OK, I am not familiar with battery-backed systems with a separate switch to turn off the light and still power the charger.
In my area it is very popular option to have battery units installed inside the wiring compartments of standard lighting fixtures. You can get these UL life safety battery units for LED, Flouresent and HID fixtures. Here is some info from one maker
http://www.bodine.com
These units require a 24/7 feed for the battery but have a lead for switching the lamps on and off.
I apply the KISS principle when it comes to life safety.
Rock and roll all night and party every day?
Kidding, I understand the principle.
The more stuff you have that can make the lighting not come on the less safe it is.
No doubt, but at the same time sometimes the fixtures used for emergency lighting are the same ones used for general lighting in stores, schools, offices etc. so being able to turn them off is in my opinion a good reason to add some stuff.
Check out these
http://www.functionaldevices.com/lighting-controls/ul924.php
http://www.hubbell-automation.com/products/bypass_relays/
http://www.wattstopper.com/products...y-lighting-control/elcu-200.aspx#.U5XbaHZhsTA
Again this is a single posting from the OP with no further detail.
That is a good point, in our own minds we look at it from our own experiences and tend to assume that the OP was doing the same type of work we do.
The simplest way to ADD emergency lighting and lighted exit signs to a facility is to use simple battery backed up packs with batteries and lights as a unit. To add exit signs is to use the packs with attached exit signs. If you labor costs anything, it is probably also the cheapest. [Hang the pack, tap into the power feed for the area lighting, done.]
I agree that is the simplest and cheapest way but this is where I applied the KISS principle. I just assumed no qualified person would be asking about switching off the feed to an emergency battery pack. That could be a mistake of mine.
There is the more complicated problem of knowing where to put the lighting and the exit directional signs, accounting for the proper footcandles, and accounting for a single failure while still maintaining lighting levels.
In my experience almost no one voluntarily decides to add emergency lighting, they have usually been directed to add emergency lighting and / or Exit signs in spots X, Y and Z by an inspection official. YMMV.
If you are using a low voltage central battery, you have to contend with voltage drop problems in the circuits and the zillion relays to detect local power failures [if they still install such systems.] If you use remote heads with small battery packs you still have to look out for voltage drop.
The last central battery unit I put in that operated at 12 volts was in 1984, as you say voltage drop is an issue. Now we are installing central battery units that have inverters on board that allow us to distribute at 120, 208, 277 480 etc. Some of them are large
http://www.dual-lite.com/products/trident_trn_series/?pbid=405323
This overcomes voltage drop issues but it is installed just like a generator system, all circuits must be run independently of other circuits. Its an expensive installation but an option if the project cannot use a generator for some reason.
If they already have an emergency generator one would be very surprised that emergency lighting was not already provisioned.
I still find inspectors such as fire officials asking for more at yearly inspections. Particularly if the lay out of a room has changed or its an old building that they want to bring up to modern standards without hurting the owner with too much work at once.
It will be interesting to here from the OP.