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.
Understanding Robotics: LiPo Battery
A battery does more than set the robot voltage. Its internal resistance affects what happens when motors demand current suddenly. Every battery has some resistance inside it, even when it is healthy.
Current flowing through that resistance causes some energy to become heat and reduces the voltage reaching the robot. A pack with lower internal resistance holds its voltage better during a hard acceleration. This is why two packs with the same printed capacity can make a robot perform differently.
Older packs often develop higher internal resistance. They may seem fine with a small load but struggle when a drivetrain starts moving.
Series connections raise voltage, while parallel connections raise available capacity. In a series pack, the same current passes through every cell. One weak cell can therefore limit the whole pack.
Cells in parallel share current, but they must be at nearly the same voltage before being connected. Battery balance matters because no two cells age in exactly the same way. A balance charger measures individual cell voltages through the small balance lead.
It slows or redirects charging for cells that are reaching full charge first. Charging only through the main power lead cannot correct an imbalance between cells.
Robot builders should size the whole power path, not only the battery. Wires, connectors, circuit breakers, motor controllers, and switches must safely handle the expected current. Thin wires have more resistance, so they waste energy as heat and create extra voltage drop.
Loose connectors create resistance too. A connector that becomes hot during use needs attention. A fuse or breaker protects wiring from a fault, but it does not make an undersized wire safe.
Motors can draw much more current at startup or when stalled than while spinning freely. A robot that pushes against a wall, lifts too much mass, or has a jammed mechanism can create these high currents.
Good handling prevents most LiPo problems. Charge packs on a nonflammable surface in a clear area, using the correct charger program and a safe charging bag or container. Stay nearby while charging.
Do not charge a pack that is swollen, punctured, unusually hot, wet, or damaged after a crash. Swelling means gases have formed inside, which is a warning that the pack is failing. After a match, let a warm battery cool before charging it.
For storage longer than a short break, use the charger storage setting rather than leaving the pack full or empty. Label packs with their purchase date and track their condition. Retire packs that show repeated imbalance, swelling, damaged wires, or poor performance under load.
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.