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Twin batteries

Old 08-31-2013, 10:53 PM
  #1  
Mesaflyer
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Default Twin batteries

Hello,
I searched and have found a few discussions on this. I am putting together a twin, two motors, two ESC's, two batteries. Have found that some people Y the batteries together and power up both ESC's, and some keep the batteries separate, and then run each battery to each ESC'S. What are the pros and cons of each way? Really would like to know what most people do and why.
Thanks
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Old 08-31-2013, 11:57 PM
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DHC Beaver
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I'm going with the Y connection on my current twin build,so that if one battery drops faster than the other,I won't have an "engine out" scenario.Even 2 packs of the same batch can have slightly different discharge rates,so i'm not taking any changes.
I have flown a few 2-stroke twins,and if one died,it was usually curtains for that plane.
Slight variations in motor speed don't affect the flying qualities of a twin.
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Old 09-01-2013, 12:17 AM
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rcers
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Originally Posted by Mesaflyer View Post
Hello,
I searched and have found a few discussions on this. I am putting together a twin, two motors, two ESC's, two batteries. Have found that some people Y the batteries together and power up both ESC's, and some keep the batteries separate, and then run each battery to each ESC'S. What are the pros and cons of each way? Really would like to know what most people do and why.
Thanks
First welcome.

Second - there are not ANY advantages to running separate battery packs that I can think of. There are several con's however.

Issues:
  • One battery is not as capable and you fly a tad too long and you have one motor quit before the other
  • Battery failure and one motor quits
  • Different charge state of the battery is lessened with parallel
It is really about keeping both spinning.


Mike
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Old 09-01-2013, 01:03 AM
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hayofstacks
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the only way I think it.could be considered safe is in a pusher puller setup when the plane uses a receiver battery, and it will fly on one motor.

one reason I like twin batteries is you double the rated c ratings when using two of the same battery. should give you more peak power and a lot higher amprage available for both motors
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Old 09-01-2013, 01:08 AM
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Using two packs is fine - in fact it works well as you can use the single packs in different models. The question is if you run those connected in parallel vs separately.

Parallel is the way to go.

Mike
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Old 09-01-2013, 05:20 AM
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Definitely parallel rather than separate packs for each motor.

Anything that reduces the chance of "single-engine-out" is worthwhile.
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Old 09-01-2013, 01:28 PM
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Originally Posted by hayofstacks View Post
the only way I think it.could be considered safe is in a pusher puller setup when the plane uses a receiver battery, and it will fly on one motor.

one reason I like twin batteries is you double the rated c ratings when using two of the same battery. should give you more peak power and a lot higher amprage available for both motors
That was my reasoning .... but way way back - I posted a thread about hooking up two packs in parallel to gain extra capability in AMPs draw to motor .. and was shot down by many who said I wouldn't get higher amp capability ...

I accept that I only get total capacity of the packs - but I would have expected that 2 x 20C packs could provide more oooomph than the individual 20C rating ... I was hoping for 30C or more as a result.

Anyway - back to OP ... I agree with others - do not run separated - single engine out is not nice !!

Nigel
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Old 09-01-2013, 03:40 PM
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Dr Kiwi
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I accept that I only get total capacity of the packs - but I would have expected that 2 x 20C packs could provide more oooomph than the individual 20C rating ... I was hoping for 30C or more as a result.

Whoever gave you the advice about amp draw from parallel packs is wrong. Obviously having two packs in parallel gives you double the capacity, and thus double the potential amp discharge.

Your terminology/understanding of C-rate is slightly astray. C-rate (20C in your example) is a fixed characteristic of the particular pack. With two packs in parallel the C-rate will remain 20C, but since you have doubled capacity, the potential maximum amp draw (still at 20C) will double.

