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Using Smaller LiPo Packs To Make Big Packs

Old 12-15-2007, 11:27 AM
  #1  
Sam_K
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Question Using Smaller LiPo Packs To Make Big Packs

I'm kinda just day dreaming here but I had an idea and I'd like to know if this is a silly idea or not?

The basic idea is to buy several small capacity LiPos, all the same brand and same capacity, and then combine these in various combinations of parallel and series in order to make up whatever pack you need for whatever model you want to fly.

For example let's say I bought four 2S 1200mAh packs and four 3S 1200mAh packs. Using these packs I could make any of the following.


2S x2 in parallel = 2S 2400mAh
3S x3 in parallel = 3S 3600mAh
2S + 2S in series = 4S 1200mAh
( 2S + 2S in series ) x2 in parallel = 4S 2400mAh
( 2S + 3S in series ) x4 in parallel = 5S 4800mAh
(3S + 3S in series) x2 in parallel = 6S 2400mAh

Plus many more options, I'm sure you get the idea.

I guess the first and obvious comparison to make is the comparative price and weight of these combined packs versus buying a single large pack. I'll use FlightPower EVO-25 LiPos as an example and take US dollar prices and pack weights from their official website.

A 2S 1200mAh FlightPower EVO-25 LiPo is $32.99 and weighs 73 grams, this gives us 72.7mAh per dollar per cell and 32.8mAh per gram per cell.
A 3S 1200mAh FlightPower EVO-25 LiPo is $49.99 and weighs 105 grams which gives 72.0mAh per dollar per cell and 34.28mAh per gram per cell.

Combining these packs 4 parallel sets of 2S and 3S in series gives us a 5S 4800mAh pack which cost $331.92 and weighs 712g. I will pause here for a moment to acknowledge that this will require a wiring harness to connect them all up and I don't know how much all the wire and deans plugs would cost and weigh. Ignoring that for a moment, this works out as 72.3mAh per dollar per cell and 33.7mAh per gram per cell.

The nearest Flight Power pack to the pack described above would be the 5S 4900mAh pack which costs $294.99 and weighs 607g. This works out to 84.0mAh per dollar per cell and 40.36mAh per gram per cell. OK so buying the dedicated 5S pack is cheaper and lighter, I knew this to begin with, but our combined pack is more versatile. Combining packs has given us a 16% increase in cost and a 20% increase in weight. I guess that is quite a bit of extra weight, but then again is the kind of model that is going to use a 5S 4800mAh pack really going to feel that extra 105g?

I guess a big challenge is going to be charging. I could make a custom balance connector lead so that packs connected in series could be charged together. The packs all have the same size and make of cell so that shouldn't be a problem. But what about when there are packs in parallel? I don't know how I would balance charge cells that are in parallel. I would guess that the packs would have to be separated and charged individually.

The balancer I currently use supports up to 6 cells, so I guess any combined pack with more than 6 cells in it is going to have to be split up to be charged. So the 5S4P (20 cells!) combined pack described above would take 4 times longer to charge than the dedicated 5S1P pack as I would have to do 4 separate charges to get them all topped up (assuming I charge a 2S and 3S together in series on each charge).

Now, all of this so far sounds like a lot of hassle BUT the big clincher comes in the following scenario. Let's say I have several models I've been flying for a while now that are all 2S and 3S power systems. I have a bunch of batteries that I use to fly them, I like having several of each to minimise charging at the field. Now lets say that for my next model I may be looking at my first really big plane and the power system would need a 5S battery. Suddenly, combining my existing batteries doesn't seem like such a silly idea because I can have a 5S battery for no more than the cost of a few deans plugs and some wire. An extra 100g of weight (+wire and connectors) seems a small price to pay to save $300 on buy a new 5S battery. Also, if a cell goes bad in my combined pack it would be a lot cheaper to replace than if a cell went bad in the 5S1P pack.

