It's all a function of the Amp-Hour ratings of the batteries, for the most part, regardless of battery type. A 20 Watt bulb will draw about 1.6 amps at a true 12 volts, or 1.38 amps at 14.4 volts (10 cells vs. 12). Most 12 volt bulbs will run fine, if not better, at 14.4 volts. 20 Watts divided by whatever voltage you want to use equals the amps required. A 1.6 amp-hour 12 volt battery, will run your 20 watt bulb for an hour, but only in a perfect world. The best bet would be to attempt to triple the amperage draw of the bulb in the amp-hour rating of the battery (1.6Amp x 3 = 4.8Ah) batter. That way you could expect a solid 2+ hours of light in varying conditions.
Charging quality is as much, if not more, important than battery quality. A high quality charing system will get the most power out of fair batteries. A crappy charger can not correctly charge great batteries, and may even cause permenant damage. Not something you asked about just an editorial note....
Batteries off-gas when charging. Many DIY canister light builders use a screw & O-Ring to seal a vent hole on the battery compartment during the dive, and remove the screw during charging & storage to vent and dry the battery box. As far as an explosion goes, a hot surface would probably need to be soooo hot. soft plastics and other things would be a first indicator. Some switches spark when they make or break, so if the light is not vented during charging, and your switch is on the battery box, sparking may be a consideration.
Hope this helps,
G
