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Old Jun 28, 2012, 11:55 AM
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Different Lipos in Parallel and Series

Hey guys, I have a quick question...

Can I wire 2 different 3s lipos in a Parallel or series circuits and fly with them?
Would the only problem be the packs not discharging equaly? I have a 2200mah and a 1600mah pack.

Thanks guys,
Nighthawk
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Old Jun 28, 2012, 12:26 PM
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The parallel arrangement should work; the series arrangement needs cells to be of similar capacities. In a series circuit, the current through each cell is equal, therefore each cell would lose capacity at the same rate, the 1600mAh cells being empty first.
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Old Jun 28, 2012, 03:23 PM
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Personally I would not do it.

Parallel the ESC would see the highest voltage and use that as a cutoff. That would make it VERY easy to over discharge the 1600 pack.

Series the 1600 pack would work harder that the 2200 and maybe over heat. You might get away with this because the cutoff voltage would be based on a 6s battery. The smaller battery would reach cutoff quicker but since the bigger pack would be well above cutoff you still might overdischarge the little pack.

Mike
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Old Jun 28, 2012, 03:25 PM
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Quote:
Originally Posted by AZcrashpro View Post
Personally I would not do it.

Parallel the ESC would see the highest voltage and use that as a cutoff. That would make it VERY easy to over discharge the 1600 pack.

Series the 1600 pack would work harder that the 2200 and maybe over heat. You might get away with this because the cutoff voltage would be based on a 6s battery. The smaller battery would reach cutoff quicker but since the bigger pack would be well above cutoff you still might overdischarge the little pack.

Mike
+1 this sounds right to me. I wouldn't do it because as he says in either configuration I think you risk over discharge of the 1600.
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Old Jun 28, 2012, 03:52 PM
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Quote:
Originally Posted by AZcrashpro View Post
Personally I would not do it.

Parallel the ESC would see the highest voltage and use that as a cutoff. That would make it VERY easy to over discharge the 1600 pack.

Series the 1600 pack would work harder that the 2200 and maybe over heat. You might get away with this because the cutoff voltage would be based on a 6s battery. The smaller battery would reach cutoff quicker but since the bigger pack would be well above cutoff you still might overdischarge the little pack.

Mike
you definately can use them in parallel as long as they are the same cell count, one pack will not empty before the other if wired in parallel, it doesnt work that way, if you wanted to use a 5000 milliamp 3 cell battery and a 100 milliamp 3 cell battery in parallel, it would work fine, you cannot use them in series however because they have to be the same capacity.
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Old Jun 28, 2012, 04:09 PM
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Quote:
Originally Posted by AZcrashpro View Post
Parallel the ESC would see the highest voltage and use that as a cutoff. That would make it VERY easy to over discharge the 1600 pack.
Wrong. In parallel they're at the same voltage, as per the laws of physics. This makes it impossible to overdischarge the smaller pack. This is also why it is perfectly safe to charge lipolys of the same cell count in parallel, regardless of relative capacity.

They may be used in series as long as the discharge current and capacity is kept within the limits of the least capable pack. Discharging all the way to LVC would most assuredly destroy the lower capacity pack so one must time discharges if mixing packs in series.

keithy4129 is absolutely correct.

Mark
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Old Jun 28, 2012, 05:53 PM
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Originally Posted by mrforsyth View Post
Wrong. In parallel they're at the same voltage, as per the laws of physics. This makes it impossible to overdischarge the smaller pack. This is also why it is perfectly safe to charge lipolys of the same cell count in parallel, regardless of relative capacity.

They may be used in series as long as the discharge current and capacity is kept within the limits of the least capable pack. Discharging all the way to LVC would most assuredly destroy the lower capacity pack so one must time discharges if mixing packs in series.

keithy4129 is absolutely correct.

Mark
That is true voltage remains the same in parallel but consider this. If you assume like you say that you can not overdischarge the 1600 and fly to the 80% most people say is safe of the 3700mah pack you create in parallel and use 2960mah per flight. All things being equal that is 1480 per pack. You have now over flown the 80% rule of the 1600 pack and gone to 92.5%.

