I did a search of this forum, and did not find much on rechargeable flash lights, and nothing very recent.
I had my bedside flashlight crap out on my recently, so I'm looking to replace it. I see a lot of rechargeable flashlights when I look and I'm wondering about them. One of the problems I had with flashlights over the years was the tendency for alkaline batteries to corrode the inside of my flashlights (that's what happened to my current flash).
The new unit I'm looking at uses a 18650 Rechargeable Li-ion battery. Are Li-ion batteries subject to the same corrosion problem as alkalines? My bedside flash may sit unused for months at a time, should I leave it on charge, or charge it up on some sort of regular schedule, or will it hold a change long term? Any other tips for the care and feeding of a rechargeable flash?
Alkalines cause corrosion because they can release corrosive gas due to the materials used in the battery. I wouldn't recommend using them in anything you care about.
Rechargeable lithium ion 18650 cells make up most of the battery packs (the 18650 tubes are arranged in a series inside of the pack) that you use for power tools, laptop computers, etc. 18650 batteries can be safe, or not, depending on who made them and how they are used.
There are "protected" and "unprotected" cells. Unprotected cells can be shorted out easier causing an explosion or fire. Protected cells have a gadget inside the wrap that manages how quickly power can go into or out of the battery, but the protection gizmo makes the battery very slightly longer and they don't fit in all lights. Protected cells sometimes won't work in the most powerful lights because they have limited output (how they stop from over-discharging during a short) although they should work in anything that isn't ridiculously demanding.
Many of the 18650 batteries you see on ebay are old junk removed from 'recycled' battery packs and can be dangerous and unpredictable. Many of the name-branded 18650 are cheap cells of questionable quality. I highly recommend buying high powered batteries from a reputable company. I use and recommend Orbtronic 18650 batteries, the ones I bought are Japanese cores with Japanese protection circuits.
In order to avoid a short situation with high powered batteries never carry or store them loose. The best carriers have threaded caps that fully enclose the battery and protect it from submersion without taking up much additional space. I use carriers from CountyComm and Oveready.
18650 batteries don't do well in heat, do not leave them out in a hot car. They also slowly degrade over time regardless of how you care for them, so plan on replacing them every 5 years or so but they can last longer. They degrade faster if left fully charged so I store mine at 3.4V except for the battery in my EDC light and a spare that I keep in a fully enclosed carrier.
There are all kinds of chargers, from cheap to expensive, simple to elaborate. I recommend spending enough ($30 or so) to get one that measures capacity (to monitor degradation) as well as voltage. Then get an inexpensive single cell charger (like the XTAR MC1Plus) for travel.
I hope this helps.
Edited to Add: Like someone else said, if you're fine with AA and AAA power levels you can use nimh AA and AAA rechargeables, they're way easier to use and a lot less to worry about. Eneloop brand used Japanese cells the last I checked. I believe they do better in heat and don't degrade as much over time, but you'd have to do the research to confirm. You can get a 4 pack of Eneloop AA or AAA with a smart charger for $15-20