Batteries

From Combat Robot

Batteries are devices made of plastic, metal, and chemicals that store energy in the form of chemical potential energy.

Considerations

When looking at batteries, it is important to note their specifications. There are many factors that need to be taken into consideration when choosing the right batteries for the job.

• Voltage - Each type of battery chemistry has a "nominal cell voltage". This is the voltage that one battery "cell" will ideally produce under average load. In order to obtain a voltage matched to the motors aused in a bot, multiple cells can be linked together in a series circuit.

• Energy Capacity - The length of time that a battery can output a certain amount of current defines energy capacity. Capacity is generally measured in amperes * hours (Ah) or miliamperes * hours (mAh). Batteries with greater Ah ratings will be able to run a robot longer than those with lower ratings. To obtain a higher energy capacity in a battery pack, sets of cells may be wired in a parallel circuit.

• Energy Density - Some bettery chemistries "pack in" greater energy capacity per unit of weight than others. Because weight is such a key consideration in building a combat robot, it is important to use the lightest suitable batteries that posess enough energy to run the bot for the time required.

• Current Rating - A battery pack has a certain amount of internal resistance that limits how much current can flow to the rest of the bot. Generally, a higher capacity cell can deliver more current than a lower capacity one, but cells of similar capacity can have greatly varying current ratings.

Classifications

Several common types of battery chemistries used in combat robots are:

• Sealed Lead Acid - Able to dump enormous amounts of amps, but weighing a great deal for their capacity, SLAs were once commonly used in Heavyweight and Super heavyweight designs where there was a call for enormous amounts of amperage to get 340 pounds of mass rolling around with a 15HP Etek, or when a 100-pound weapon is being spun up in a few seconds. Commonly available in 6V and 12V, SLAs required only simple charging systems and were easy to maintain.

• Nickel-Cadmium - NiCads are an old standby in bots. They're cheap, plentiful, and can provide solid amperage. Cells are nominally 1.2V per cell (1.3 to 1.5 volts at full charge, and about .9 volt at discharge), and range from very small energy capacity like 250mAh to large 3000mAh cells and beyond.

• Nickel-Metal Hydride - NiMH cells are a newer type of cell, with greater energy density than NiCads and no 'memory effect'. Cell voltage is aboute the same, but the maximum charge rate is lower

• Lithium-Polymer - LiPo's have the highest energy density of all common battery chemistries. These cells are also high in price, making them prevalent mainly in the "insect" classes, where speedy evolution has quickly driven builders to the higher-powered parts. Cell voltage is nominally 3.7V, and capacities are available up to ~6Ah recently. While great care must be taken when charging all types of batteries, mischarging of LiPo's is particularly dangerous as they can actually burst into flame! Discharge rates higher than specified for the cell can also cause damage from overheating.

• Alkaline - Alkaline batteries are not generally used in robots because they cannot supply high amperage and cannot be recharged once depleted.

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See also: Battery charging