Now that we have all had electrical 101, we can get back to the question at hand...
The diy battery backup is as simple as it seems. Likely there is a relay on the input side of the controller where the normally closed position is connected to the 12V jack, such that when 24V is applied, the relay goes to the normally open position allowing 24V to the controller. Upon loss of power, the relay should fail to the normally closed position which will have a 12V battery hooked to it.
It's easy to test, just hook a 12V battery to the 12V port on the controller while the 24V plug is disconnected....you'll find that the controller powers up...and you're good to go. The Tunze setup is just the same, they just don't sell a shiny super whamidyne setup like Ecotech, choosing rather tell you how to hook a battery to it.
While this is speculation, I would hope that Ecotech uses a 12V regulated Lithium pack, given the price tag of the unit. One can build a reasonably similar unit with NiMH or lead acid batteries for a fraction of the cost of the $160+ that that one retails for....a Lithium based setup would be smaller, last longer per charge, and provide a much longer service life do to the fact that a lithium battery doesn't have the memory like other batteries....which would almost justify the parts. A decent Lithium battery pack will run $50, and a charger $30-40...which would explain the retail of $160. After all, they need to pay the bills.