My general advice: buy a brand name charger for Li-Ion/IMR batteries.
Get the best charger that you can reasonably afford from a reputable e-cig vendor (RTD Vapor, Illumn, Orbtronics). Statistically most battery incidents occur while batteries are charging. A higher end charger will have better built-in protections. Some will have built-in voltage meters which are pratical/useful, but not a necessity.
Pila,
Xtar,
Nitecore, and
Efest are the most recommended brands. I recommend batteries be charged on a flame-resistant surface: stove top, marble countertop, metal baking pan, pyrex glass dish. Do not charge batteries if you are not physically present to keep an eye on them.
Many of today's higher-end box chargers have
"intelligent" technology designed for the charger to shut down when the battery reaches full voltage (4.2 volts), however electronics can and do fail. Don't trust the safety of your home & family to a faulty electronic circuit or charging cord.
Rest batteries after charging
One commonly-reported factor in almost all the incidents we hear of where batteries failed violently while in use is that they were taken directly off the charger and then used immediately, at which point they failed.
Because of this, we think it may be a good idea to rest batteries after charging them. This advice will not be found in the usual 'reference bibles' on batteries, but we see more and different reports than others. Therefore we now advise:
Do not use batteries directly after charging them. Use a battery or batteries you previously charged, and that have rested for several hours. This is especially important if using a stacked pair for higher voltage, as statistically the risk is far higher.
Pila has had the reputation for making the best charger within the last few years, but hasn't updated their single model with today's popularly common features like real-time voltage meters. A charger with "intelligent charging" will allegedly be a safer charger, using special algorythms and not over-charging past 4.2 volts. Some of the higher end chargers can allegedly revive an over-discharged battery with special algorhythms. Some may not be able to charge 18350 size batteries without spacers, or be able to handle the size of a 26650 battery.
My personal experience: I purchased a Pila in 2012 when I bought my first mod. It lasted for about two years with heavy use; then one of the charging bays stopped working for no reason. By that time, I had nearly a dozen mods requiring batteries of different sizes, so I needed a new charger with at least four charging bays.
Second charger was an Xtar 6 bay charger. It was fine except that it used flimsy springs in the bays, and I couldn't keep 18350 batteries to stay in the charger without popping out. Overall I was disappointed in the design.
I next got a Nitecore Intellicharger i4 (four charging bays), which has been in heavy use for over a year now. It uses metal sliders to hold the batteries and charges batteries from 18350 to 18650.
