https://charge4.harkwood.co.uk/
Have one of these with which I have been very pleased.
Profiles can be loaded for specific devices as required.
Thanks 
Those are quite heavy duty and impressively built boxes.
I eventually got around this 5.25V requirement by building my own 5.25V power unit. However, I used the Lenovo tablet from 2013 until 2019 and then went “backwards” to a Samsung T705 8" tablet which doesn’t have this stupid issue, and does everything needed including acting as a backup phone for my phone, runs all the VFR charts, runs EasyVFR, and unlike the Lenovo tablet it never shuts down because it uses a slow CPU
The Lenovo win8 tablet was absolutely needed only to run the Thuraya XT satphone, which had winXP and win8 USB drivers, but that phone got obsoleted by the much better Golze ADL150 system which is neat and just keeps working.
Nowadays TrueBluePower makes
https://www.truebluepowerusa.com/products/aviation/voltage-converters/tc120-series/
https://www.truebluepowerusa.com/products/aviation/voltage-converters/tc240-series/
that are adjustable from 5V to 18V, as well as USB power ports with USB-PD support that will output up to 2×60W at 5, 9, 15 or 20 V (depending on the USB-PD negotiation).
https://www.truebluepowerusa.com/products/aviation/usb-charging-ports/ta360/
They ain’t cheap.
There are some impressive voltage converters there, at amazing prices… However I didn’t see anything adjustable, to get 5.25 or so. There is a 6V one so one could get 5.25 with a low dropout stepdown regulator after it, with a minimal power dissipation.
I did actually manage to get the Lenovo T2 tablet to work adequately from my existing 5.00V power supply, by re-wiring the lead with a thick wire and as short as I could get away with. It doesn’t actually charge during flight, with the screen on, but it doesn’t discharge which is perfectly good enough.
Thanks for a good discussion on this topic with some helpful links to useful products. I used to have a problem powering my Samsung 7.7 tablet moving skymap.de (really like that product) when the crossed out battery symbol came on while using most of my 12/24 volt chargers that I bought/tried from E-bay sellers. In my own search for decent and affordable USB charging solution for our 28 volt Helicopter Cockpit I have come across this product http://daveonlinestore.com.au/car-audio-accessories-c-132/car-usb-chargers-c-132_395/3-x-usb-port-car-charger-for-phone-or-tablet-p-1663.html out of Australia (have not found it anywhere else online). It supplies a total of 4 Amp to a dedicated Ipad, Android Tablet and Smart Phone usb outlet. I think realistically you can only use one, maybe two of these outlets at a time.
It costs about 24,-$ Australian and appears to be well built and is a solution for 12-28 Volt Aircraft power range and gives options for the Android and Mac user.
To be honest I still sometimes get the crossed out battery symbol when I plug it in but if I turn the power supply off and on again (after the charge cable is plugged into the tablet) it usually goes away. I assume it has to do with it sensing a lower voltage/poor connection during the plugging in process (as some mentioned before the quality of cable seems to matter).
Here is also a range of options to maybe address the 5.5Volt requirement. I have not tired any of these.
http://www.aliexpress.com/store/group/CPT-Car-Power-Converter/224790_251040802/2.html
That's an interesting product.
The various options highlight the "fact" that there is no way to have a single USB port for charging both apple and non-apple devices, because apple need the specific resistor values whereas others need something different. It's a right PITA.
If I was installing this in a panel I would have it switch selectable i.e.
the apple resistors on the data pins, or
the data pins shorted, for Lenovo - or possibly the google/samsung tablets?
Try this: http://goo.gl/xbf3gO
We've just received one for our Eurofox panel. They're handmade to order, and the supplier is helpful and knowledgeable.
That Trueblue thing looks great, would love to have one of those in my panel (i do have a 12 V outlet and a double USB in the armrest but i might need that one for my Iridium receiver soon ..) But $ 399 ...?
Yes that looks a LOT better. How much is it?
It contains the regulator so won't be cheap.
The Lenovo tablet does actually need 5.25V to draw the full current. And yes one needs a thick cable (non USB - just two thick wires) to charge properly. The Lenovo power unit is 5.49V open circuit which is even worse... I can however charge it fine from this which is 5.25V open circuit.
I think Lenovo have done a mega cheat but it is the only product which does everything else I need.
Also the Ipad needs the specific resistor values across the data pins, whereas Lenovo don't. We have an old thread on all that here somewhere...
