With a little bit of effort, the Cateye HL-500 can be modified to give greater light output. You only need a couple of simple tools and a budget of around SGD25 or less, depending on your ability to salvage old electrical stuff. Here’s a list of the stuff you need.

Part
Cost
Notes
Cateye HL-500 (the one that takes 2 C-size batteries and comes with a Krypton or Halogen bulb) None if you already own one. Otherwise about $15? Actually, this project may only make sense if you already have one of these lights lying around and you feel that it is not bright enough for you, but really hate to throw it away.
Maglite MSA4 bulb or Philips HPR50, 5.2V, 0.85A bulb About $7. Any equivalent bulb will do. But unless it is halogen or krypton, it will not be bright enough. The Philips bulb is better than the Maglite. Both can be hard to find.
Wires Free, just re-use old stuff lying around the house. Make it thick. Else you’ll loose power in the lines.
Crocodile clips Less than a dollar each. Buy the better ones for good contact and durability.
Yuasa NP4-6 sealed lead acid battery About $15. So far, this the only Japanese battery that’s cheap. Taiwanese version is $12. But…
DC power supply Free. You need to salvage some from old equipment. Using one from an old Motorola phone charger. Just nice because it provides 400mA at 7.5V. Open circuit voltage around 8V… if you can’t find, then you need to get one from Sim Lim.
5 ohm resistor Can’t remember. The white rectangular ones. Need to put one in series with the positive terminal when charging the battery.

You will also need a soldering iron, solder and a fine drill. After deciding that you want to do this, go find the bulb first. That’s the hardest part. Next, get the DC power supply. The battery is easy to get from Sim Lim. There is a shop in the basement that sells nothing but batteries.

Step 1: 
Remove original bulb. Put in your new “turbo” unit.

Step 2:
Wire up the stuff for your new souped up battery. Drill a small hole in one corner of the casing (the part that doesn’t get removed when you change batteries. The hole should be large enough for two wires to go through. Cut a length of a pair of wires that will run from your light to your water bottle cage. Strip the ends. One end of the first wire should be soldered to point 1 on the picture. The other end should be soldered to point 2. Then cut a short length of wire (say 5cm) and solder it from point 3 to point 4. Solder your crocodile clips to the free ends of your long wires. The crocodile clips are for attachment to the battery contacts.

Step 3:
Reassemble your light. Clip the crocodile clips onto the battery terminals and switch on your light. Everything should work.

Step 4:
Set up the charger. Strip some wire of the ends of the cable. On the positive wire, solder in your 5 ohm resistor. Then solder one crocodile clip to the other end of the resistor. Solder another crocodile clip to the negative wire. To charge, make sure you put the positive clip to the positive battery terminal. Otherwise…

Step 5:
Mount the battery into your water bottle cage. Somehow, mine snaps into place. But if your cage has different design, you may need to get some Velcro straps to hold it in place. Mount your light and wire up and presto!

Key to photo:
1-4: Battery contact solder points on stock HL-500
5: Battery
6: DC power supply
7: 5W resistor
8: The bulb (Philips)
9: The HL-500 Special Edition (hehe, with electronic power control via switching power supply and with battery low comparator.)

Specifications:
Power: 5.08W measured at 6.05V.
Low voltage threshold: 5.95V with no load
End of charge: 7.5V (with charger connected)
End of charge: 6.3V (after being disconnected for 24hrs from charge)
Disconnect your battery when your light is connected and voltage drops to 5.85V.

Talk about this guide in the forums!