All about batteries

This page attempts to explain batteries: the different types that you will encounter in robotics and some terms associated with them.

There are five types of rechargable batteries used in robots. Words in red are explained in the glossary below.

Lead Acid (Pb) These are like car batteries, although they come in smaller sizes than that. They aren’t used in antweights, except sometimes to power 12v battery chargers. Each 12v lead acid battery consists of 6 (2 volt) cells wired in series.

Nickel Cadmium (NiCd) These are smaller and lighter than Lead Acid, they can be used in remote control transmitters and receivers. They have a very high discharge rate but suffer from memory effect. Each NiCd cell has 1.2 volts, so eight wired in series would give a voltage of 9.6v.

Nickel Metal Hydride (NiMh) These look the same as Nickel Cadmium, but they don’t suffer from the memory effect. Although their discharge rate isn’t as high, they are more than good enough for antweight robots. Each NiMh cell has 1.2 volts, so four wired in series would give a voltage of 4.8v.

Lithium Polymer (Li Po) Not to be confused with Lithium “button” batteries, these are much smaller and lighter than previously used batteries for the same power storage and output. Li Po batteries are rectangular and if their metal film is pierced then the battery will not work. They have a very high discharge rate and are the preferred battery for antweights. They must be used with care, as over discharging or over charging can lead to the battery not working, and in the worst case, a fire. One symptom of when a Li Po is “blown” and will no longer work, is that the film cover swells, and the battery can be squashed and is no longer firm. Care must be used when handling these batteries. They need their own special type of charger. Each LiPo cell has 3.7 volts so two wired in series would give a voltage of 7.4v.

Lithium Ion (Li Ion) These are chemically similar to LiPo batteries, but physically the cases are harder. They are used in mobile phones and laptops. Lion battery cases are cylindrical and harder than Li Pos and are more resistant to physical damage than Li Pos. So far these have not been used in antweights. Care must be used when handling these batteries. They need their own special type of charger.

Charging

Tx/Rx charger  Sometimes come with a kit, or can be bought separately. Sometime just for NiCd but can be NiMh as well. Charging ampage cannot be altered.

Li Po charger  Available in many formats Usually require a 12v power source. Cheaper ones do not have balancer sockets. On board electronics vary the output voltage to give a perfect charge.

Universal charger  Usually four multifunction buttons at the bottom. A little complicated to use at first, but they usually charge all the types of battery mentioned here. Price ranges from £40 to hundreds. Usually require a 12v power source. Sometimes includes balancer sockets

Charge bag  Fireproof bag for safe charging of Lithium batteries Some (heavyweight) competition organisers insist that these be used when charging Lithium batteries

Glossary

• Amps and Ampage Amps is a measure of the amount of current that flows from the battery
• Balancer Batteries in series can have different cells at different voltages. If they are all then charged in series then one of the cells may be overcharged and become unusable. A balancer connects each cell in the battery to the charger so that it can ensure that all cells are individually charged correctly. Batteries with this facility have extra leads and chargers have balancer sockets.
• Battery More than cell one wired in series is a battery.
• Cell One cell all alone is called just a cell.
• Charge rate The speed at which a battery can be charged. Charge rate is measured in C (that is the charge capacity of the battery). A battery with a capacity of 100mAh will have a charge rate of 100mA. Lithium batteries should not be charged at a rate higher than 1C.
• Discharge rate The speed at which a battery can release its energy. The higher the better when you’re designing high ampage robots like spinners. Discharge rate is measured in C (that is the charge capacity of the battery). A battery that holds 100mAh and has a discharge rate of 5C will be able to discharge 500mA (although the actual discharge rate may be a lot less, depending on the robot’s motors etc).
• Memory effect Some types of batteries hold less charge after they have only been partially discharged before being charged again.
• Milli-amps (mA) one thousandth of an amp. A typical antweight battery will be discharged at between 50 and 500mA during combat, depending on the number of motors and weapons.
• Milli-amp hours (mAh) The capacity of a battery. A typical antweight robot will have a capacity of between 100 and 300 mAh.
• Parallel cells would be connected together side by side, with each battery’s positive ends connected together, and each negative end connected together. This would increase the current flow but not the voltage.
• Power source Some chargers require a 12v power source as input voltage rather than 240v mains voltage. A 12v lead acid battery can perform this function, or power supplies can be purchased. It’s also possible to use a modified PC power supply, or a power supply from an unused household device e.g a radio or printer.
• Series cells would be connected end to end, with each battery’s positive end connected to the next battery’s negative end. This would increase the voltage but not the current flow.
• Volts and Voltage is the potential difference, or “pushing power” of a battery. If all other things were the same, then a battery with twice the volts would deliver twice the amps.