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Old 10-20-2012, 10:48 AM   #26
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Originally Posted by r_kopka View Post
There seems to be some misunderstanding.

Weight distribution stays valid.

With a two pack system (independent of the number of motors but for each comparision the same number) you need exactly the same ESCs - the current will not lessen.

The C-rate must be the same if not more. A 2Ah pack with 10C gives you 20Amax. Two packs with 1Ah 10C give you 10A x 2 = 20Amax
But only if the cells are equal. If one is worse the current will be taken more from the better one. That brings a problem if the cells are also used standalone.

For an example I took two cells from the same label and the same shop:

LemonRC 2200/3s/35C - 173g €26,90
LemonRC 4500/3s/35C - 370g €45 -> half: 185g €22,5
so you gain 24g or 6% and loose 20% in cost with the two pack solution which has 2% less capacity.

Dymond 2400/3s/30C 202g 26,90
Dymond 5000/3s/30C 426g 46,90 -> half: 213g €23,45
so you gain 22g or 5% and loose 15% in cost with the two pack solution which has 4% less capacity.

Yes, the two pack solution is lighter but not cheaper. The gain is only some %.

I noted that but it will lead to different results if one compares unequal cells.

It also depends on what you intend. 5min WOT is a safe assumption. For a pylon or some other model that needs mostly WOT you get the correct result. But if the model is more like a trainer which runs most of the time at lets say 50% you get about 10min of flying time. That my be OK too but leads to too big batteries if only flown for 5min each time. A safe but heavy solution.

RK
It's like any debate of differing views - we can always find examples to prove our case, and my saying that, is not saying you or I are better at this than other.

If I was to take more time in my examples - I could show supporting data for my case, as you have for yours.

I was giving rough illustration .. next time I'll spend more time on it !!

I think the OP has plenty to go on and will no doubt come to his own conclusion and I hope the thread whichever way he decides has helped him.

For myself - healthy debate makes me re-look at my ways of doing things and I am always willing to consider alternatives. That's the beauty of forums to read other views. There are more ways to 'skin a cat' as they say.

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Old 10-20-2012, 12:57 PM   #27
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Well, you asked for advice, several options have been given, now you exercise your option to choose.

BTW, I would compare 5000mah and 2 x 2650mah and remember to use the 80% capacity rule, 10C at 1000mah is 10A but you should plan on using 8A from it. And I don't think anyone is buying less than 20C batteries these days
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Old 10-20-2012, 06:48 PM   #28
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Originally Posted by npowell28 View Post
Ok guys. Should i use 2 x 11.1v 1800mah lipos or just one for my twin motor cargo plane?

Cheers

Neil
If your ESC's have built in BEC's (Battery Elimination Circuits) as most smaller ESC's have, it is not a good idea to directly parallel the two BEC's from the two ESC's for receiver power. Some of these BEC's don't like that.

So, two options exist.

First, disconnect one red wire from one of the ESC servo connectors by sliding out that red wire from the connector.

Or second, use a pair of Shottky diodes (Not available from Radio Shack!) to isolate the two ESC BEC's from each other. These diodes are not expensive, shipping costs will exceed their value.

Take a look:
http://www.wattflyer.com/forums/showthread.php?t=67094

This way, you will have dual BEC power for redundant DC power to your receiver and its servos.

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Old 10-20-2012, 07:29 PM   #29
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Am I correct that the diodes merely provide redundancy if one BEC fails? There seems to be no mechanism for load sharing.

It's never too late to have a happy childhood.
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Old 10-21-2012, 01:28 AM   #30
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A diode is sort of like a one way valve. It does not allot power to back feed into a curcuit. That way you could run two bec's without one failure resulting in a crash.
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Old 10-21-2012, 02:04 AM   #31
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Originally Posted by pmullen503 View Post
Am I correct that the diodes merely provide redundancy if one BEC fails? There seems to be no mechanism for load sharing.
These uBEC's (Switching Power Supply Battery Elimination Circuits) usually do not like to be directly parallel connected. They can potentially fight each other. (Not good.)

