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Parallel same voltage diffrent mAh ok? or no?

Old 05-28-2011, 06:33 PM
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K CLOSE
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Default Parallel same voltage diffrent mAh ok? or no?

Hi guys and gals,
From the research I've done it seems as if paralleling same voltage packs of various mAh is ok. What hasn't been made clear to me is if it is a situation where it's a Communist 'from each according to its ability" or a "your chain is only as strong as it's weakest link". I ask because I would like to parallel two 11.1v packs- one 1300mAh and one 2200mAh in an "L"shape for balance reasons. But if my flight times will end up being based on the 1300 battery rather then the total 3500 its a waste. Ill just buy some 3300's. But the further appeal of having the 1300's and the 2200's is that they will work in like 6 or 7 other planes. Any 3300's I but will only fit one.....
Survey says.......
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Old 05-28-2011, 08:58 PM
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That will be o.k. but you can't really rely on dragging the full 3500mAh out. If you're conservative and limit it to about 70% (which is good practice even with individual packs) then you'll have no trouble.

Steve
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Old 05-29-2011, 11:47 PM
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OK, I am the last person to come to mind when the phrase "electricaly savvy" is uttered, but when hooked together don't both packs have to have the same capacity (mAh rating) in order to prevent one pack's voltage from dropping too low before the other is depleted?

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Old 05-29-2011, 11:57 PM
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Walt Thyng
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You are always bound by the lowest parameter. In a parallel pack of differing capacities (mah) you will be limited by the lowest capacity pack.
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Old 05-30-2011, 12:01 AM
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Originally Posted by FlyWheel View Post
OK, I am the last person to come to mind when the phrase "electricaly savvy" is uttered, but when hooked together don't both packs have to have the same capacity (mAh rating) in order to prevent one pack's voltage from dropping too low before the other is depleted?

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That is what I have been told also.

have a good one
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Old 05-30-2011, 03:23 AM
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Originally Posted by Walt Thyng View Post
You are always bound by the lowest parameter. In a parallel pack of differing capacities (mah) you will be limited by the lowest capacity pack.
Walt
See, now I've read that this is always true and that it's only true when series wiring packs. I've googled,etc and found varying responses on parallel. Some say never ever mix cells, others have stated that the cells provide power proportional to their size and ability.
In a 1000mAh parallel with a 2000mAh for a total of 3000mAh, the first pack provides 1/3 of the demanded power and the second pack provides 2/3's - almost like two different sized gas tanks with a built in fuel ratio valve at the joint.
I came away confused, so I posted my query here. It seems reasonable that the "weakest link" scenario would be the logical choice, but electrons do funny things sometimes....
What are we basing our opinion on? Surely some fellow watthead tried this and found out for sure at this point. Dr.Kiwi ever do anything on this topic?
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Old 05-30-2011, 05:39 AM
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Originally Posted by K CLOSE View Post
Hi guys and gals,
From the research I've done it seems as if paralleling same voltage packs of various mAh is ok. What hasn't been made clear to me is if it is a situation where it's a Communist 'from each according to its ability" or a "your chain is only as strong as it's weakest link". I ask because I would like to parallel two 11.1v packs- one 1300mAh and one 2200mAh in an "L"shape for balance reasons. But if my flight times will end up being based on the 1300 battery rather then the total 3500 its a waste. Ill just buy some 3300's. But the further appeal of having the 1300's and the 2200's is that they will work in like 6 or 7 other planes. Any 3300's I but will only fit one.....
Survey says.......
One way to find out if two parallel battery packs are properly sharing the load is to measure the current output on both of them. Problem is, inserting a meter in series with one of the battery packs, such as an Astroflight Whattmeter, will result in changing the actual current, due to the meters wiring resistance. It's also somewhat of a pain in the (behind) to connect up one of these meters in a dual battery setup.

What will work, and work very well, is to pick up one of those Sears Craftsman #82369 AC and DC clamp on ammeters. This meter allows real quick checking of either AC or DC currents through a conductor simply by clamping its jaws around one of the battery leads.
http://www.sears.com/shc/s/p_10153_12605_03482369000P

And if both batteries show identical currents or at least similar currents, from before taking off, to after landing, you're good to go, at least with the batteries you've selected.

If you should do this, it would be a good idea to post your results. A lot of wattflyer readers would be interested in what you find.

(I did check parallel currents on my A123 cells, but they are all identical in voltage an Ampere Hour capacity.)
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Old 05-30-2011, 07:07 AM
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Larry3215
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This keeps coming up over and over again

To be absolutely safe make sure all cells are perfectly matched.

On the other hand, as long as the two packs are even somewhat similar, you wont have any problems.

On the other other hand, mixing together packs that are very dissimilar gains you almost nothing so its not really worth doing.

The more dis-similar the packs are in Ir, the greater the chance you have of over discharging the weaker (higher Ir) pack. That also means you have to be more conservative as far as max discharge amps and max discharge capacity.

Thus - my conclusion that using packs that are very different is a waste of time as well as being more risky.

IF your application has a max discharge rate that is well below the combined total of the two packs, then there is no risk at all.

So, if your only after duration and not extra raw power, then your quite safe paralleling even very different capacity/Ir packs.
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Old 05-30-2011, 07:10 AM
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By the way, one of the "good guys" in rc, RDBlakesly and I were among the first to do controlled tests on this very issue a number of years ago. So I have some idea of what I speak
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Old 05-30-2011, 02:45 PM
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Larry is correct, no problem as long as the current from the pack having the lowest C rating is not exceeded; this means that the maximum current you can safely pull instantaniously from the combined cells is twice the C rating of the lowest rated battery.
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Old 05-31-2011, 03:35 AM
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Thanks for the help guys. I really did search for a good while before posting Larry so sorry for bringing up what I guess is a recurring question. I think I'll give it a go and see how the voltages check out after the flight. I have several Turnigy batteries, I was going to Parallel a 1300 with a 2200 both rated at 20-30C in a plane that calls for 2000mAh but needs a bit more nose weight. I like to add weight with adding flight time whenever I can. I will try it out as I am now comfortable that I'm only risking the batteries not the airframe there in. Thanks guys.
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