One of the downsides of modern devices is that you have to be constantly monitoring battery usage when you're out and about. Once upon a time, you could still take photos without a battery in your camera. But with today's digital cameras, that doesn't work so well. I always have a bunch of spare batteries on hand, but it's still a good bet on a shoot that I can run out, especially when I'm shooting with GoPros. And then there are all the other devices--smartphones, tablets, or GPS trackers, for starters.
It's easy enough to keep everything fully charged at home or in the studio, but it takes a bit more planning when you're out on the go. And especially so when you're on the go in remote areas where relying on several hours of AC power is risky. So it's good to have options.
Many newer cars include a USB outlet, but they're usually low power. And you can't count on a rental vehicle having one. So I find that having a car charger with me comes in very handy. They're cheap, and one of the few things that car manufacturers have standardized over the years has been the humble cigarette lighter that became the DC power outlet. And that adds another good option to charge your devices--I've even used them in cabs. You can get dedicated car chargers for camera batteries, etc, like these Pearstone duo battery chargers. But so many devices now charge by USB that a regular USB charger for the car comes in very handy.
Powergen Dual Port Car Charger
It's easy to find basic USB car chargers. Even a lot of gas stations now sell them. And I've tried a bunch. They all do a pretty simply job, but even with something this basic, some do it better than others. Some of the cheap ones I've picked up just stop working. If they do keep working, most chargers come with one USB slot that kicks out only a slow trickle of power.
I've settled on the Powergen Dual Port Car Charger (model PGCHCAR42A-WH), and it's small and light enough that I just keep one in my camera bag as well as keeping one in my personal car. There are a couple of things that make this one stand out from most of the others.
The first is right there in the name. This is a dual charger, which means it has two USB slots and you can charge two devices at once.
The second is that this is more powerful than your average car charger. Most car chargers kick out 2.4 amps. But this one puts out up to 4.2 amps. A lot of devices don't need that much, but it also means you can charge two 2.1 amp devices (like iPads) at once.
The two USB ports are not the same. The one labeled "A" is designed for Apple products like iPads and iPhones. The other, labeled "NA", uses the standard protocol and is designed for everything else. The manual warns that "using the wrong port may work, but charging could be much slower." So if you don't use Apple devices, this isn't your best option.
The other important catch is that the unit's total output is up to 4.2 amps. So if you're using both ports, they'll both draw from that same total available pool. In most cases that shouldn't be a problem, but it's worth noting in case your devices require a lot of power.
There's also a status light. It doesn't reflect battery charge status--it's just letting you know that the charger is correctly plugged into the DC socket.
It doesn't come with any USB cables, so if you don't have spares on hand you'll need to get those separately. A good option is something like the Belkin 7-in-1 Cable Travel Pack, which combines many of the USB or ethernet plugs you're likely to need in a convenient travel pouch. But make sure it has all the plugs you need--many of them don't come with some of the micro USB options, and I haven't seen any that come with a Lightning connector for the current generations of iPhones and iPads. Otherwise, you can of course buy the cables separately, like this Lightning cable for the iPhone 5 and 5S and newest iPads.
And while this is a small point, I find it handy that it comes in white. I've left many black ones in rental cars before in the rush to catch a flight or train. The white option at least stands out a bit against the dark dashboard and makes it just that little less likely I'll forget it. (So far so good!)