Hang out on the Battery/Electronics forum.
I enjoy 'battery/cell talk' more than the lights themselves.
Only one way to learn and that's to read a lot and ask questions.
Baby steps.
Go to HKJ's site:
http://www.lygte-info.dk/info/indexBatteriesAndChargers UK.html
You can read about how he tests batteries, cells and chargers, as well as AC adapters and power banks.
You can click on a specific battery/cell section, go to the 'comparator,' select two choices, click off the current drains that he tests them at and which mimic your device in question and then oogle at and study the results.
Rinse and repeat.
If I had a dollar for every hour I've spent reading there alone, I'd have a lot of money for a new monster light!
Chris