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Old Jul 06, 2006, 06:27 AM
Giz
Tony Rogers
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Here are a couple of examples to support my view about the weight.

Hyperion Yak 54 - .40 size. Stock setup with 5s2p 4200 lipo. Mine weighs 81 oz. The nearest equivelent pack would be 6s2p A123s which would boost the weight up to 93 oz and cost a similar amount to the lipo. The lipo pack gives 10 minute flights with reasonable (20%) reserve and pulls upwards of 70 amps. 7s1p A123s might work (if they could provide the 70+ amps) and would weigh about the same as the lipos but the flight time would be down to 5 or 6 minutes, which is a bit short for me.

Steven Aero Cap 232 .40 size. 4s2p 2650 lipo or 4s2p 4500 lipo weighing 44 and 49 oz respectively. 4s2p (with a larger prop) could give similar power but the weight would increase to 56 oz. 5s1p (with a smaller prop) is quite a nice option (because you could get two packs out of one Dewalt pack) providing the same power at 48 oz. Duration would be maybe 70% of the 2650 lipos. I fly the 2650s for 9 minutes so that would be
6-7 minutes.

I have flown my Yak at 86 oz and 81 oz and I much prefer the lighter version. 5s1p in the Cap could possibly temp me given that I could get two packs and recharge in 30 minutes on my charger (max 5.5 amps).
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Old Jul 06, 2006, 07:09 AM
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Quote:
Originally Posted by Giz
Stock setup with 5s2p 4200 lipo. Mine weighs 81 oz. The nearest equivelent pack would be 6s2p A123s which would boost the weight up to 93 oz and cost a similar amount to the lipo. The lipo pack gives 10 minute flights with reasonable (20%) reserve and pulls upwards of 70 amps. 7s1p A123s might work
A 5S2P pack of good-quality 15-20C cells is going to set you back significantly over $200, with the added issue of any pack that size becoming a liability if any one cell fails, or weakens, early.

I'd take the far cheaper, and far safer, 7S1P A123 pack and live with the slightly shorter duration, and a pocket full of extra cash!


Also, large Li-Po packs like that are a major fire hazard, as it's impossible to properly balance hard-wired parallel cells, and the sheer amount of flammable material in a large pack is very scary. (it's been shown that even in an enclosed environment, loose adjacent smaller Li-Po packs will not ignite, but all the cells in a large pack will most certainly ignite either simultaneously, or in sequence!)
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Old Jul 06, 2006, 09:04 AM
Giz
Tony Rogers
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Quote:
Originally Posted by meteor
...

Also, large Li-Po packs like that are a major fire hazard, as it's impossible to properly balance hard-wired parallel cells, and the sheer amount of flammable material in a large pack is very scary.

...
Just a minor point but the hard-wired parallel cells will balance each other automatically and the series sets of parallel cells will be balanced by the balancer.

And yes, the $200 did hurt!
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Old Jul 06, 2006, 12:37 PM
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Quote:
Originally Posted by Giz
Just a minor point but the hard-wired parallel cells will balance each other automatically...

Yup, "balance" each other to death!

One weak cell is propped up by one good one, in an ever-increasing spiral of loss, until the pair fails together. (the most common failure mode of laptop battery packs...)

It's very rare that a paralled pair of cells will fail/decay at the same rate.


A weak cell in a parallel pair causes the stronger cell to deliver far more current than it is rated for, and this will lead to a quick death of the cell.

It's particularly dangerous in high "S" packs, as it's harder to detect a single cell-pair departure from a safe LVC level. (the delta between the pack open voltage and pack LVC is large enough to allow any cell-pair to drop substantially below the safe single cell LVC level without triggering an LVC shutoff...)


Endless fun!
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Old Jul 06, 2006, 01:33 PM
Giz
Tony Rogers
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Quote:
Originally Posted by meteor
Yup, "balance" each other to death!

...


Endless fun!
Isn't that the same situation that can happen to series cells though? Does it matter whether a cell is balanced by its parallel pair or by a balancer? By weak cell I presume you mean lower capacity and/or higher internal resistance.

You may well be right but I have not heard of it being a problem before. In fact I have seen R D Blakeslee's tests of discharge two cells in parallel of different capacities which seemed to work just fine. Maybe another way of looking at it is that the stronger cell protects the weaker one?
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Old Jul 06, 2006, 03:02 PM
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Quote:
Originally Posted by Giz
...R D Blakeslee's tests of discharge two cells in parallel of different capacities which seemed to work just fine. Maybe another way of looking at it is that the stronger cell protects the weaker one?

The problem with that test is that the amp loads are far below the sum of the individual "C" rating capability. (less than 5.5C for the pair...)

