# Lithium Battery Selection Guide

When selecting a lithium battery, the following points are generally considered:

## Voltage

The voltage of a lithium battery is represented by number of cells in series + s. The rated voltage of a single lithium battery is 3.7V, fully charged it is 4.2V, and discharged it is 3.5V. Therefore, the voltage relationship for lithium batteries with different numbers of cells is as follows:

Number of Cells Rated Voltage (V) Voltage Range (V)
2s 7.4 7.0 – 8.4
3s 11.1 10.5 – 12.6
4s 14.8 14.0 – 16.8
5s 18.5 17.5 – 21.0
6s 22.2 21.0 – 25.2

## Capacity

The capacity of a lithium battery represents how much electrical energy it can store. For example, 2000mAh means it can discharge at a current of 2000mA for one hour.

## Discharge Rate

The discharge rate of a lithium battery is used to indicate the ratio of the battery's charging and discharging current ( Maximum Discharge Current = C * Capacity). For example, for a battery with a capacity of 1000mAh, if the discharge rate is 1C, then the discharge current is 1000mA; if it is 10C, the discharge current is 10000mA.

Choosing a sufficient discharge rate is important. During the startup or direction change of the motors in a robotic system, or when trying to achieve the transient response requirements of the system under PID control, a large instantaneous output current may be required. If the current cannot be provided, it may cause a voltage drop, resulting in voltage fluctuations in the control part, or even a microcontroller reset.

How to calculate the maximum current of the system? Generally, priority should be given to power-consuming components such as motors. Since motors are inductive loads, the current during startup can reach 4 to 7 times the rated current. Therefore, in the design, it is assumed that during the motor startup, the current is 7 times the rated current.

After the overall calculation, leave a margin of 20%.

## References and Acknowledgements

Original: https://wiki-power.com/ This post is protected by CC BY-NC-SA 4.0 agreement, should be reproduced with attribution.

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