AA's vs Sub-C's ?

[BrianG]

Yeah, another battery-related discussion... I know Li-Polys would be better than NiMH in everything but price, but for those still using NiMH cells;

Maybe this has been discussed before (search turned up nothing relevant), but I have a bunch of NiMH AA 2300mAh batteries laying around and was wondering about the advantages and disadvantages of using these instead of the normal sub-C cell packs.

For a little background, I did a little measuring and calculating:

AA's (Duracell)
- 2300mAh
- Cell Diameter = 14mm
- Cell Height = 49mm
- Cell Volume = 7542.9 mm^3
- Cell Weight = ??
- A pack of 14 AA cells, wired in series, in 2x7 arrangement takes up 134456 mm^3 of space.

Sub-C's (GP3300 cells)
- 3300mAh
- Cell Diameter = 22mm
- Cell Height = 42mm
- Cell Volume = 15965.6 mm^3
- Cell Weight = ??
- A pack of 7 sub-C cells, wired in series, in 1x7 arrangement takes up 121968 mm^3 of space.

AA Advantages of using 2 AA cells in parallel vs 1 sub-C cell
- 39% increase in battery capacity/runtime, providing manufacturer ratings are not exaggerated.
- 48% increase in surface area to aid in cooling.

AA Disadvantages of using 2 AA cells in parallel vs 1 sub-C cell
- 5.8% decrease in battery cell volume (actual voltage/current generating volume)
- 10.2% increase in battery dimensions (tray real-estate) needed.

I figure two packs of 7 GP cells could theoretically be replaced by two packs of 14 AA cells. Each of the 14 AA cells in one pack would be wired in series for a full 14.4v. Then, both packs could be wired in parallel. Despite the two disadvantages I could think of, would the increased capacity offered by the AA cells be useful? Also, what is the discharge rate of AA cells? Weight could also be a consideration.

Price is also a factor. Two Sanyo 2500mAh cells cost about $6.50 (quoted from BallisticBatteries.com), while a single GP3300 cell runs for $5 from rc-monster.com. That's a 30% increase in price.

Ideas/thoughts?

[Serum]

Yeah, you forgot the most important thing... ENERGY DENSITY!

The power/energy they can deliver per gram of battery (in this a good lipoly is superior)

It IS all about energy density. the thing is; those cells aren't made to deliver high power (current) the sub-c's can deliver..

Thos AA's are nice to power a camera or a clock, but your RC will thanks you for using the GP3300..

[nbcaznmaster]

yep, if you try to do that, your AA will overheat.

[BrianG]

Originally posted by Serum
Yeah, you forgot the most important thing... ENERGY DENSITY!

Yeah, but I didn't have that parameter. That's why I measured the cell dimensions so I could approximate density from volume since that is a major factor. I guess I'd have to weight them as well. Also, I kinda took this into account by paralleling two AA's to replace one sub-C. Two batts will have to work half as hard as one (half the current).

nbcaz: Of course they'll overheat if I use one AA per one sub-C. However, there is a higher surface area to volume ratio in two AA's than one sub-C, so cooling will be improved.

It comes down to how the cell is built internally, which I'm not about to destroy cells to find out. :)

[Nick]

If you use 2AAs for every 1Sub-C, going to be more volts...

AAs are still 1.2v.

[BrianG]

Two AA's in parallel for every sub-C. Sorry, should've made that clearer...

[MetalMan]

AAs aren't meant to supply a lot of amps. Sub-Cs have their application for a reason, and it's going to take at least 5 AAs in parallel to be able to give the amp draw that a sub-C can.

[maxxdude1234]

The GP 'A' size 2000's could be a good run for your money if your looking for a lighter battery packs, but can't afford lipo. They can handle any amp draw a GP3300 can.

[Nick]

GP do small 'A' size 2200mAh. Not sure about 2000?

[maxxdude1234]

GP do a 4/5A 2000mah battery, and a 4/5SC 2200mah.....just to clear that up.

http://www.cheapbatterypacks.com/mai...gid=custompack ....scroll down the page....

