When you say "Devices" are you referring to you comms equip such as a cell or smartphone? If so, I have never had a problem with them drawing down my vehicle battery. WTBS these devices seem to charge faster if you are in airplane mode or the device is off. I have also use a very small 75W inverter (draws minimal) and use a 110V charger- seems to be faster.
I have a back up a portable pack (emergency one I have at home or I bring on the boat etc. it also has booster cables etc. has a built-in in 110V inverter, USB etc. There are several types of these out there
There are also USB battery packs (various sizes/capacities) they can be charged from a home outlet/or your 110 when genny is running, or off the vehicle
LOTS OF OPTIONS