In your case, you are probably running the 2100's at near maximum amperage for each cell. Therefore a "weak" cell means that the good one is pushed far beyond its capabilities.

I think you'd find that the vast majority of people use multiple "P" packs to arrive at suitable amperage, rather than to extend duration. (even if it's not needed, most will probably run close to max... that's human nature!)


Gotta keep the data in context!
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Old Jul 06, 2006, 07:16 PM
Giz
Tony Rogers
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Sorry to everyone if this feels like it is getting too off topic but I am very interested in the answer to the issue that meteor has raised.

Meteor, I appreciate your points and I am not at all sure of my own opinion on this.

If you compare the demise of two packs containing a "weak" cell, one 5s2p and one 5s1p, which dies first and worst? The weak cell in the 2p pack will itself be partially protected by its pair whilst the healthier cell in the pair will have to work harder and may be operated beyond its discharge rating. In the 1p pack, the same (or nearly the same) current will be pushed through the weak cell by the overall pack voltage causing it to get hotter (because it has a higher internal resistance) and possible overheating. Which of these two scenarios is preferably seems open to doubt. Both are not very good.

If you are correct, your suggestion is the first that I have seen that would suggest that the best way to make a 5s2p pack would be from 2 5s sticks in parallel rather than 5 2p pairs in series. Most pack assemblers seem to use the latter method to simplify the addition of balance connectors and the balancing process.

I'll try to stay on-topic now!
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Old Jul 06, 2006, 07:56 PM
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I think it comes down to personal opinion. My opinion is that I'd rather have a parallel pack fail since the cells should be cheaper since it costs less to produce more cells than it costs to produce one (quantities).
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Old Jul 06, 2006, 09:11 PM
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Quote:
Originally Posted by Giz
the best way to make a 5s2p pack would be from 2 5s sticks in parallel rather than 5 2p pairs in series.
2 5S sticks with 10 balancing leads is clearly the best and it makes it easy to detect a bad cell. Its also easy to revert to 5S1P when a cell goes bad. The improvement may not justify all the extra connections. "Best" is always subjective.

Or just use A123 cells which are self-balancing and forget about it.

Rick.
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Old Jul 07, 2006, 09:55 AM
HammerHead H2o Racing
Deltona, Florida
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in our r/c boats, especially in oval racing and [SAWS] 1/16th mile straightline time trials we draw lots of amps...

in structured r/c boating our rules have required us to run sub-c batteries and the battery of choice at the moment seems to be IB4200 SHV's...

in a look to the future, the I.M.P.B.A. is allowing the testing of lithium chemistry batteries in three provisional [voltage] classes for '2006

N-3 [2s] Q-1 [6s] and T-1 [10s]

in our testing of the a123 M1 cells we're running 2s 2p, 6s 2p, and 10s 2p [parallel packs] to get an honest 4200 mAh of run time...

we're looking at Li-Ion vs NiMh and see many advantages in the M1 cell over the sub-c's we've been using in our current rule structure...

we can run a123 cells in parallel at less weight than equal voltage sub-c's in series so the 2300 mAh rating of the M1 cell is not at issue except expense wise...

the M1 cells seem to take the high amp loads [70-100 plus amps] we throw at them with ease and come back for more...

if the cycle life of these cells is anywhere near the mfg's stated claims i see a welcomed place for them in r/c boating!

energy density wise, there is no question Li-Po batteries are superior to the current crop of Li-Ion cells but in the [high amp-draw marine environment] of r/c boating, safety issues need to be addressed as well...

time and more testing will tell...

Ron
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Old Jul 07, 2006, 10:33 AM
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Quote:
Originally Posted by waterdog49
in our r/c boats, especially in oval racing and [SAWS] the M1 cells seem to take the high amp loads [70-100 plus amps] we throw at them with ease and come back for more...
Just to be clear, you are running 2P packs so that's 35-50A per cell? I don't think individual cells can really do 70A peak as some have suggested without major voltage loss.

Thanks,
Rick.
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Old Jul 07, 2006, 04:20 PM
HammerHead H2o Racing
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Quote:
Originally Posted by rpage53
Just to be clear, you are running 2P packs so that's 35-50A per cell? I don't think individual cells can really do 70A peak as some have suggested without major voltage loss.

Thanks,
Rick.
correct... as i stated, all our testing has been 2s, 6s, and 10s-2P

Ron
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Old Sep 07, 2006, 02:24 PM
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The discussion leading to further graphs started here:

http://www.rcgroups.com/forums/showp...&postcount=580

- RD
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Old Sep 07, 2006, 06:14 PM
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The bursts caused no discernible degradation:
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Old Sep 07, 2006, 06:53 PM
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That's pretty awesome IMO. How many cycles has that cell gone through now? 34? Or have you been using more than one cell for testing?
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