@Max, but it's not exactly something you can slip in your back pocket, is it?
Well, clearly not, but many engineers have equipment with USB ports that can be used as chargers. Did you actually think those USB post s were there for inserting flash drives? So if you're in the lab all day, you can charge your phone or the portable charger for your phne such as those shown above.
BTW, I replaced one of my kitchen duplex outlets with one that has one AC mains outlet and two USBs for charging. Now I can make tea and charge my phone in one location.
My wife and I do a lot of Geocaching, and we prefer to look for the ones hidden out in the woods. Running around in the woods with the backlight active on your smartphone will quickly take a toll on the battery. This is more of a problem now that many smartphones don't have replaceable batteries, so I started a quest for a solution.
If you search on "Emergency Cell Phone Charger" on eBay you'll find a bunch of items that are ridiculously cheap. Out of curiosity I tried a few. I can report that:
The ones that use a single AA battery can only deliver 100 milliamps or so at 5 volts. The ones that use two AA batteries will deliver about 300 ma. Both of these are too wimpy to charge a tablet or large-screen smartphone. The ones that hold 4AA batteries are another story. They will do about 700ma and do a decent job of charging a smartphone or tablet. I keep one of these in the car, one at home, and one at the office. These are best strictly as emergency chargers since they are intended to use non-rechargeable batteries. The upside is that the shelf life is several years. I didn't bother to try rechargeable AA batteries because I am guessing that I would get much poorer performance.
We did get a more expensive version that contained a Li-PO battery (rechargeable, of course), and it works great. It weighs a ton, but will even provide a couple hours' worth of charge for a 10" tablet. I don't have the brand name of that one with me, but I'll try to post it later. This ended up being the one we take with us in the backpack.
@Sanjib: Hi there! Actually, Active-Semi makes the SoC ICs that are now used in many powerbank products out there in Asia. (Active-Semi is not exactly a powerbank end-product manufacturer). These SoC enable the design of powerbanks with all essential functions integrated in a single-chip. Helps bring down the size (total system size, or alternately allow the use of larger batteries) and system cost.
One of our customers built us a few samples with Active-Semi name on them as mementos! Have to admit they are a great hit (and not to mention, real lifesavers when you run out of battery in the middle of that long conference call you took from your cell phone :-)