Batteries for rarely-used S5-IS

[requa]

I have a Canon S5-IS I use almost exclusively for taking photos of new employees for their ID. When we converted to our new ID system, it seemed like I'd be doing lots of these so, like any photographer, I got a charger and AA NiMH batteries. Now I pretty much just use the camera when we hire a new employee (3 or 4 per year). Today I tried to charge my batteries, but they would not take a charge. I suspect that since they generally sit for months at a time between usage, NiMH is not the right technology. Bearing in mind that I will face a brief burst of usage a year or so down the line when multiple IDs expire at the same time (30 or so photos), what type of battery would be the best for a camera that is used only occasionally, though sometimes for multiple shots within a week? Lithium disposable? NiMH low discharge (assuming my current charger can charge them)? NiZn? Bear in mind that I work for a government agency with requirements to justify any immediate expenditures -- sometimes it's much easier to buy a more expensive short-term technology than to invest in something that makes sense over the long term. Today, I dropped in a set of 4 "Pro" Duracell alkaline batteries.