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Well, if you're not overly concerned with efficiency you can make a simple one by hanging an LM7805 5Vdc regulator off a battery pack of sufficient current and voltage overhead(probably 6 x 1.2Vdc rechargeable) and it'll give you a decent 5 volt regulated supply, probably better if you add a couple capacitors, maybe an LED and a switch. There are regulators with lower voltage overhead needs. Trickiest part is making sure you get the polarity right on the micro-usb connector/cable. Charging could be done with whatever charger you already have by swapping the batteries out.
I've been considering building one of these to recharge my cell phone, tablet, etc. during the occasional power outage. I'll probably go with a lead acid/gel cell 12 volt system for flexibility(tablet needs 12Vdc). It'd also be easy to recharge by hooking it directly to my car battery during an outage while driving around. But the thought of throwing away 7 volts+ through a resistor/regulator combo makes me want to design a DC-DC converter for the 5Vdc output.
There are at least a couple of kits out there(Google it) that step two AA batteries up to 5Vdc, but the cheapest ones still cost more than I'd prefer to spend. Of course, 2 AAs @ 2500maH gives you 3V x 2.5A or 7.5W @ even 75-80% efficiency you get over 1000ma @ 5Vdc. I'd like to try a 4 cell pack (4.8 nominal, but nearer 5.6Vdc fully charged), but so far I've chickened out. Back in the day you could count on +/- 0.25 under over being safe, but I'd just about bet most devices could handle the initial extra overhead, and the 5.6 is measured with no load. Loaded, it would drop off to 5.25, or less, quickly.
I may wise up and buy the one I linked to, and just drag the tablet along and charge it in the car or the office if I need.
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