It may surprise people, but my main expense was not actually primary cells – it was rechargeable batteries. Because of all my runtimes, I would go through a lot of charge cycles – and I didn’t want that affecting the runtime results. So after a certain number of cycles (typically 40-50 for Li-ions), I would recycle the batteries and buy new ones.
Although there are lot of different battery chemistries, the most common type for high-powered flashlights remains standard ICR chemistry (LiCoO2) with protection circuits – although there are of course other formats out there for specific purposes (e.g., high-drain uses). Whatever the chemistry, these come in a variety of cylindrical sizes, based on their original purpose (e.g., the ubiquitous 18650 was originally used in laptop packs, 16340 was a replacement for CR123A, etc.). The numbers tell you the physical characteristics: the first two digits are width (in mm), the next two are length (also in mm), and the “0” refers to the fact they are cylindrical.
As an aside, for a reviewer, my key concerns were consistency and reliability. I would actually verify every new battery that arrived to confirm that it performs within the range of early samples (using a standardized set of lights that I would run though my lightbox). Capacity outliers are discarded, although that is pretty rare with my main source of batteries – AW. His 2200mAh capacity 18650 and 750mAh RCR protected cells were remarkably consistent in rated capacity over time (14500 less so, but still reasonable – those saw a slight increase in capacity over time).
The same process was true for NiMH Eneloops for my AA/AAA lights. I would buy multiple packs of 4xAAA and 4xAA every time they went on sale here, and go through them even faster than Li-ions (probably more like 20-25 cycles on average, maybe less). The reason for this is the occasional over-discharge event (i.e. running a light down to off, or nearly there). This is damaging to LSD NiMH – I would recycle those cells any time one of those events occured. As a result, it got the point the local pharmacy staff began asking why I bought so many batteries!
You can learn more about various battery technologies at Battery University, a free online resource. It is not as active as it used to be, but there is also the online battery and electronics subforum of candlepowerforums that can also be a very useful resource in terms of flashlights.
One of the key issues when it comes to rechargeable batteries is how to best go about charging them. Again, I strongly recommend you check out the battery charging section of Battery University. There you will find a detailed set of pages on the various methods for different types of chemistries.
Back in the day, I had done a number of reviews of individual chargers, but it was never my specialty. Similarly, I had done some comparison reviews of different types and makes of primary batteries. But I recommend you stick with reputable online battery sites for more info.
Fortunately, things are let better now with the new 21700 battery format; many batteries (or the lights they are in) have built-in USB-C charging ports. So you don’t need to deal with third-party external chargers, or worry about losing custom cables/ports, etc. It’s a real great convenience now. As long as you educated yourself on basic battery use and safety, you should be fine.