Inside the battery
A lithium cell stores energy in chemistry: lithium that would rather be in the cathode is pushed into the graphite by the charger, and when you let it go back, it sends its electron round your circuit. Every picture below is computed while you watch.
What it is
Two electrodes that can each hold lithium (graphite, and a metal oxide or phosphate), a porous separator between them, and an electrolyte that lets lithium ions through but not electrons. The difference in how much each electrode “wants” the lithium is the cell’s voltage: about 3.6 V for NMC, 3.2 V for LiFePO4.
V = OCV(SoC) − I · R Energy ≈ V · Ah 1 C = the current that empties it in one hour
How it’s made: an 18650 cell
Inside a working cell
On discharge lithium ions leave the graphite, cross the separator and slot into the cathode, while their electrons go through your load. On charge the charger pushes them back. The voltage you measure is the open-circuit voltage for the state of charge, minus (or plus, when charging) the drop across the internal resistance, which grows in the cold.
A 3 Ah cell. The voltage curves are typical shapes for each chemistry, not one maker’s cell. Each ion drawn stands for a huge number of real ones.
What goes wrong: plating, shorts and thermal runaway
Overcharge a cell, or charge it too cold or too fast, and lithium deposits as metal on the anode: needle-like dendrites that can grow through the separator. That is an internal short, and heat. Past about 90 °C the cell’s own chemistry starts making heat; past the separator’s melting point the electrodes touch; past the cathode’s limit it releases oxygen and the electrolyte burns. The BMS exists to stop each step before it starts.
Illustrative: real runaway temperatures vary with the cell and its state of charge. Do not try this with real cells.
Other ways cells die
- Over-discharge: below about 2.5 V (NMC) the copper foil starts to dissolve; recharging it can short later.
- Heat and full charge: stored hot and full, a cell loses capacity fastest. Keep it cool and around half charged for storage.
- Deep cycles: using 100 % of the capacity every day wears it faster than using 80 %; LFP takes it far better than NMC.
- Mismatched cells in series: without balancing, the weakest cell is over-discharged or overcharged first. That is the BMS’s other job.
Use it on the bench
- Battery internal resistance — a battery’s internal resistance and sag (about it)
- Cells in series and parallel (LiFePO4) — cells in series and parallel (about it)
- Li-ion CC/CV charger from an LM317 — a CC/CV charger for Li-ion (about it)
- Cell protection: DW01 + 8205A — cell protection: the DW01 and 8205A (about it)
- Lead-acid against LiFePO4: usable capacity, voltage and life — lead-acid against LiFePO4 (about it)
Check yourself
Exam-style questions (ECE, EE and NC-II reviewers ask these). Each answer links to the scene that shows it.
1. During discharge of a Li-ion cell, lithium ions move:
Ions inside, electrons outside: the separator lets ions through but not electrons, which is why the electrons must do work in your circuit. Watch the ions scene. See the scene ▸
2. A cell with an open-circuit voltage of 3.7 V and 50 mΩ internal resistance delivers 4 A. Its terminal voltage is:
V = OCV − I·R = 3.7 − 4 × 0.05 = 3.5 V. The 0.2 V lost becomes heat (0.8 W) inside the cell. See the scene ▸
3. Why does an LFP (LiFePO4) cell’s voltage say so little about its charge?
LFP’s two-phase chemistry holds the voltage nearly constant from about 10 % to 90 %. BMSes count the charge (coulomb counting) instead. Compare NMC and LFP in the ions scene. See the scene ▸
4. A Li-ion charger works by:
CC/CV: 4.2 V for NMC, 3.65 V for LFP. It stops when the current falls to about C/20. See the scene ▸
5. Why is charging a lithium cell below 0 °C dangerous?
Cold slows the ions entering the graphite; at the surface they deposit as metal. Plating is permanent and can short the cell later. See the scene ▸
6. What starts thermal runaway?
Past about 90 °C the SEI breaks down, then the separator melts (~130 °C) and the cathode decomposes, releasing more heat and, in NMC, oxygen. The BMS’s job is to keep it far from that. See the last scene. See the scene ▸
7. Why are LFP cells favoured for home solar storage?
LFP’s cathode releases little oxygen even when abused, so runaway is much milder, and it tolerates thousands of full cycles. It is heavier per kWh than NMC. See the scene ▸
8. What is the SEI?
Formation builds it on purpose; it consumes a little lithium (the first-cycle loss). Its slow growth over the years is one reason capacity fades. See the scene ▸
Inside the other parts
The resistor · the capacitor · the inductor · the transformer · the diode and LED · the transistor · all the parts