Let me start by saying that the point of this question is to figure out if I can do this with items on hand instead of buying a UPS. I have a cable modem and wireless router which require 12V 750mA and 12V 1000mA respectively. In the event of a power outage, I'd like to be able to manually disconnect them from their wall warts and then connect both (in parallel) to one battery which will have been kept on a "battery tender" up to that point. In very short tests, each device seems to work fine when powered individually from a 12V sealed AGM battery that I have on hand. I'm assuming that the modem and the router have their own internal regulators to some working voltage lower than 12V, but I don't really want to crack them open to see. What's the worst case scenario if I just hook the two devices to the 12V battery in parallel and let them run until one or both stop working? Maybe each device will turn off when the battery voltage falls below the device's regulator's cut off voltage? Maybe cut off voltages will never be reached and the battery will fully discharge? Is there significant risk that either end would damage the devices, or will their (assumed) regulators handle the situation gracefully? I understand that full discharge isn't ideal for the battery but one website said an AGM will still get something like 150 cycles even when each cycle is a 100% discharge. Power outages aren't frequent, so 150 cycles is plenty. Is there anything else I should be considering? Thanks!