[uthe54]

I think you want to look for a deep cycle lead-acid battery. They supposedly handle being discharged better than a typical SLA (sealed lead acid) would. You want to find a battery with a high Ah rating - ignore the CA/CCA rating since you won't be using it to turn any starters. use the math I described earlier to get your desired headlight runtime, and then you know what to look for. I think deep cycle batteres are more expensive than traditional SLAs though.

Different types of batteries are maintained differently. NiCad/NiMH batteries (often found in power tools and some digital cameras) do have "memory effect" so its better to cycle them down completely before recharging them. This is not the case with Lead Acid batteries. Think of your car for instance, your battery ideally should never discharge lower than 90%. It just provides a burst to start the engine, then is instantly recharged with the alternator. car batteries last about 5 years doing that. If your lead battery had a memory effect, it wouldn't last long treated that way. Also, I'm sure you've had instances where you left your car headlights on by accident. It's amazing how that can kill your battery. Why? Because the battery wasn't designed to sustain current for a long period of time - plus because your starter needs a pretty good chunk of power to turn over - a battery discharged 50% of the way makes a noticable difference turning over an engine.

The better type of battery are lithium batteries. These are typically found in cell phones, laptops, and higher-end power tools. Lithium batteries have a very low self discharge rate, and can provide full voltage until it's almost dead - which is why laptops/cellphones can still run with 3% battery life, yet a flashlight can hardly run if it's battery has only 3% left. They also are pretty decent at providing bursts of power, and have no memory effect. The problem is lithium batteries are more expensive, and I'm not sure one exists that will do what you're looking for - running your headlights. Oh - one other advantage is lithium batteries are light - unlike lead batteries where weight usually isn't too big of a concern.

I could go on forever on this topic... I like discussing this stuff, but I think I answered your question.