From BEAM Robotics Wiki
- A combination of one or more electrochemical cells, used to convert stored chemical energy into electrical energy.
Most BEAMbots are solar powered, so batteries aren't usually a huge issue. Some BEAMbots, though, have sufficiently-high energy requirements that they really have to run on batteries. These tend to be more compex robots such as walkers.
Once you've made this decision to use batteries, have a couple of ways you can go:
You'll pick your batteries based largely on what's available (given your budget), whether your bot will be recharging the battery from solar cells (if so, you'll want a fairly low voltage battery), and how big your bot is going to be (9V batteries are none too light).
 Battery chemistry
A number of battery chemistries are available. The following comparison chart comes from the January, 2002 issue of Embedded Systems Programming:
|Chemistry||Vnominal (V)||Max. load current (C)||Energy by weight (Wh / Kg)||Energy by volume (Wh / L)||Operating Temperature (deg. C)||Rechargeable?||Life cycle (recharges)||Shelf life / self discharge|
|Alkaline||1.5||150||375||-20 to 55||No||-||5 years to 80%|
|NiCd||1.25||>2||45 - 80||125||-40 to 60||Yes||1500||20% / month|
|NiMH||1.25||0.5 - 1||60 - 120||180||-20 to 60||Yes||500||30% / month|
|Lithium Ion||3.6||1||100||270 - 325||-20 to 60||Yes||50 - 100||10% / month|
|Lithium polymer||3.7||0.2||120 - 160||230 - 270||-20 to 60||Yes||500||10+ years|
|Lead acid||2||0.2||30||80||-20 to 60||Yes||200 - 500||5% / month|
|Zinc air||1.4||300||1150||-20 to 60||No||-||3-12 weeks to 50%|
|Silver Oxide||1.55||130||500||-20 to 60||No||-||2 years to 84%|
Note that here load current is expressed in units of C (which is just the Amp-Hour rating of the battery divided by 1 Hour).
 Battery Availability
A number of types of small batteries are available to us:
- "Button" cells -- 1.25V each; alkaline or NiCad; these are the smallest in size, but also have a fairly low energy storage capacity
- "9V" -- available in alkaline, NiCad, NiMh; you'll need to use a voltage regulator to get the output voltage down to something CMOS can stomach (usually, 5V).
- 9V battery cells -- alkaline only. 9V alkaline batteries are made up of 6 clustered cells in a can. You can pull open this can (carefully) with pliers to pull out the small cells (which are a bit smaller than AAA batteries). Since these aren't rechargeable, I don't use them much.
- Cordless phone 'batteries -- these are easily salvaged (and in high availability, if you go through cordless phones as often as I do) NiCad batteries -- most output 3.6V, which is just right for most BEAMbots.
 Also See:
 External References
- Wikipedia Article: Battery (electricity)
- Wikipedia Article: Internal resistance
- Wikipedia Article: List of battery types
- Wikipedia Article: List of battery sizes