"Most" what?
The most usual 12 V to (USB) 5 V devices are ones made for vehicles.
I suggest that you also obtain a "USB voltage tester" and test the output voltage of any USB Voltage "converter" that you purchase before you use it to power any device.
I have had a situation where the voltage of a "new" converter varied repeatedly between 5 V and 12 V which, of course, destroyed the device to which it was connected.
Fast chargers with correct hand shaking can output up to 20 volt, however they are backwards compatible so with older equipment plugged in still work at 5 volt, I have a volt/amp meter for USB and it does vary a little on voltage +/- 0.4 volt approx.
As to charging a lead acid battery the problem is the time it takes for the battery to absorb the energy, so although a caravan may have a 30 amp charger, this is so when on mains no current is drawn from the battery, if comes directly from the charger, if I take my 90 Ah battery after being out with the caravan and put it on charge with a 3.8 amp smart charger, within the hour the charge rate has dropped to 3 amp, however to fully recharge it can take 24 hours, more if it has been left sulphated. So I will normally leave it connected to charger for a week.
The lead acid battery can be charged with a stage charger or pulse charger to 90% reasonably quickly, it's the last 10% that takes the time, and if you don't complete the charge the battery will slowly over time loss capacity. All sorts of ideas have been tried with caravans and canal boats, specially the latter, with an engine running 7 hours a day, with twin alternators pushing their power into an inverter so can deliver a pulsed charge of well over 100 amp (the pulse is so software can read battery state as voltage decays between pulses) you would think there would be no problem recharging the batteries, however there was, it simply could not complete the charge within the time. The advent of solar panels has changed that, as it has extended the charging time.
So your shed either you will need to bring battery into house once a month for a long charge, or fit solar panels. Or the battery will not last long.
For USB not only do you need a charger able to deliver in the main 2.1 amp, but also leads with correct hand shaking, I was having problems keeping items charged, and I found the leads from Poundworld would only charge at 500 mA but the Samsung leads would charge at around 1.4 amp, I have seem fully 2.1 amp when two items plugged into same outlet, but never when charging one item, iphone/pad does seem to draw more, around 1.8 amp max. Although I know the fast chargers exist, never had one, all mine use 5 volt.
I use a Lidi 3.8A charger it will do both 6 and 12 volt and two settings up to 20 Ah and up to 140 Ah, the down side is the user has to select battery size, so after a power cut it will not switch on again until buttons are pressed, with 20 Ah mode it actually switches off completely once charged with the higher Ah rate it always charges at 0.1 amp. I plugged the charger into an energy monitor to find this out, not given in instructions the Ctek charger however will auto restart charging, but will not charge 6 volt, so not one better than other just different.
Other than in caravan where the charger is more like a power supply than charger, I don't use large amp chargers any more, the problem with high amp chargers is if a battery cell goes short circuit they try to get remaining 5 cells to the 6 cell voltage so over charge the 5 good cells producing bad egg smell and hydrogen gas with will burn rather rapid (explode) the low amp charger does the same of course, but out put is so low not enough hydrogen gas is produced to cause a danger, so high output chargers should not be left for long unattended, where you can leave the low amp on for weeks, and the difference in total charging time with 90 Ah battery, 3.8 amp charger around 18 hours and 38 amp charger around 16 hours, except for starting a car or double as power supply, better with low amp charger.
