What a battery actually is
A battery is an electrochemical device that stores energy in chemical bonds and releases it as an electric current on demand. In a lithium-ion cell, lithium ions move between two electrodes through a liquid electrolyte while electrons flow through the external circuit to do useful work. When you charge the cell you push the ions the other way and reload the chemical potential.
Almost every modern application that says "battery" means a lithium-ion battery. Lead-acid is still common for 12 V automotive starters and some backup systems. Nickel-metal hydride lingers in hybrid vehicles. Flow batteries and sodium-ion are emerging for stationary storage. But lithium-ion is the default, and understanding it gets you most of the way to understanding the industry.
Inside a lithium-ion cell
Four components do the work: the cathode, the anode, the electrolyte and the separator. The cathode is the positive electrode and is where the chemistry name (LFP, NMC, NCA) comes from. The anode is the negative electrode, almost always graphite, increasingly with a small amount of silicon added to boost capacity. The electrolyte is a lithium-salt-in-solvent mixture that lets ions move. The separator is a thin porous film that prevents the electrodes from touching while allowing ions through.
On discharge, lithium ions leave the anode, travel through the electrolyte to the cathode, and slot into the cathode's crystal structure. The electrons take the long route through the external circuit, which is the current that powers the load. On charge, an external voltage reverses the process and pushes the ions back into the anode.
The chemistries you need to know
Cathode chemistry sets most of a cell's character: how much energy it stores, how safe it is, how long it lasts, and how much it costs. Three families dominate.
- LFP (lithium iron phosphate). Cheap, safe, long-lived, and free of nickel and cobalt. Energy density is lower (~160 Wh/kg at cell level). Dominant in stationary storage and the fastest-growing chemistry in standard-range EVs, especially out of China.
- NMC (lithium nickel manganese cobalt). Higher energy density (~250 Wh/kg and rising), more expensive, more thermally sensitive. The default for premium and long-range Western EVs. NMC variants are written by their nickel-manganese-cobalt ratio, for example NMC 811 (80% nickel).
- NCA (lithium nickel cobalt aluminium). A close cousin of high-nickel NMC, used historically by Tesla in Panasonic-supplied cells. Similar energy density, similar considerations.
Sodium-ion is the next chemistry to watch. It uses no lithium, no cobalt and no nickel, has lower energy density than LFP but works better in cold weather, and is starting to ship in Chinese stationary-storage products and entry-level EVs.
The parameters engineers argue about
Five numbers come up in almost every battery conversation. Knowing what they mean lets you follow industry discussions without translation.
- Energy density, in Wh/kg (gravimetric) or Wh/L (volumetric). More energy for the same mass or volume. Drives EV range and aviation feasibility.
- Power density, in W/kg. How fast energy can be delivered or absorbed. Drives acceleration and fast-charging capability.
- C-rate. Charge or discharge current expressed as a multiple of capacity. 1C empties the cell in one hour, 2C in 30 minutes. Higher C-rates mean more heat and faster ageing.
- Cycle life. Number of full charge-discharge cycles before capacity drops below a threshold, typically 80%. LFP routinely exceeds 4,000 to 6,000 cycles; NMC is usually 1,500 to 3,000.
- State of charge (SoC) and state of health (SoH). SoC is how full the cell is right now (0% to 100%). SoH is how much capacity it has left compared to new. The BMS estimates both because neither can be measured directly.
From cell to pack
A cell on its own does very little. Real applications stack many cells together and wrap them in mechanical, thermal and electronic hardware to make a usable system.
Cells come in three main formats: cylindrical (the AA-shaped cans, e.g. 18650, 21700, 4680), prismatic (rigid rectangular cans, common in EVs and BESS), and pouch (soft laminated bags, lighter but needing external compression). Cells go into modules, modules into packs. Many newer designs skip the module layer and go cell-to-pack, which saves weight and cost.
A pack includes the cells, busbars to carry current, contactors and fuses for safety, a cooling system (liquid cooling is now standard for performance EVs and BESS), a battery management system that watches every cell, and an enclosure that handles vibration, impact and water ingress. Designing this well is an engineering discipline in itself.
How batteries age and fail
Batteries degrade in two ways: calendar ageing (just sitting there) and cycle ageing (charging and discharging). Both are accelerated by heat, by high states of charge, by deep discharges, and by high C-rates. This is why a well-designed pack keeps cells in a narrow temperature window and why most EVs default to charging only to 80%.
The failure mode everyone worries about is thermal runaway: a single cell overheating, releasing flammable gas and igniting, then propagating to its neighbours. Modern pack design fights this with cell-level fusing, intumescent barriers, venting paths, and chemistries (especially LFP) that are far harder to push into runaway in the first place.
The value chain in one paragraph
Raw materials (lithium, nickel, cobalt, graphite, manganese, iron, phosphate) are mined and refined, mostly in Australia, Chile, Indonesia, the DRC and China. Refined materials become cathode and anode active materials, made by specialist chemical companies. These go into cells, manufactured by giants like CATL, BYD, LG Energy Solution, Samsung SDI, Panasonic and a growing list of Western and Korean entrants. Cells go into packs, built by automakers, BESS integrators (Tesla, Fluence, Sungrow, Wartsila) or consumer-electronics OEMs. At end of life, packs enter second life or recycling, an industry still finding its shape. Every link in that chain is a career.
Where to go from here
If you are coming from an adjacent industry (automotive, oil and gas, power electronics, materials science, finance) the fastest way to get fluent is to layer the application on top of these fundamentals: EVs, BESS, second-life, manufacturing. The vocabulary on this page is most of what you need to follow industry conversations and read company materials with confidence.
Informational and educational content only. Not professional, financial, legal, or engineering advice.