Batteries

This was the hardest part for me and there where a few questions needed to be answered before I decided what batteries to get:

1. How wide is the canister?
   - The pipe I was going to use has a radius of ~100 mm (inside, 110 outside) so the area and the battery needed to fit within that radius.
   
2. How many Watt is the bulb on? -> What bulb to get?
   - I choose to use halogen bulbs because they are cheap to get and there is some bulbs that are made to shine more with less watt. An Osram IRC 35 W equals an normal 50 W halogen lamp. You could read about that you up to 30 % in energy using IRC-lamps. Here at Baggbo-dives homepage you can read an (in swedish) comparison between different bulbs: http://www.baggbodykarna.org/article.cgi?sid=82 So I decided to use an Osram IRC 35 W because I thing I just need an lamp with 50 W to begin with...
   
3. How long should the lamp operate for?
   - I wanted it to last at least 2 dives and still have some left of the batteries because you can't run the butteries down to empty which will damage them.
   
4. What batteries to use? (The final and hardest question of these 4 questions)
   - So to get roughly 2 dives (between 1 and 1½ hour) out of my dive lamp with a 35 W Osram bulb and have about 1/3 left in my batteries after the 2 dives, how many Ampere Hours do I need the batteries to be on?
   
   Here the mathematics begins:
   P = U * I (P = Watt[W], U = Volt[V], I = Ampere [A, Ampere per hour])
   'U' is simple, 12 V is standard for the bulbs and contacts to use. The 'P' is 35 W (from the bulb), so to get the 'I' = P / U = 35 W / 12 V = 2.92 A is this bulb using per hour.
   
   So for using this bulb (35 W) for 1½ hour and have at least 1/3 left you need an battery of: 2.92 A * 1½ = 4.38 A is the bulb using for 1½ hour of diving.
   
   Adding 1/3 to that gives us: 4.38 + (4.38/3) = 5.84 A which means that you need a battery with at least 6 Ampere Hours that fits in to a pipe with the radius of no more than 100 mm.
   
   I got so confused looking at the different homepages, batteries, prices, radius and so on that I put up it all in a table: ~
   
   
   
   So from these answers and this table I will buy the two 6 V / 7 AHrs from Biltema (http://www.biltema.se/products/product.asp?iItemId=95120)
   
   This means that the battery will be 302 mm long, have an radius of 100 mm, have 7 Ampere Hours of power and will cost me 180 swedish kronor.