One of my pet peeves when traveling with camera gear is having to take a dedicated charger for each device and type of battery. Thankfully, more and more devices are settling on the USB standard. Which means that you can get away with something like a 5-port USB charger, even if you have to take several different types of USB cables.
But many camera manufacturers haven't gotten the memo yet. To be sure, there are some cameras that have the ability to charge their battery by USB, either directly in the camera itself or with a charger that accepts USB input. Smaller cameras, like GoPros, often come with USB chargers by default. But many, especially for larger cameras, of them don't, especially larger DSLRs that take more powerful batteries. For those, you often have to take along a dedicated AC charger, either the one provided by the manufacturer or a third-party variant. They work well, but they add bulk, are yet one more thing that's easy to leave behind when you're rushing out the hotel room door for a 6am flight, and, obviously, you need access to AC power to make them work.
So I've been looking into options for charging my DSLR and mirrorless camera batteries with USB chargers. And it turns out they are available, even if they don't necessarily turn up at the usual retailers or by well-known manufacturers.
This, for example, is one I've been using to charge EN-EL15 batteries for Nikon DSLRs. The charger is put out by a Chinese brand I've never heard of before.
It's pretty basic, with two cradles and a couple of LED lights to indicate whether each battery is charged or charging.
But there are a number of advantages over the usual manufacturer-supplied charger.
For one thing, it can be significantly smaller and lighter. And if you're packing light, that matters. It's not self-contained in that you still need to plug it into something to supply power to the cradle, whether that's a laptop, an AC adapter, or an external battery, but I always take a multi-port USB charger anyway because it handles a bunch of difference types of batteries, devices, and cameras. This example has the Nikon-supplied charger for a single battery alongside the USB cradle for two of the same batteries.
For another thing, the power source doesn't have to be AC power. With an external USB battery pack like this one you can charge your camera batteries on the go, whether you're on a long-haul flight on a plane that hasn't yet been upgraded to have AC outlets or bouncing around in a chicken bus in Central America. You can use a USB charger. It also means one less AC outlet adapter you need to have on hand.
There are also significant potential downsides to factor in.
For one, these usually aren't endorsed by the camera manufacturers. They're third-party devices from smaller accessory brands. That means you use them at your own risk. And there are things that can go wrong. Charged incorrectly, lithium batteries can melt or even catch fire. They may not be optimized for any proprietary aspects of the battery's electronics. There may also be reliability issues in the chargers themselves--it's very much a case-by-case basis.
For another, in some cases the charging might be significantly slower than using the manufacturer's AC charger. It depends on the input requirements of the battery itself, the output capabilities of the charger cradle, and the output capabilities of the power source you're feeding into it.
That said, the couple that I've tried have been working well for me. I've been using them for Nikon DSLR batteries as well as for the batteries of a Ricoh GR II. The chargers charge properly, turn off when the charge is complete, and I haven't had any problems with them so far. But your mileage may vary.
Where to Find Them
There are a number of different models available, from some that charge a single battery at once to others that charge two at once. I've not come across any from big brands--they tend to be ones I've barely heard of, if at all.
Each charger has to be designed for a specific battery model, so the key information you need is the battery manufacturer's model number. If you add that battery model number to a search on places like Amazon or Ebay for "USB charger" you should get some options (eg: "EN-EL15 USB charger").
Some brands, like this one put out models for different batteries (use the drop-down menu on that page to select), which makes it easier.
Tip: When looking at them, be sure that what you're getting is a USB battery charger that draws its power from USB input. There are some chargers out there that come up in a search for things like "EN-EL15 USB charger" that are AC chargers that have a USB output, which isn't the same thing.