Nicely put wendee, i would take safer out of the equation and put common sense in there . Have a read up on the charger side, cell type/brand.
Max charge V 4.2v
Min by manufacturer can be 2.5v(by this time your light probably would have blinked or shut off at 3v
Ideally keep the cell between 3.5ish to 4.2v(little leeway) as turbo probably wont hold and it will drop down at 3.5v or less so a warning there.
Dont leave on charge over night or untended.............i dont mean watch it for 3hrs whilst charging, just dont leave the area!
Ideally a V read out on the charger to get an idea of the V and usage
Learn the light, use it for a set time and check the V, get an idea of when the cell's close to around 3.5v , for example, after i use a light for a set time i top it off for the next. Be it 3.9v or 3.5v.........
Buy a matching set of 4 cells, samsung 30Q are a decent well priced option, sanyo/pany 10a 3500mah are also another option(many more)
Buy a decent charger, xtar i prefer over nitecore, both brands are a good start
Multi meter, does not have to be expensive, just a 1/2 decent one for checking V and keeping a periodical eye on them
If you drain your cell's low, top up quick and dont store for months, locking lights out help prevent accidental activation and parasitic drain where an issue.
Rough n ready little guide, there is more, that is basics for you. Have a little search on here for more info.