What is a cell in a battery
A cell is the smallest electrochemical unit that produces electricity. It contains a negative electrode (anode), positive electrode (cathode), electrolyte, and a separator to keep the electrodes apart while ions move during charge and discharge.
Cells can be primary (single-use, e.g., alkaline, zinc-carbon) or secondary (rechargeable, e.g., Li-ion, LiFePO4, NiMH, lead-acid). Typical nominal voltages are 1.2 V (NiMH), 1.5 V (alkaline), 2.0 V (lead-acid cell), and 3.2–3.7 V (lithium chemistries). In short: a cell = one chemical “engine” that powers a circuit.
What is a battery and how is it different from a cell
A battery is one cell or several cells connected and packaged to deliver a target voltage and energy. Cells in series raise voltage; cells in parallel raise capacity. A practical battery also adds conductors, a case, and often a battery management system (BMS) to monitor and protect the pack (over-charge, over-discharge, temperature, and balancing).
Everyday speech calls a single AA a “battery,” but engineers treat it as one cell; a 12 V lead-acid or 51.2 V LiFePO4 home pack is a multi-cell battery.
Cell vs battery at a glance
| Topic | Cell | Battery |
|---|---|---|
| Core idea | One electrochemical unit | One or more cells assembled |
| Typical use | AA cell, 18650 cell | 12 V car battery, 51.2 V home pack |
| Voltage/Capacity | Fixed by chemistry & size | Set by series/parallel layout + BMS |
How do series and parallel cells change voltage and capacity
- Series raises voltage: Vpack=n×Vcell; capacity (Ah) stays the same.
- Parallel raises capacity: Cpack=m×Ccell; voltage stays the same.
- Energy: E [Wh]=V×Ah.
Simple battery math you can reuse
12.8 V 100 Ah LFP (4S1P)
V=4×3.2=12.8 V; C=100 Ah; E=12.8×100=1280 Wh.
Double the capacity (4S2P)
Two identical 4S strings in parallel → V=12.8 V; C=200 Ah; E=2560Wh.
51.2 V 100 Ah rack battery (16S1P LFP)
V=16×3.2=51.2 V; C=100 Ah; E=5120 Wh=5.12 kWh.
High-voltage 102.4 V module (32S1P LFP)
V=32×3.2=102.4 V; if C=100 Ah then E=10.24kWh.
What is the difference between battery voltage and cell voltage
Names you’ll see
- Cell voltage: the per-cell nominal value (typicals: LFP ≈ 3.2 V; NMC/Li-ion ≈ 3.6–3.7 V; NiMH ≈ 1.2 V; Alkaline ≈ 1.5 V; Lead-acid ≈ 2.0 V).
- Battery (pack) voltage: the series count × per-cell voltage (e.g., 16 × 3.2 V = 51.2 V).
Nominal vs working range
- Nominal is a label for comparison (e.g., 51.2 V).
- Working range is what you measure in use (e.g., an LFP 16S pack might operate roughly 40–58 V depending on BMS limits and load).
- Cut-off voltages (per-cell min/max) are enforced by the BMS to protect cells.
Voltage ≠ state of charge (SOC) in a straight line
- LFP has a flat plateau, so voltage alone poorly predicts SOC in the mid-range.
- Packs often need coulomb counting + periodic balance; voltage is more useful near the top/bottom of the curve.
C-rate, temperature, and internal resistance matter
- Under high load (C-rate), measured voltage sags due to internal resistance (IR) and recovers at rest.
- Cold temperatures increase IR → more sag; high temps can raise apparent voltage but stress the chemistry.
Where you measure from
- A handheld meter across the pack reads pack voltage.
- Cell-level voltage is read through a BMS or balance leads; uneven cells reduce usable capacity and trigger earlier cut-off.
Practical cheatsheet
- Want more voltage? Add cells in series.
- Want more runtime (Ah)? Add parallel strings.
- Want safe, consistent packs? Use a BMS to watch per-cell limits and balance them.
