What external batteries can I get to charge a tablet during long road trips?

I use an external charger for my phone. One of its big advantages is that I can use it to charge my phone without making my phone hard to use (it's about the same size as the phone and they stack together nicely) and then charge the charger without having to attach my phone to the car or wall or computer or whatever I'm charging the charger from. As MeNoTalk mentioned in a comment, typically external chargers can charge your device multiple times (or can charge multiple devices if you're a sharer) from a fully-charged charger.

That said, if you are in a car this is just not an issue. Your tablet should easily recharge fully in an hour or less on a typical car charger. I can't imagine there would be more than 4 tablets in the car. So even if all of you were all using your devices full time (unlikely) the charger socket would sit empty about half the time. If you are charging and you get to half full and someone panics in the back seat that they're about out of power, unplug and run on battery for a while to let them charge, then finish your charging. I've handled this long ago with two laptops that had only about 2 hours battery when used to play media from a spinning CD or DVD (that chews battery) and we had no crises, so I am sure that modern tablets playing media from SSD and not pulling down a lot of data over the SIM card will last more than long enough for a single car charger to look after you all.

So the problem with external batteries is that you also have to charge them somehow. The one you linked needs power from a plug or something similar. So depending where in the world you want to travel, a solar charger may be better suited for you:

There are special solar charger for laptops, for example this one:

http://www.solarbag-shop.de/deutsch/solar-power-packs/netbook-notebook/notebook.html

Bigger solar charger systems have batteries that are capable to hold up to 33Ah. So you should be able to charge your laptop around 10 times with one full battery.

I have this jump starter for my car - Booster PAC ES2500 900 Peak Amp 12V Jump Starter. I bought it at NAPA but it's also available from Amazon for $110. It has a cigarette lighter port that can be used to charge electronic devices. Works really well for a long car trip, especially with a dual USB car charger. We are a family of 5, so one little cigarette lighter just won't cut it on a long trip. Certainly not tiny and portable option, but I already had one in the car and think it's great. It can be left on the floor in the third row of our van and the kids in the back can connect to and it and keep on playing their devices. It also works very well for it's intended purpose - to jump start cars. :) Highly recommend!