Ohm's law!

The issue is that the bulb won't explode but the battery might.

That projector bulb that you have is probably rated 12V at some wattage - like 80 or 120 watts. Ohm's law can give you the current required by working the formula E*I=W, it your 12 V (E) lamp uses 120 watts of power that would take 10 amps of current to light it up.

Batteries are rated in their output voltage, watt hour and current capicity. Get one of those three wrong and the battery is going to melt down. You can't start a 12 volt 80 amp car starter motor with a 80 volt 12 amp battery.

Typically when you connect a battery like a 9V square battery to something like a 12 volt halogen lamp is that the battery sees the lamp as too much of a short circuit - the lamp tries to draw too much power, too many amps from the battery and it gets hot enough to pop/explode. Those small batteries are made for high voltage (9V) and tiny current devices like radios. They supply wattage measured in tenths of a watt.

With enough wattage the battery lasts much longer. The little SureFire lamps use like 15-20 watts worth of power heating their batteries up really fast. Try connecting your 12V lamp up to your car battery and you'll more than likely be all right, just have a pair of jumper cables handy if that thing is like 200 watts.

Soldering a regulator in the circuit would help if you had a 9V bulb and a 12V battery, but not in your case.  A diode prevents electricity from traveling one way whil allowing it to travel the other way, this also will not help you.

Just hook it up and see how much light you get for how long, but with a 50W bulb I doubt you get much, if anything out of it.  Put the battery on a glass plate or something first, just in case, but I don't think there is much danger of it exploding.  It should just drain quickly.

Does it look like this ?

[img]www.gelighting.com/eu/business/images/p_dicroic.gif[/img]
[img]store4.yimg.com/I/elights_1672_3526289[/img]

If so, these lamps are designed to be powered with a transformer matching the lamps wattage AND voltage. This type of lamp is made for lighting fixtures and they require a constant voltage AND wattage input to work properly. Another draw back is the fact that if you touch the halogen capsule itself with your fingers the oils in your skin are enough to cause the lamp to melt when ignited. The capsule may also rupture or,in the worst case senario explode. This is do to the intense heat these little dudes create when burning.