Reddit mentions: The best mp3 player chargers

Honestly, if you have to ask this (and certainly this generally), you should probably think twice before attempting it. Keep in mind that faulty wiring and poor circuit design can easily lead to fires, and these happen all the time, even with "professional" Chinese USB wall-warts.

I highly, highly, highly recommend that you don't attempt to build this from discrete parts (ie transformers, rectifiers, voltage regulators, etc) - I'm an EE, and I wouldn't even consider doing this myself if I wasn't planning on putting 15+ hours of design in just to ensure reliability and safety, as well as another couple hours of testing and verification, ESPECIALLY if I was giving it to someone else. Improper design can, at worst, cause a serious fire (due to thermal failure or internal short circuiting) or electrocute the user with 110v mains due to improper isolation, and, at best, simply not work properly or destroy the device due to poor power regulation.

All that said, Apple has a unique charge scheme for their devices that require placing voltages on the USB data lines to signal the maximum charge current that the device can take. Most Android (and other) devices are signaled that a dedicated charger is present by shorting the data lines together. Between more nuanced things like this and the safety issues I mentioned earlier, your safest bet is to buy and utilize an off-the-shelf USB adapter.

I suggest you buy an inexpensive charger like this and find a way to work it into the clock. Although the form factor might not be completely ideal for you, your chance of making anything smaller yourself are slim to none. Try to keep the case on it if you can, but if it needs to come off in order to fit, make sure you take it apart carefully (a little hot air and prying always works), and then insulate the entire device from both itself and other things using electrical tape.

Then, plug it in to one of these. You shouldn't be trying to remove the existing metal tabs from the charger (or soldering / attaching bare wires to them, either), so you need a plug. The output of that will then have to go into some quick-connect tabs that will let you crimp in this wire. The other end of that wire will then get cleanly soldered to the safest access point from the 120v going into the clock electronics - after which point the wire needs to be strapped down and strain relieved, then both contact points cleaned and coated in hot glue.

Yeah, it's an ugly hack, but it's the safest thing you're gonna be able to get done - anything else would require a lot of reading and learning to ensure that you aren't going to shock someone or start a fire.

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interesting idea. I am however, very reluctant to ever leave my phone anywhere but my own house or pocket regardless of how secure i think it might be.

I've had one of these for years and bring it with me on vacations/city trips/train rides etc. and it's never failed to give me that little extra boost of juice late at night before i can get back to my hotel room.

If you go the power bank route, pick a unit which supports charging via Quick Charge 2.0 like this one (requires a compatible charger like this one, or the charger that comes with the Motorola Droid or other modern phones).

Charge time shortens from 8+ hours to maybe 2. Lithium batteries are also much lighter than lead-acid, even with the 5V regulation circuitry you'll find in a USB power bank.

To get the Watt-hourage of a power bank, multiply its amp-hours by 3.7V (nominal voltage of a lithium cell) -- so a 10,000mAh bank would have 37Wh of energy. A similarly-sized lead-acid battery would be around 5-7Ah (× 6V = 30-42Wh) and weigh considerably more.

EDIT: I just read that your equipment is a sound meter, so consider whether a switch-mode power supply would introduce electrical noise that could interfere with your sound meter. If that's the case, a linear regulator with a large heatsink might be the way to go -- or you could just use 4x rechargeable AA NiMH cells (the Low Self-Discharge type, of course), which won't require regulation, so long as the device works down to 4.8V (NiMH cells have a nominal voltage of 1.2V).

Can you compare something like a Belkin or like a Samsung one?
As long as its 1A at 5V, it shouldnt matter right?

Yes I have tried that. Fast charging stopped working with the stock power brick so I bought this one.

https://www.amazon.com/gp/product/B00X4U1NFO/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1