Martin,

Your specs are gonna be pretty difficult to meet... I think your best bet would be 4/3 A Sanyo NiMh rechargables in an 8 - 9 cell configuration running a regulated boost circuit to keep your voltage at a constant 12v. Your theory of using CR123 cells is feasible, but it will take a heck of a lot more than 4 cells and you'll still need a regulated boost circuit - expensive batteries. Read on and see why I say NiMH:

Depending on how close you need to be to 12v, with any chemistry you'll either need to add a few cells for a higher fresh voltage or include a boost/regulator circuit to keep output pumped up 12v as you use the batteries. Battery voltage starts dropping as soon as you start using them.

CR123 cells start at 3v and are usually considered exhausted around 2v. They typically have about 1500mah capacity, so that means you will need 8 of them, not 4, to get 3000 mah, and at the end you will be looking at 8v, not 12v. (Two 12v series packs wired in parallel gives you 3000mah at 12v) 8 cells come in about 4.8 oz naked, so your weight limit seems attainable. Lessee, that's about $8 - $10 a pop at discount mail order price. It may not be supremely reliable, though, because once in a while a single CR123 will go dead-nuts flat compared to its in-series companions.

L91 AA 1.5v lithiums have pretty close to the ampacity you spec - 2900 mah. That's at a steady discharge rate of 200ma. About 0.5oz each, it will take 9 to start out at 12v and the pack will be down to about 9v when the cells are consider to be exhausted. That's about $18 to reload and the naked cells add up to only 4.5 oz (1/2 oz each).

Alkalines look interesting at first. A standard E91 AA has 2850mah, standard E93 C has 8,350mah, E2 X91 AA has 3,125mah, and X93 C has 9,185mah - not too shabby. BUT those ampacities are at a discharge rate of only 25ma. They simply will not deliver anything close to that at a high discharge rate (they will benefit from "resting", but you'll never get those ampacities at high discharge rates no matter how you do it). And these cells are considered exhausted at 0.8v, so your "flat" voltage will be only 7.2v. Figure about 0.8oz each for the AAs and about 2.3oz each for the C - I figure you can make something work with a regulated boost circuit and 9 C cells at reasonable discharge rates, but just the naked cells will weigh more than your 1lbs. Some wizard can fiddle with it and figure out efficiencies, discharge rates, etc - maybe you could do it with fewer cells (boost circuit).

NiMh rechargeables look interesting. A gonzo AA NiMh is rated for 2100mah at 420ma discharge, starts at 1.2v fresh and at C/5 is down to 1.0v. About 1 oz., you can make your weight limit and have an ampacity of 2/3 what you want (10 cells, 10oz naked). C cells run close to 3oz each in case you were wondering... and the average ones have little more capacity than the gonzo AAs, although they can run upwards of 4,500mah. Better would be Sanyo 4/3 A size cells at 3800mah and 52 grams each (that's about 1.8oz, so 10 cells are a tic over 1lbs - but back to the boost circuit - you could drop one or two cells)

Build/have built a magic box using Sanyo 4/3A NiMh cells and it can be reliably done. Recharging is a whole other topic and this is already too long a post.

Disclaimer - no affiliation with Sanyo or anyone else. You can get larger capacity 4/3A cells from Sanyo, Varta, and others - not surprisingly, they weigh more...

HTH,

Tom