# Battery Basics

## Highlights

•     Batteries are defined by their milliamp hours (mAh), discharge rate (C), and number of cells.
•     Battery charge limits: 3.5 V/cell minimum, 3.7 V/cell nominal, 4.2 V/cell maximum
•     When cutting battery wires, never cut them at the same time
•     Lithium Polymer batteries get hot during operation. Do not put them in a confined space or wrap them with insulation.
•     Water will not put out a lithium fire.
•     Charging a battery past 4.1 V/cell or draining below 3.5 V/cell can be harmful for the battery.
•     Swelling in a LiPo battery can be a sign of an imminent fire.

## Notes

There are many different types of batteries, all of which are useful in different applications. The most commonly used battery for RC applications are Lithium Polymer batteries because of their high energy density. This means that the battery stores a large amount of energy in a small package (low weight).

All batteries have an anode, a cathode, and an electrolyte. The anode is an area with a surplus of electrons, giving it a net negative charge. The cathode is the opposite of an anode: it has a deficit of electrons, and therefore has a positive net charge. An electrolyte separates the anode and the cathode to prevent electrons from flowing directly from the anode to the cathode when the battery is not connected to anything. Once the circuit is closed, electrons can flow from the anode through a wire and the circuitry it’s connected to, then back into the cathode. Charging a battery works in the opposite fashion, electrons are forced out of the cathode into the anode so the battery can be used again. Please not that not all batteries can be recharged. A battery should never have its anode and cathode connected directly to one another or the battery will short and can cause a fire.

The three main attributes that describe a battery are its voltage, capacity, and discharge rate. Batteries typically have multiple cells in them, each one having a nominal voltage rating of 3.7 V. Each cell in a battery adds 3.7 V to the total battery voltage, so a battery with 11.1 V has three cells. Capacity is typically measured in milliamp hours (mAh) and indicates how much current can be discharged from a battery in an hour. A 3000 mAh (or 3 A) battery can discharge at 3000 mA continuously per hour. The discharge rate, shown as 25-50C in the image above, can be used to determine the maximum amperage the battery can discharge at any given time. The range of discharge rates can be interpreted as 25C continuously discharged, while supporting a burst of up to 50C. $Capacity * Discharge Rate = Maximum Amperage Discharged$, so a 3000 mAh battery rated at 50C has a max discharge of 150,000 milliamps or 150 Amps. It is important to note that a battery will not last very long at its maximum discharge rate. The 3000 mAh battery with a maximum discharge rate of 150,000 milliamps will only operate for 1.2 minutes at max discharge.

$Battery Duration (minutes) = \frac{{Capacity (Amps)}\times{60}\frac{mins}{hour}}{Discharge Rate (Amps)}$
Note: It is important to match a battery with your RC application. While it will be rare for a battery to be operating at max discharge rate for long periods of time, the max discharge rate must be at least the maximum amps your vehicle will pull at any time.

LiPo batteries have a maximum charge of 4.2 V/cell and a minimum charge of 3.5 V/cell. It is important to never charge any battery cells past their maximum voltage or to discharge them below the minimum voltage. Both of these actions can cause a battery to malfunction, and may lead to a fire. When charging a battery make sure both the main plug and the voltage plugs are connected so that a charger can ensure no cell exceeds 4.2 V. The charger must also be set to the correct type of battery (LiPo in this case), the number of cells, and the capacity. Additionally, a voltage monitor should always be used on batteries. The monitor will buzz whenever a cell reaches 3.5 V and the battery needs to be changed.

Always be careful when handling lithium polymer batteries because if they are mistreated they can catch on fire. Both discharging a battery below 3.5 V then subsequently recharging it, as well as charging a battery over 4.1 V can cause them to catch fire. Additoinally, hard impacts or punctures can cause a LiPo to swell up, which is a sign of an imminent fire. If the connectors on a battery need to be charged, never cut both wires at the same time. This will cause the wire cutters to complete a circuit between the anode and cathode, causing the battery to short and catch fire. If a LiPo catches fire do NOT put water on it, that will only spread the fire. Instead cover the battery in sand or use a chemical fire extinguisher.