What types of battery cells are used today
Cylindrical (e.g., 18650, 21700)
- Pros: rugged can, great cycle consistency, mature automation.
- Use: tools, laptops, e-bikes, some EV modules, portable ESS.
Prismatic (rectangular cans)
- Pros: high packing density in racks/vehicles, fewer interconnects.
- Use: home/industrial racks (e.g., 51.2 V modules), many EV/ESS systems.
Pouch (laminate foil)
- Pros: very light/thin, flexible footprints.
- Watch-outs: needs strong mechanical support and swelling management.
- Use: consumer electronics, some EV and UAV packs.
Want a selection deep-dive (sizes, thermal behavior, cost, and use-case mapping)? Spin off a dedicated page (e.g., Types of Battery Cells: Cylindrical, Prismatic, and Pouch Compared) and link to it here.
FAQ
Is an AA battery a cell or a battery?
AA is a single cell. We often call it a battery in daily life, but technically it’s one cell.
How many cells make a 12 V, 24 V, or 48 V battery?
- Lead-acid 12 V: 6 × 2.0 V cells in series ≈ 12.0 V.
- LiFePO4 “12 V class”: 4S = 12.8 V.
- LiFePO4 “24 V class”: 8S = 25.6 V.
- LiFePO4 “48 V class”: 16S = 51.2 V (common for home/C&I).
Why does a 9 V battery use multiple small cells inside?
To reach ~9 V in a compact case: alkaline 6 × 1.5 V; NiMH 7 × 1.2 V (sold as 8.4 V nominal).
Can a single cell be called a battery in daily language?
Yes, but in engineering we reserve battery for an assembled unit (one or more cells plus connections/protection).
What is the difference between a primary cell and a secondary cell?
Primary = single-use (alkaline, zinc-carbon). Secondary = rechargeable (Li-ion, LiFePO4, NiMH, lead-acid).
What is the cell symbol vs the battery symbol in circuit diagrams?
A cell is one long + one short plate; a battery shows multiple plate pairs in a row.
Why do packs need a BMS when multiple cells are used?
The BMS monitors per-cell voltage/current/temperature, balances cells, and guards against over-charge, over-discharge, and short circuit.
How many cells are inside common batteries?
| Item | What is it? | Typical cells inside | Nominal voltage | Notes |
|---|---|---|---|---|
| AA alkaline | Single primary cell | 1 × 1.5 V | 1.5 V | Called “AA battery” in daily life, but technically a cell. |
| AA NiMH | Single rechargeable cell | 1 × 1.2 V | 1.2 V | Reusable cell for cameras, toys. |
| 18650 Li-ion | Single rechargeable cell | 1 × 3.6/3.7 V | 3.6–3.7 V | Cylindrical cell used in tools and laptops. |
| LiFePO4 cell | Single rechargeable cell | 1 × 3.2 V | 3.2 V | Common building block for energy storage. |
| 9 V alkaline | Small battery | 6 × 1.5 V in series | 9.0 V | Inside are six tiny 1.5 V cells. |
| “9 V” NiMH | Rechargeable battery | 7 × 1.2 V in series | 8.4 V | Often sold as 8.4 V; seven cells inside. |
| 12 V lead-acid | Battery | 6 × 2.0 V in series | 12.0 V | Car starter and UPS. |
| 12.8 V LFP | Battery (4S) | 4 × 3.2 V in series | 12.8 V | Often called “12 V LiFePO4.” |
| 25.6 V LFP | Battery (8S) | 8 × 3.2 V in series | 25.6 V | Two times the 12.8 V pack voltage. |
| 51.2 V LFP | Battery (16S) | 16 × 3.2 V in series | 51.2 V | Home and C&I energy storage standard. |
| Laptop pack | Battery pack | Multiple 18650/21700 in series-parallel | 10.8–14.8 V | Includes BMS and sensors. |
| ESS rack pack | Module/pack | 16S/32S LFP, often paralleled | 51.2/102.4 V | Module → rack → cabinet, with BMS and busbars. |