"We have found all the iOS and Android devices charge and operate no matter what the voltage of our battery
(plugged in via a 12v USB Adapter). I've read they only need 6v to 12v."
The 12v-to-USB adapters today are almost ALL dc-to-dc convertors which have fairly low overhead, low loss, and yes, they should be quite happy to work from a 10v dead battery
all the way up to a 14.4v alternator
. How far beyond that will just depend on the luck of the draw. There is junk on that market as well as fine product.
My friend went through four of the two-buck adapters in six months. I spend more like $5-10 for a known brand name with a higher power spec, and so far none have failed.
"The laptops will charge very slowly if they are turned off and the battery is producing less than 13v. They will slowly discharge if turned on, plugged in with less than 13v." Part of that will be the power rating of the adapter. The USB standard only calls for 500mA but many smartphones require twice that much and computes obviously more. SO, you look for chargers rated at least 1 Amp (1000mA) and can easily find them rated with one 1A port and a second 2A port on the same adapter, still only ten bucks.
from a USB port...tha is kinda pushing things at best, since the USB port in the laptop
is fused and may not tolerate charging
USB3 standards call for much higher power capacities though, so if you see any devices supporting USB3, go for them now, even if your gizmos don't support it yet.