This little olight battery charger uses magnets to attach the ends of the battery which allows it to be very small. It can handle 1.2 volt Ni-MH AA and AAA batteries as well as the who family of 3.6/3.7 volt Lithium-Ions. Using the ubiquitous USB port to power the charger means your phone charger or laptop usb power can be used to provide power, meaning you don’t have to carry a separate device.
There are some limits to the device, such as only being able to charge a single battery at a time, and with a maximum output of 750mA and only when feed with at least 800mA, so if you use a standard 500mA port, you should expect only about 375mA output, so charging times might be longer than a dedicated charger.
These limitations are more than acceptable for travel charger that fit easily in your bag taking up virtually no room or weight. I have used it off a simple battery bank that you might recharge a phone off of, and off my laptop post, both worked very well. The charger has a simple red light when charging, a green light when done but gives no detailed status like a dedicated charge like the Nitecore D2/D4 will.
If you travel with a flashlight, this charger is a great little investment. I can see this charger and spare battery that you rotate nearly indefinitely.