A robot is not necessarily a machine with arms or legs or even wheels. If the robot is a stationary machine as in case of robots like arm(welding and assembly line) robots, then the power supply is usually as simple as having transformers or SMPS etc. then using regulators for supplying the required currents.
If however the robot is a mobile autonomous robot, then we need to consider batteries. There are various types of batteries each with their own pros and cons. eg. Lead acid batts are longer lasting but are really heavy etc. Here is some info. as taken from the robot builder’s bonanza book.
Zinc batteries are the staple of the battery industry and are often referred to simply as “flashlight” cells. The chemical makeup of zinc batteries takes two forms: carbon zinc and zinc chloride. Carbon zinc, or “regular-duty,” batteries die out the quickest and are unsuited to robotic applications. Zinc chloride, or “heavy-duty,” batteries provide a little more power than regular carbon zinc cells and last 25 to 50 percent longer. Despite the added energy, zinc chloride batteries are also unsuitable for most robotics applications. Both carbon zinc and zinc chloride batteries can be “rejuvenated” a few times after being drained. See the section “Battery Recharging” later in the chapter for more information on recharging batteries. Zinc batteries are available in all the standard flashlight (D, C, A, AA, and AAA) and lantern battery sizes.
Alkaline cells use a special alkaline manganese dioxide formula that lasts up to 800 percent longer than carbon zinc batteries. The actual increase in life expectancy ranges from about 300 percent to 800 percent, depending on the application. In robotics, where the batteries are driving motors, solenoids, and electronics, the average increase is a reasonable 450 to 550 percent.
When you think “rechargeable battery,” you undoubtedly think nickel-cadmium—or “Ni-Cad” for short. Ni-Cads aren’t the only battery specifically engineered to be recharged, but they are among the least expensive and easiest to get. Ni-Cads are ideal for most all robotics applications.
NICKEL METAL HYDRIDE
Nickel metal hydride (NiMH) batteries represent one of the best of the affordable rechargeable battery technologies. NiMH batteries can be recharged 400 or more times and have a low internal resistance, so they can deliver high amounts of current (read more about internal resistance and current in “Battery Ratings,” later in the chapter). Nickel metal hydride batteries are about the same size and weight as Ni-Cads, but they deliver about 50 percent more operating juice than Ni-Cads. In fact, NiMH batteries work best when they are used in very high current situations. Unlike Ni-Cads, NiMH batteries do not exhibit any memory effect, nor do they contain cadmium, a highly toxic material.
While NiMH batteries are discharging, especially at high currents, they can get quite hot. You should consider this when you place the batteries in your robot. If the NiMH pack will be pressed into high-current service, be sure it is located away from any components that may be affected by the heat. This includes any control circuitry or the microcontroller.
A lot more material is available on the internet about batteries and battery care and recharging. From my experience however this is what I have to share.
For very small and short lived projects, you might consider simply using a step down transformer and using a regulator with it. The adapter is reuable and is cheap and is easy to use and store.
For mid-sized projects with time limits, I recomend using zinc or alkaline batteries. They are cheap and easily available and give you mobility.
For long lived projects that don’t require immediate ‘put the battery on the board’, consider going for a 12V lead acid battery. They dont need frequent recharges and give you mobility more or less.
For more permanently mobile battery solutions that I used, I used rechargable Ni-MH batts. Lots of em. clubbed with regulators. I bought them in bulk and I usually have one pair in the recharger all the time cause of my usage.
I simply put two packs together and on a small PCB put an 7805 with the necessary components and a whole lot of connectors. And this is what I use.