The security system for Diablo VT's 1998 and later includes a radio control system that must be used to start the car.  While it is possible to open the car door with the "regular" physical key, the engine will not start without pressing the security key radio control unit. There seems to be no way to simply bypass this unit. I have tried for example opening up the center dash unit of the car where the security system resides (under the radio), pulled the fuse hidden on the underside of this unit, only to find the car will not start. 

So if you are traveling out of town and the battery in your security lock system runs down you have a problem. I keep a second security key system hidden in my car just in case. Alternatively you can change the battery as described below "on the fly".  It's not difficult but you have to be handy with your hands.

Here is one way to go about it.  With two razor blades pry off the metal clip cover of the unit (fig 1). Use two blades one on each side. Fig 2 shows the unit with the clip removed. Next (fig 3) use a screwdriver to pry open the plastic unit itself. Note you really have to force them apart as they seem to be stuck together with silicon rubber cement to keep the unit waterproof.  Fig 4 shows an opened unit.  Next you will have to remove the circuit board. Again it is stuck in there with silicon rubber cement. It is soft and can be scraped away with a wooden toothpick. Do not use a metal a metal object!  The removed circuit board is shown in figure 5.  Finally note the two 3V coin batteries. With a wooden toothpick slide them out from under the spring holding them in place. When replacing with identical batteries, remember the + side is up.  Finally reassemble the unit. If you have a little silicon rubber cement all the better to render the unit waterproof.  I am told these units have batteries that last about 5 years