Power

The power system of a robot provides energy to the other systems on the robot. Generally this will be electrical energy and the power source of the robot will be a battery.

## Considerations

The main thing that needs to be considered when designing the power system are the requirements of the other components of robot. Each circuit will require a particular voltage level and have a particular current demand. The power supply you choose will have to have the correct voltage output and be able to supply the sum of all the current demands.

### Different Voltage Levels

It is less complex to have all circuits operating off the same voltage level as the power source. However, sometimes this cannot be the case and additional circuitry must be used to provide a new supply at a different voltage level. Linear regulators or switch mode power supply modules (BECs) can be used to this.

Additionally these circuits can ensure a constant voltage level as the output voltage of most batteries change as they discharge.

### Current Draw

Each component in your circuit can require a range of current values to operate. For example, a brushed DC motor that is stalled will draw 0.5A. However, when it is freely rotating it will only draw 0.1A.

Hence it is advisable to design for the *worst case scenario* and assume maximum current draw for all components. Then ensure that your chosen supply is rated for this current draw (plus a little extra to account for errors).

If you include any additional power circuitry you will need to account for how much current this will use. A linear regulators input current is the same as its output current while a switching regulator's input current will be different to its output.

### Power Source

Most combat robots will use some form of battery to provide portable electrical energy. The size and type of battery needs to be chosen carefully. There a range of battery technologies to choose from each offering various relative advantages in terms of size, energy capacity, price and safety.

Battery life is determined by the energy capacity of a battery. As the rate of energy a robot consumes will vary depending in how it uses its motors it is difficult to estimate how long a battery will last. One method is to use a "worst case" analysis again. This involves assuming the robot's circuitry will draw maximum current all of the time. Then in reality the battery will last longer than this time.

Another thing to consider when choosing a power source is its re-charge rate as you will need to ensure you have enough spare batteries to ensure your robot is always ready to fight.

### Protection

An often overlooked aspect of the power system are the protection elements which protect the robot's circuitry, its users and the power source from damage.

Elements such a fuses and low power indicators are essential to ensure no harm comes to the robot from over discharging the battery or from short circuits.

Elements such as power switches and power indicators protect the user and others by allowing users to see when the robot is active and control if the robot is active or not.

## Li-Po Batteries

Li-Po batteries are one of the popular battery options for RC model enthusiasts. They are low-cost, have a high power density and can have high current outputs. Hence ideal for powering combat robots.

### Safety

There is a price to pay for having such a good energy density. These batteries are very sensitive to over discharging or charging, discharging or charging too quickly, impact, puncture and temperature. If put under too much strain these batteries have a reputation for catching on fire violently along with releasing all types of toxic fumes. Hence it is imperative that they are treated with respect.

To avoid Li-Po fires it is good practice to follow the following guidelines.

* Always store li-pos in a hard container in a fireproof bag out of direct sunlight. Li-Pos should also be put on a storage charge before storing and should not be connected to anything (including battery alarm).
* Always charge li-pos with a genuine balance charger in a fireproof bag.
* Stop using or charging a Li-Po battery if it feels warm or swollen.
* Always use a Li-Po alarm to ensure that the battery does not become over-discharged.
* Do not puncture or compress the Li-Po battery
* Do not attempt to open the battery or expose the individual cells inside.
* Do not short circuit the battery. A suitable fuse should be included to stop your circuit drawing excess current.

### Voltage (S number)

When selecting a Li-Po careful attention should be paid to the s number as this indicates the nominal voltage output of the battery. S indicates the number of 3.2V battery cells arranged in series. Hence the output voltage of the battery is 3.2×S number.

### Capacity

The capacity of Li-Pos is generally given in milliamp hours. This indicates the number of milliamps the battery can supply for one hour. To calculate how long the battery will last connected to your circuit (in hours) divide the batteries capcity in mAh by the draw of your circuit in mA. To convert to minutes multiple the result of this by 60.

### C rating (Max current)

The C rating of a battery indicates how much current the battery can be charged or discharged at. To obtain this rating in Amps multiply the batteries C rating by the its capacity in Amp hours (Ah) . Most capacities are given in mAh, divide this by 1000 to convert to Ah.

The maximum discharge current should be larger than the maximum possible draw of your circuit.

Additionally you should protect the battery by including a fuse inline with the positive supply. This fuse should be rated at or below the maximum discharge current of the battery. The fuse's rating will also need to be larger than the maximum draw of.