[BrianG]

Thanks for the info people. I wasn't really looking for weight savings really - I just had a bunch of NiMH AA's laying around and was looking at the Ah capacities and got to wondering. Since I haven't been able to find the max draw rate of AA's, I thought I'd ask the knowledgeable people here. :)

I suppose I could take some of those AA's, hook up a resistive load, and seeing how the voltage drops at 5A, 10A, and so on. Maybe even use a few transistors and make a variable load circuit. However, this wouldn't be totally accurate as a motor is a mix of mostly inductive, a little capacitive, and some resistive for a sometimes complex impedance - and the signal feeding a motor is really square wave AC. Since I was planning to run two AA's in parallel to approximate a sub-C, I can just double the values. Since sub-C cells can be subjected to load currents upwards of 70-90A (or more), I'll have to load the AA's with 35-45A. To be fair, I should perform this test with a sub-C as well and note the difference in drop of voltage. Unfortunately, I don't have any more loose sub-C cells. :(

Another reason why I was toying with this idea is I was reading a thread in a different forum (sacreligious, I know) about using D cells and people said they have more capacity, but do not release heat as well due to their bulk. I also know that sometimes using a bank of small capacitors in parallel is better than one large one of the same total value due to the transient response to loads. I thought maybe these two factors may apply equally here.

[BrianG]

Well, I did a little testing and I guess it answered my question.

I figured since I planned on using two AA cells in parallel to replace a single sub-C cell, that, for this test, I would halve the 30A discharge rating most places use to rate their GP3300 cells. So, I wired five 0.33 ohm resistors in parallel for a total of 0.066 ohms. This would draw an 18A load (provided the battery voltage stays at a constant 1.2v, but of course that won't happen). Once I hooked up the resistor pack across the AA cell, the battery voltage dropped to 0.84v. That equates to a 12.73A draw. Unloaded, the AA cell measured 1.364v. Assuming a linear internal resistance curve (which it's not), that equates to 0.041ohms of internal resistance.

A "zapped" sub-C cell supposedly maintains 1.17+ volts at a 30A draw. I can't imagine a non-zapped cell would drop much more than that (maybe 1.0v?) at a 30A draw. For this test to be conclusive, I'd have to try this test, except decrease the resistive load to 0.044 ohms (eight 0.33ohm resistors in parallel) to force a 30A draw, on a non-zapped GP3300 cell just to be sure. While I'm at it, I'd like to try this on a zapped cell as well to verify the claims. But, as I said before, I don't have the cells to test on. Poo.

If I had a GP3300 zapped, a GP3300 un-zapped, and equivalent zapped and un-zapped AA cells, I'd like to try the test again with incremental loads for a better test and feel for battery efficiency.

[Gustav]

What about using GP1100s in parallel?

They seem as energy dense as subCs, as gp2200s are(more so even).
A pair of GP1100s in parallel may give you the same performance as 1 GP2200 at the same weight plus the increased surface area you desire?

(I don't know,i'm asking really.I know the flyers use them)

[BrianG]

I was really looking at a solution to replace a set of GP3300 cells with greater capacity, or equal capacity for the same weight and/or real estate. GP1100's, even in parallel, wouldn't give me that. They may have a high energy density, but the capacity isn't there.

The AA cells I was looking at were 2300mAh, so I figured two in parallel would give me 4600mAh. But, as my test displayed, they seem to have higher internal resistance so they aren't good for high discharge rates. They might be an ideal alternative to GP3300's if I needed a lower discharge rate (like in a 1/18 scale R/C) since there is more capacity (ie: longer runtimes) overall.

I would be able to get the high discharge rates if I were to parallel three or four AA cells and get a really nice runtime, but weight and real-estate would be much higher.

[Dafni]

BrainG, thank you for this info!

Originally posted by BrianG
....I thought I'd ask the knowledgeable people here. :)

Looks like you are a pretty knowledgeable person yourself! :D