Lithium-ion battery cells. If you go with a larger cell size, say A123 (brand name) 26650 (size) cylindrical cells which are 3.3V each (Lithium Iron Phosphate chemistry) you will need four cells. Cells weigh 76 grams each, so four cells is 304 gr = 10.7 ounces. This will give you around 4.2 amp hour.

http://www.a123systems.com/lithium-ion-cells-26650...

The voltage out of these cells remains constant at 3 to 3.3 volts until only less than 5% of the charge is left then quickly fall to about 2V when about 1% of the charge is left.

The one issue is that they need to be kept above -30C or you can't get the energy out. So, you will have to insulate them some, and make projections of flight-time, thermal mass, heat loss, etc to determine how much insulation you will need.

I'm assuming your using them for a one-way trip, so balancing circuits, charge monitoring, over-current protection circuits heater-circuits for low temperature operation can all be skipped. These cells have a very, very low internal impedance, so you will not need any super caps. These four cells alone would be enough for a single short crank to start a car.

This size and chemistry-type battery cells are used in some Dewalt power tools packs. You can find them on eBay.

If you are actually making a product, then you might contact me as I work at a company that uses these to make aircraft battery packs. Sorry - I'm not allowed to give out samples.

The 18650 is a slight lighter battery 39gm : http://www.a123systems.com/Collateral/Documents/En...
but you can get them with over 5Ah capacity https://www.ebay.com/sch/i.html?_from=R40&_trk...

TTFN (ta ta for now)
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IR; There are zero 18650 cells with more than 3400mAHrs. Any advertised as having more than that are complete lies and actually tend to have less than 1000mAHrs. Even so 3400mAHrs is impressive.

Keith Cress
kcress - http://www.flaminsystems.com

Look into lithium *primary* cells, non-rechargeable. I believe that they have excellent energy density. Downside might be having to build-up a battery pack multiplied by the number of fairly expensive, one-time use, cells. They're used in aviation safety products. I'm assuming they might be lighter, but not sure.

Another thought is to dispense with the 12v requirement and use a little DC-DC converter with fewer cells. You'd still need the same energy, in fact a bit more, so the A-hour spec would be adjusted as required.

Thank you for all the advice, I forgot to mention that we will be trying to retrieve the payload afterward. But all this info has helped immensely. Is that true itsmoked? Cause that would be awesome if they are 3400mAH.

I suggest you stick with ONLY;
LG
Panasonic
or
Sony

The LG 18650 MJ1 3500 is 3500mA.

Sanyo purportedly just came out with 4250mA but you should be suspicious of a company stating a value considerably higher than everyone else in a physics-constrained product that is years mature.

Keith Cress
kcress - http://www.flaminsystems.com

What's critical is that you get 12V * 4Ah, so if you have a boost regulator between your battery and your load, you can potentially squeeze out a little more capacity.

TTFN (ta ta for now)
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IRStuff,
Your ebay link pulled up mostly Ultra Fire brand batteries. Not so sure that is a good name for a battery with all of the battery fires happening.. Lol

"...a weather balloon up to 100000ft."

Better double-check the ambient temperature at 100,000 feet (likely below -40°) versus the effect of such low temperatures on the battery capacity. I suspect that this may be an issue worth considering.

If it is, then think about adding insulation and maybe a source of heat (e.g. paraffin wax with a phase change) as such solution may (?) be lighter than extra batteries. Even the waste heat from the electrical load may need to be conserved within the thermal envelope. Solar gain might assist. A thermal analysis may be required. Or include a temperature sensor or two within the battery and payload, and record or transmit the data so you'll know if is, or isn't, a problem.

VE1BLL, Definitely has a point though most balloon trips are pretty short time and some insulation should do the trick.

Another issue is altitude. Some electronics have pretty low altitude limits.

UltraFire seems to be considered one of the most successful battery re-wrapping scammers.

Keith Cress
kcress - http://www.flaminsystems.com

Thank you again for the further info, Ve1BLL you are correct on that the insulation is key. our team is getting together to figure this out this week, so hopefully we decide to do something similar to this because of how light it is. If not looks like it might be a lithium based battery that was about 14 oz.

Worth mentioning that the "prismatic" lithium cells, the rectangular Li-ion slabs often used in tablet computers, are often packaged in what amounts to a sealed flexible baggie. They are probably (?) not suitable for your project due to the low air pressure at extreme altitude. I'm basing this on some recent experience where such cells inflated (due to some mild malfunction, not altitude in my case) and the inflation of their pouches pushed the LCD screen right out of my tablet.

Meh, just punch a couple of holes in the pouch, problem solved

Dan - Owner
http://www.Hi-TecDesigns.com

Brings back college memories. I was on a high altitude balloon project and temperatures caused the batteries to fail, which left our circuit to separate the balloon from the payload, inoperable. As such, we had to retrieve the payload and balloon a few days later several hundred miles away. It was heavily insulated due to the extreme cold (don't recall how cold but believe around -40 to -50F, I would verify). However, it wasn't enough. Battery technology has come a long way since then but pay attention to the temperature, as already pointed out.

I think space temp is around -270C. You won't technically be in space at that altitude but almost.