The two diodes isolate the DC output of the two BEC's from each other. It's not very likely, but if one BEC's output should short out, the diodes would prevent the other BEC from being pulled down by the first unit.

Wired up per the schematic above, you can unplug one ESC from the receiver, and the other ESC's BEC will still power everything. If your ESC's use those linear BECs, the two units tied together with the isolating diodes will provide some measure of reliability if one BEC overheats and shuts down.

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Old 10-21-2012, 03:12 AM   #32
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Originally Posted by kyleservicetech View Post
These uBEC's (Switching Power Supply Battery Elimination Circuits) usually do not like to be directly parallel connected. They can potentially fight each other. (Not good.)

The two diodes isolate the DC output of the two BEC's from each other. It's not very likely, but if one BEC's output should short out, the diodes would prevent the other BEC from being pulled down by the first unit.

Wired up per the schematic above, you can unplug one ESC from the receiver, and the other ESC's BEC will still power everything. If your ESC's use those linear BECs, the two units tied together with the isolating diodes will provide some measure of reliability if one BEC overheats and shuts down.
The only thing that springs to mind is the voltage drop experienced across diodes ... the voltage even with the small drop across the Diode should still be within Rx / servo power range ... but should you start pulling more power - would it create a dip that exceeds lower volt limit ?

Sorry to be an **** !!

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Old 10-21-2012, 04:23 AM   #33
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Originally Posted by solentlife View Post
The only thing that springs to mind is the voltage drop experienced across diodes ... the voltage even with the small drop across the Diode should still be within Rx / servo power range ... but should you start pulling more power - would it create a dip that exceeds lower volt limit ?

Sorry to be an **** !!

Nigel

****???

Not true, those folks that don't ask questions forever stay less informed!

That's why I selected those Shottky diodes. These diodes have 1/2 the voltage drop of a Silicon diode.

Take a look at the attached JPG of these diode characteristics. Note that they have a very brief peak current rating of 100 Amps!

Just a note, for a lot of $$$$, you can now buy a silicon diode with a 2500 Volt and 6000 Ampere continuous current rating!!!


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Old 10-21-2012, 05:21 AM   #34
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Fair do's ....

I only asked because when boaters used diodes to control charging of multi battery banks - they had 0.7V drop per diode and that was enough to make system untenable. So we got into cheating alternator regulators by adding a diode there to make alternator boost its output by 0.7V ... or better still we used fast action electronic relays instead which of course had no voltage drop.

Cheers
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Old 10-21-2012, 06:38 AM   #35
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Originally Posted by solentlife View Post
Fair do's ....

I only asked because when boaters used diodes to control charging of multi battery banks - they had 0.7V drop per diode and that was enough to make system untenable. So we got into cheating alternator regulators by adding a diode there to make alternator boost its output by 0.7V ... or better still we used fast action electronic relays instead which of course had no voltage drop.

Cheers
Nigel
Yeah
We had an old motorhome that did that with the "house" battery. Didn't work well there either.

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Old 10-21-2012, 07:57 AM   #36
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Originally Posted by kyleservicetech View Post
Yeah
We had an old motorhome that did that with the "house" battery. Didn't work well there either.
In UK we have a couple of specialist companys that provide kits to boost charging etc. One being Sterling ... another Adverc.

Designed for RV / Boating ...

Personally I didn't like them as they took charging past the normal level right up into the highest max they could get ..
It wasn't just compensate for the diode drop ,... it was way more than that .. Average alternator system will click out at about 93 - 95% level ... they were pushing up as close to 100% as they could get ...
A couple of boating guys I knew had batterys literally cooked by these boost systems.

My preferred was the 'cheat the alternator' by adding a diode across the regulator terminals so the alternator detected a lower voltage and stepped up that 0.7v to compensate. That way you didn't push the batterys so hard.

Apologies for getting of topic !!

Nigel

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