Assuming you only fly to the capability of the 1600 pack you would be fine. I never assume anyone will not make a mistake and overfly the 1600 pack. I also don't believe in giving advice that set people up to have problems which is what this will do. WAY to easy to overfly the 1600 pack.

Can you do it yes, would I suggest doing it no.

Mike
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Old Jun 28, 2012, 05:57 PM
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Originally Posted by AZcrashpro View Post
That is true voltage remains the same in parallel but consider this. If you assume like you say that you can not overdischarge the 1600 and fly to the 80% most people say is safe of the 2100 pack you will use 1680mah per flight.

So where is the extra 80 mah coming from for the 1600 battery?
It is coming from the 2100mAh pack.

A 1600mAh pack and a 2100mAh pack in parallel becomes a single 3700mAh pack and should be treated as such. Similarly, a 100mAh pack in parallel with a 3600mAh pack also becomes a 3700mAh pack.

Quote:
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WAY to easy to overfly the 1600 pack.
In series, yes. In parallel, both packs will be at the same voltage so it will be absolutely impossible to overdischarge the smaller pack without also overdischarging the larger pack.

Mark
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Old Jun 28, 2012, 06:06 PM
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Quote:
Originally Posted by mrforsyth View Post
It is coming from the 2100mAh pack.

A 1600mAh pack and a 2100mAh pack in parallel becomes a single 3700mAh pack and should be treated as such. Similarly, a 100mAh pack in parallel with a 3600mAh pack also becomes a 3700mAh pack.



In series, yes. In parallel, both packs will be at the same voltage so it will be absolutely impossible to overdischarge the smaller pack without also overdischarging the larger pack.

Mark
Please see my edited post, we crossed.

Mike
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Old Jun 28, 2012, 07:43 PM
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Ok, read your post again and my point remains - it is physically impossible to overdischarge one battery that is part of a parallel set without also overdischarging the second battery. The total pack capacity is the sum of the packs in parallel, NOT a doubling of the lowest capacity pack, as is frequently incorrectly theorized.

The following thread deals with parallel charging but the same concepts apply to packs that are discharged in parallel: http://www.rcgroups.com/forums/showthread.php?t=932319. Carefully read through the first post a few times and it will make sense.

Cheers,
Mark
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Old Jun 28, 2012, 08:29 PM
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When wired in parallel, you can't possibly over discharge 1 pack.
They are in parallel and almost by definition the voltage is the same in each pack no matter what.
it is not a weak link thing like in series, they are ONE with the universe.
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Old Jun 29, 2012, 03:07 AM
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The critical factor in paralleling packs of different sizes is that they will automatically share the total current not EQUALLY as some people imagine but in proportion to their capacity (it's a bit more complicated but that's roughly how it works).

So if you run a 2000mAh and a 1000mAh pack in parallel and draw 30A total then 20A will come from the larger pack and 10A from the smaller....so they empty at the same time .

Neat isn't it ?

Steve
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Old Jun 29, 2012, 09:22 AM
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[QUOTE=slipstick;22027696]The critical factor in paralleling packs of different sizes is that they will automatically share the total current not EQUALLY as some people imagine but in proportion to their capacity (it's a bit more complicated but that's roughly how it works).

So if you run a 2000mAh and a 1000mAh pack in parallel and draw 30A total then 20A will come from the larger pack and 10A from the smaller....so they empty at the same time .

Neat isn't it ?

Not to take this off topic but a quick question ,what charge rate would you use if parallel charging 2s-850mah-900mah-1320mah @1C =3.1A????
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Old Jun 29, 2012, 09:31 AM
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Not to take this off topic but a quick question ,what charge rate would you use if parallel charging 2s-850mah-900mah-1320mah @1C =3.1A????
Yes. See link in post #10 above for more information.

Mark
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Old Jun 29, 2012, 11:09 AM
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Thanks Mark is was P charging the same packs and doing another cycle for the others,now i see it's quite ok to charge packs of disimilar mah but the same cell count obviously.But i have a question for you e.g if paracharging 2c1000-2s 2000@ 1c=3amps charge how does the charger know to deliver the correct amount to each pack and not deliver 1500mah to each at it sees a single 2s 3000 pack?
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