What I would really like to know from you guys though is if there are any hidden engineering gotchas in combining lot's of LiPos like this. Would the discharge characteristics of the combined pack be markedly different than the one big pack?
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Old 12-15-2007, 12:55 PM
  #2  
jb48
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I think your idea makes sense IF YOU ALREADY OWN the numerous pack, and for any reason you are not ready to buy big packs. I am in a similar situation than your and did about the same thing. For one plane I have two 2S 2500 mAh 20C Polyquest and for another plane I have two 3S 2500 mAh 20C Polyquest. Last Winter, Spring and Summer I scratchbuilt my bigger airplane to date (8 pounds) and combined, serie and parallel, all them to make me a 5S2P 5000 mAh. 29 oz of batteries (23 oz) and 6 oz of wires and connectors (Zero Loss). It`s heavy and a nightmare to charge with my old version Triton 2 and LBA6 Hyperion only balancer! It works but now I cannot fly my other planes on the same day that I fly my «big» one. Now, if I can save enough for buying two one-pack big lipos...
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Old 12-15-2007, 01:18 PM
  #3  
coro
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I have recently written a lot about series combining of separately used packs.
http://www.wattflyer.com/forums/showthread.php?t=27752

They are probably used differently, in different models/power setups (e.g. 2s and 3s). So they become a bit different.

In addition to what have been said: Series charge requires also more POWER from the charger - they are limited by power, not only by maximum current.
e.g. 5s pack will reach 4.2V*5=21 volts. If Your charger is ordinary small one, with maximum capability of 50Watts, it means that highest current it is capable of charging 5s pack is 50Watts / 21 Volts = 2.4Amps
For You, using 1200mAh packs, charging at 1C, it is sufficient even for 5s2p. However in general, it may become limiting rule.
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Old 12-15-2007, 03:03 PM
  #4  
Sam_K
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Thanks guys that's some good feedback!

I understand what you're saying about using all your batteries at once jb48.

My charger is a 50W charger, so charging big packs like that is going to be very slow now matter how I do it. It would be no different if I was using a single large pack though, I just need a better charger if I'm going to be using that kind of power.

I have since read elsewhere that you can charge cells in parallel as if they were 1 cell. So provided you have a charger than can pump out enough juice it should be possible to charge the combined 5S4P pack in one hit.
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Old 12-15-2007, 09:13 PM
  #5  
whitecrest
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Default Combining lipo packs.

I made up a special wiring harness using JST connectors so I could connect two 800ma 2S Lipo in series. I had started using 2S lipos but found that performance for this model was much better with at least 3S or greater, especially after switching the motor from 1600 to 1000 kv.

It is convenient to connect the two cells at the field via the harness, and I charge them separately. Reprogramming the ESC for 4S only takes a few seconds. Performance on 16 volts has been very good, and I can continue to use my six 2S lipos. Also, the model handles a bit better in windy conditions with the weight of the additional cell.
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Old 01-21-2009, 04:25 PM
  #6  
benny
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Default i need help

hi i'm a new user of this forum
i'm french so xcuse me if my english is sometimes approximative

my question does not directly concerned modelisme
but i found here a part of the answers i'm looking for
i would like to make a 3.7v lipo pack with a 11.1v 1800mah
i have from my belt cp
the pack is made of 3 cells of 600mah each (i guess) i've not unsoldered the cells for the moment
i want to plug them in parallel to get a higher capacity pack with 3.7 v tension
to use it with a small gps tracking device
this device already have a lipo pack but 2100mah made of 1 cell
with a 3 wires plug ( black yellow red ) like a phone battery
what i need to know is if i could use a big pack instead the one i allready use?
what the difference between a 3cells pack plugged in parallel and a 1 S pack (3.7v in the 2 cases)
is the diameter of wire important ?
the battery of my tracker is plugged with 3 thin wires 1-2 mm
and my belt pack is wired with big silicone wires 5mm
thank you
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Old 01-21-2009, 05:41 PM
  #7  
slipstick
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An 11.1V 1800mAh pack contains three 1800mAh 3.7V cells wired in series. If you take the pack apart and rewire the three cells in parallel you will have a 1S (3.7V) 5400mAh pack.

The wires only have to be large enough to deliver whatever current the device plugged into them needs. So they don't need to be any bigger than the existing thin "tracker" wires.

Steve
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Old 01-21-2009, 09:00 PM
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benny
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thank you very much steve
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