California
Posts: 4,472
Default

Originally Posted by Red Scholefield View Post
Great answer Steve, but it seems that there are still many out there that believe water flows uphill and one pack will charge another of an equal number of cells.
hmm... wonder if resistance of one pack was lower, if it would act like water trying to maintain equilibrium... maybe it would charge the other pack to some extent....

SK

PS, those little red Jst plugs Cant output the higher voltage so If your trying some test you'll need to remove those and put deans on both... otherwise it would limit the flow of electricity...

SK
ministeve2003 is offline  
Old 11-04-2008, 10:23 AM
  #49  
BEX
Member
 
BEX's Avatar
 
Join Date: Oct 2008
Location: South Africa , Pofadder
Posts: 113
Default

Very informative thread, a lot has been discussed but I have more questions.
I need a 6S 4000 mAh pack. The cost of buying a new one is too much but i have 2 x 3S 2200 mAh 25C packs and buying 2 more is reachable.
So I can make up a 4400mAh 6S2P pack, perfect for my need.

The 2 existing packs has done about 10 fligths each and have always been charged with a balancer and never abused.

The 2 new Lipos will be of a different make as I cannot get any more of the APLUS.

According to the info given in this thread when you put 2 packs in series they must have the same capacity but when you use them in paralel it is not that important.

Now i have 2 possible ways to connect the pack ,
1. use one new and one old pack in paralel to make a 3S2P pack , and connect 2 of these in series.


2. connect 2 old packs in series to make 6S1P pack and connect this in paralel with the new packs that are also made into a 6S1P.



My thinking is that option 1. gives an "balance point" between the 2 paralel packs and should work better than option one , but I would really like to hear what you guys say about that.
BEX is offline  
Old 11-04-2008, 01:00 PM
  #50  
dditch66
Member
 
Join Date: Jun 2008
Posts: 161
Default

option 1 does seem better than 2.
I know a couple heli folks that are wiring them like option 2.
They are using the sam brand/spec packs on all 4.
I'd say just make sure your cut off voltage is not set too low and keep your wire leads short and your junctions low resistance.
dditch66 is offline  

Quick Reply: Parallel Batteries?


Contact Us Archive Advertising Cookie Policy Privacy Statement Terms of Service

Copyright 2018 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.

Page generated in 0.12300 seconds with 14 queries