The biggest problem with these was that they didn't hold much juice. There simply aren't many milliamp-hours in a 9V battery. So people tried using banks of AAA or AA cells, often in larger packages.
Commercial manufacturers soon followed, using deviced powered by rechargeable lithium cells: https://www.google.com/search?q=portable+USB+charger&tbm=shop
And they served - until this latest generation of tablets came out. The problem is that USB is only rated for 500 mA, and the battery capacity of the modern tablets has grown to the point where you simply can't charge them in a reasonable time over USB.
I've experienced this as a long-time Nook user. My Nook color came with a special cable that would act as a normal USB when plugged into a USB port, but charged at a higher rate when plugged into the Nook's charger. It would charge in a couple of hours plugged into the Nook charger, but took more than eight when plugged into a USB port. A couple of years later, I bought a Nook Tablet, and it took nearly 14 hours to charge off of USB.
Still, I managed. The battery life on either was good enough that it'd last me through a day, except for when I was attending Science Fiction Conventions, where I might spend 16 hours in a day looking things up online, downloading books from authors I was listening to, etc.. But if I carried one of the portable chargers in my back, and kept charging throughout the day, I could manage to get through with some juice left.
About a year ago, I bought a Nook HD+, and it wouldn't charge off of USB at all. It's cable was USB on one end, and a special connector on the other, so I thought I might get a bit of charge out of it, but I've tried with several commercial chargers - even those that promised a higher-current charge, and nada.
And that's when I had my brainstorm. I had a car charger, that plugged into the cigarette lighter receptacle in a car, and ran off of 12V DC. Would that work, if I ganged up enough batteries to supply 12V? I picked up a cigarette lighter receptacle at Radio Shack, grabbed an 8-AA holder out of my pile of stuff, bought up some new alkalines, and gave it a try.
It worked, sort-of. Eight freshly-charged alkalines provided something over 13V - enough to supply a 12V demand. (I wasn't worried about over-voltage, because automobile electrical systems are notoriously noisy and unstable, and routinely vary from 9V to 14V or higher. I knew that the car charger already had to have a voltage regulator inside.)
The problem? The output voltage of an alkaline cell drops off rapidly as it discharges. I let my Nook charge overnight, and in the morning the output of my 8 cells had dropped to under 10V, my Nook had stopped charging, and I'd only accomplished about a 30% charge.
So I tried it with some expensive Energize Ultimate Lithiums. And they worked. I managed a full charge. overnight,, and was still showing over 12V in the morning. I tried again the next night, and the remaining energy added about 20% to my Nook's charge before the lithiums were expended.
I had a solution - except that lithium batteries are horridly expensive. EIght of them cost me over $20, and they aren't rechargeable.
So my final attempt - nickle metal hydride rechargeables. These are expensive, as well, but being rechargeable their per-use cost is low. They only put out 1.2V, instead of the 1.5V of the others, but their output drops off far less quickly. I hoped that 12 of these would be able to supply a 12V demand for long enough to fully charge my Nook.
And so it turned out.. I twisted all the various wires together on my desk and let the Nook charge overnight, and it was full charged in the morning. Again, there wasn't enough left in it to charge the Nook a second time, but that's not a problem.
So I took my cigarette lighter receptacle, my battery holders (one 8xAA and one 4xAA), a switch out of my pile of stuff, and a Radio Shack project box, and packaged it all together.
I would expect that this box would work as well with other tablet manufacturer's car chargers.
I won't provide detailed instructions - it's simple enough. I twisted the wires together, soldered the joints, and covered them with heat-shrink tubing.
Two pieces of information you might need to know:
- The hole into which the cigarette lighter receptacle fits is 1-1/8". I drilled it with a spade bit.
- The positive terminal of a cigarette lighter is the contact at the center, at the base of the hole. The sides of the receptacle are ground. Wire it up backwards and you are certain to short-out your charger.