An 18650 lithium-ion battery pack is a common power source for robots because it stores a lot of energy in a small, rechargeable format. Each 18650 cell is a cylinder about 18 mm wide and 65 mm long, with a typical nominal voltage of 3.6 V or 3.7 V. A robot often needs more voltage, more capacity, or more current than one cell can safely provide.
Engineers solve this by connecting cells in carefully planned series and parallel groups.
Series connections add voltage, while parallel connections add capacity and current capability. A pack labeled 4S3P has 4 cell groups in series, and each group contains 3 cells in parallel. Nickel strips, welds, balance wires, and a battery management system, called a BMS, help the pack deliver power safely.
The BMS monitors cell voltages, limits dangerous current, balances groups during charging, and protects the robot from overcharge, overdischarge, and short circuits.
Key Facts
- One 18650 Li-ion cell has a typical nominal voltage of about 3.6 V to 3.7 V.
- Series voltage adds: Vpack = Nseries x Vcell.
- Parallel capacity adds: Capacitypack = Nparallel x Capacitycell.
- Pack energy is estimated by E = Vpack x Ah, measured in watt-hours.
- A 4S3P pack made from 3.7 V, 3000 mAh cells has Vpack = 4 x 3.7 V = 14.8 V and capacity = 3 x 3.0 Ah = 9.0 Ah.
- A BMS protects the pack by controlling overcharge, overdischarge, overcurrent, short circuit risk, temperature limits, and cell balancing.
Vocabulary
- 18650 cell
- A rechargeable cylindrical lithium-ion cell about 18 mm in diameter and 65 mm long, often used in battery packs for robots and electronics.
- Series connection
- A circuit arrangement where cells are connected end to end so their voltages add together.
- Parallel connection
- A circuit arrangement where matching cell terminals are connected together so capacity and current capability increase.
- Battery management system
- An electronic protection board that monitors and controls a battery pack to keep it within safe voltage, current, and temperature limits.
- Cell balancing
- The process of keeping series cell groups at nearly the same voltage so no group is overcharged or overdischarged.
Common Mistakes to Avoid
- Adding capacity for cells in series, which is wrong because series connections add voltage while the amp-hour capacity stays the same as one series group.
- Adding voltage for cells in parallel, which is wrong because parallel cells keep the same voltage while their capacities add.
- Mixing old, new, or different-capacity cells in one pack, which is unsafe because weaker cells can overheat, overdischarge, or become unbalanced.
- Skipping the BMS or balance wiring, which is dangerous because lithium-ion cells can be damaged by overcharge, overdischarge, short circuits, or uneven group voltages.
Practice Questions
- 1 A robot pack is built as 3S2P using 3.7 V, 2500 mAh cells. Find the nominal pack voltage, capacity in Ah, and energy in Wh.
- 2 A 6S4P battery uses 3000 mAh cells with a maximum discharge current of 10 A per cell. Find the nominal voltage using 3.7 V per cell, total capacity, and maximum theoretical pack current.
- 3 A robot motor sometimes draws large current bursts. Explain why a parallel cell group can handle this better than a single cell, and describe one way the BMS helps protect the pack during the burst.