Lithium polymer batteries, often called LiPo batteries, are common in robotics because they can deliver high current from a small, lightweight pack. This makes them useful for drivetrains, arms, flywheels, and other systems that need bursts of power. Understanding LiPo ratings helps teams choose a battery that is strong enough without overloading wires, motors, or speed controllers.
Good battery habits also improve robot reliability during practice and competition.
A LiPo pack is made of cells connected in series, so the total voltage depends on the number of cells marked by the S rating. The C rating estimates how much current the pack can safely provide, based on its capacity. Under heavy motor load, the battery voltage can drop temporarily, which is called voltage sag and can cause slower motion or controller resets.
Safe charging, balancing, storage voltage, and physical inspection are essential because damaged or misused LiPo packs can overheat or catch fire.
Key Facts
- Nominal LiPo cell voltage is about 3.7 V per cell.
- Fully charged LiPo cell voltage is about 4.2 V per cell.
- Pack voltage = number of series cells × cell voltage, so a 3S pack is about 11.1 V nominal.
- Maximum continuous current = capacity in Ah × C rating.
- A 2200 mAh 25C pack has maximum current = 2.2 Ah × 25 = 55 A.
- Storage voltage is usually about 3.8 V per cell for LiPo packs.
Vocabulary
- LiPo battery
- A lithium polymer battery is a rechargeable battery that stores energy in lithium based cells and can provide high current for its size.
- S rating
- The S rating tells how many cells are connected in series, which determines the pack voltage.
- C rating
- The C rating describes the safe discharge rate relative to the battery capacity.
- Voltage sag
- Voltage sag is the temporary drop in battery voltage that happens when a large current is drawn by motors or other loads.
- Balance connector
- A balance connector gives a charger access to each cell so it can keep all cell voltages equal during charging.
Common Mistakes to Avoid
- Ignoring the S rating when choosing motors or controllers is wrong because a higher cell count raises voltage and can exceed a component's limit.
- Treating mAh as current is wrong because mAh measures capacity, while current delivery depends on capacity in Ah multiplied by the C rating.
- Charging without balance mode is wrong because individual cells can drift apart in voltage and one cell may become overcharged even if the total pack voltage looks safe.
- Using a puffed, punctured, or hot battery is wrong because physical damage or swelling can indicate internal failure and a serious fire risk.
Practice Questions
- 1 A robot uses a 4S LiPo pack. What is its nominal pack voltage if each cell is 3.7 V?
- 2 A 1500 mAh LiPo battery has a 40C continuous discharge rating. What is its maximum continuous current in amperes?
- 3 During a pushing match, a robot slows down and its controller briefly resets even though the battery was charged before the match. Explain how voltage sag could cause this and name one way to reduce the problem.