Batteries in Bicycle Lighting

General Characteristics

• With batteries, you can have light without making the pedaling any heavier.
  
• There's light when the bike is not moving.
  
• Batteries need regular recharging or replacement.
  
• A high-capacity battery or plenty of spare batteries are real heavy.
  

Battery Types

• Disposable Alkaline: For occassional use, low self-discharge, poor performance in cold weather
  Easy to find in most places. Need no care, just throw away when depleted. In third-world countries, the quality of disposable batteries is often poor, so they deliver very little light for a rather short time. In this case, use rechargeable batteries.
• Rechargeable Lead Acid: Most economical for high power (5-50W), very heavy, poor performance in cold weather
  These batteries are rather cheap for their capacity. You can even find inexpensive used ones at computer shops (they are sitting inside UPS' and are preventively changed every few years). They need to be overrated, as deep-cycling them reduces their life substantially. If you need 5Ah, buy one with 10Ah. They need simple constant-voltage charge. Chargers / universal regulated power supplies are commonly available.
• Rechargeable NiCd or NiMH: Most economical for low power (1-5W), still somewhat heavy, poor performance in cold weather, high self-discharge
  These and their chargers are widely available. Modern high-capacity cells often pack the same capacity into a much smaller and lighter pack than those used 10 years ago. AA-cells are usually the best value-for-money and offer the most energy for their volume. Today, NiMH is the battery technology that's most widely used in bicycle lighting.
• Rechargeable Li-Ion: Somewhat expensive, hence most adequate for low-power systems. Rather light, good cold-weather performance
  

  Li-Ion batteries used to be very expensive and complicated to handle. There's a moderate risk of explosion when they are treated wrongly. Nowadays, they are widely used in cellular phones. Cheap, Chinese-made replacements can be found at online stores and night markets. Some of these cheapies are not bad. Often they include a charge-/discharge control circuit that prevents explosion when being charged by 5V constant voltage. The circuit also eliminates the risk of destruction from deep-discharge. I purchased a number of identical Li-Ion batteries for a popular Nokia phone, then connected them in parallel. Works fine, holds a lot of energy for its weight. See the picture to the left.