One 20C 2500mAh pack... max. amp (2.5 x 20) = 50A
Two 20C 2500mAh in parallel = 5000mAh.. max. amp (5.0 x 20) = 100A.
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Old 09-01-2013, 05:10 PM
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Mesaflyer
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Hey guys, thanks for all the replays. This site has been very helpful to me regarding electrics. I have always wanted to start flying twins, but wanted to go electric. Bought two used arfs at a swap meet as my "test" planes (Mustang and Mosquito). Have the Mustang just about ready to fly, so the Mossie will be next. After that have a few twin kits to build. I will be using a Y to tie the batteries together.
Thanks again.
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Old 09-02-2013, 03:46 AM
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solentlife
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Originally Posted by Dr Kiwi View Post
I accept that I only get total capacity of the packs - but I would have expected that 2 x 20C packs could provide more oooomph than the individual 20C rating ... I was hoping for 30C or more as a result.

Whoever gave you the advice about amp draw from parallel packs is wrong. Obviously having two packs in parallel gives you double the capacity, and thus double the potential amp discharge.

Your terminology/understanding of C-rate is slightly astray. C-rate (20C in your example) is a fixed characteristic of the particular pack. With two packs in parallel the C-rate will remain 20C, but since you have doubled capacity, the potential maximum amp draw (still at 20C) will double.

One 20C 2500mAh pack... max. amp (2.5 x 20) = 50A
Two 20C 2500mAh in parallel = 5000mAh.. max. amp (5.0 x 20) = 100A.
I am well aware of C basis and is why I was surprised at the answers I had before.

One of the reasons given was that in calculation C rate with mAh gives higher number but battery construction is still only suitable for the lower amp draw. Your example of dragging 100a out of the 50a capable peak packs is questionable.

Nigel
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Old 09-02-2013, 05:27 AM
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I find that everyone else should agree you will get more power by doubling the c rating, unless one battery is plenty for the motors.

read on heads up on their batteries. they show with almost every single 3 cell that hooking them up in parallel will double the c rating.

I had two tired 2200mah 20c batteries. one at a time I could only pull 280-300 watts. hooking both up gave me 380 watts it may not exactly double output, but it will give you more power.
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Old 09-02-2013, 03:11 PM
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Dr Kiwi
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One of the reasons given was that in calculation C rate with mAh gives higher number but battery construction is still only suitable for the lower amp draw. Your example of dragging 100a out of the 50a capable peak packs is questionable.

Not questionable at all.. just logical. Effectively what you have done by parallel connection of two packs is to make a single larger capacity pack.. thus at 20C you can now get twice the current capacity. Are you telling me that a 3s2p pack of 2500mAh cells (total 5000mAh) is no better than a 3s 2500 pack? .....since one 'unit' would give you 50A (20 x 2.5), the two are capable of giving you 50A each.... 2 x 20 x 2.5.....total = 100A.
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Old 09-02-2013, 03:16 PM
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read on heads up on their batteries. they show with almost every single 3 cell that hooking them up in parallel will double the c rating.

Being pedantic again.... the C rating is NOT doubled... the capacity is doubled.... thus the potential current draw is doubled.

Connecting packs in parallel does not magically make 20C packs 40C.
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Old 09-02-2013, 05:19 PM
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sorry for my wording, but it does make two 20c packs operate as a single 40c pack.
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Old 09-02-2013, 05:45 PM
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Still not strictly correct, 'cos your two 20C packs (let's say each has 2500mAh capacity), when joined in parallel, now give the "united pack" a total capacity of 5000mAh... we cannot say your new "pack" is 40C [and deliver "C-rate x capacity", or 40 x 5000 = 200A], because each 2500mAh "unit" can only still only deliver its original 20C... 50A each, 100A total.

The capacity of the pack is doubled, NOT the C-rate of the cells.
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Old 09-02-2013, 06:42 PM
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oops, your right..I see what I was doing wrong now. sorry.
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Old 09-03-2013, 04:53 PM
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OK ... that makes sense .. the 20C remains but capacity increase allows greater amp draw.

BUT in practice ... I never like to drag out theoretical max amps from a pack - I do it with my EDF's and they do not last long before packs fail.

Nigel
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