Lead Acid batteries (image above) are the workhorse batteries of industry. They are incredibly cheap, rechargeable, and easily available. Lead acid batteries are used in machinery, UPS's (uninterruptable power supply), robotics, and other systems where a lot of power is needed and weight is not as important. Lead acid batteries come in 2V cells, that means you can have a battery with an even number of volts. The most common voltages are 2V, 6V, 12V and 24V.

Pros: Cheap, powerful, easily rechargeable, high power output capability.
Cons: Very heavy, batteries tend to be very large bricks because energy density is very low.

Prices: A 12V lead acid battery with 7Ah of charge should run about $25.
Power Density: 7 Wh/kg.