Battery Glossary: 30 Terms That Matter
Thirty terms that carry the industry’s real meaning — each defined in plain language, each linking to the page that goes deeper. Definitions are original to this site, written against the engineering references in the linked pages.
Fundamentals
- Amp-hour (Ah)
- A capacity unit: one amp for one hour. A 100 Ah battery can deliver 10 A for about 10 hours — at its rated (C20) rate and before usable-depth adjustments. Full explanation →
- Watt-hour (Wh)
- An energy unit that removes the voltage ambiguity of Ah: Wh = Ah × voltage. 100 Ah at 12V is 1,200 Wh; the same 100 Ah at 48V is 4,800 Wh. Full explanation →
- CCA (cold cranking amps)
- The current a battery can deliver for 30 seconds at −18°C while staying above 7.2V (SAE J537). The starting battery’s headline rating. Full explanation →
- Reserve capacity (RC)
- Minutes a battery delivers 25 A before falling to 10.5V — the “engine off, lights on” measure, roughly 1.5–1.7× the Ah rating in minutes. Full explanation →
- State of charge (SOC)
- How full the battery is now, as a percentage of its current capacity — estimated by voltage, coulomb counting or a model. Full explanation →
- State of health (SOH)
- How much of the battery’s original capacity remains — the aging number that falls slowly with every cycle and every month. Full explanation →
- Depth of discharge (DoD)
- How deep the battery is cycled. Lead-acid is designed around 50% DoD for longevity; LFP around 80–90%. Full explanation →
- C-rate
- Current expressed as a multiple of capacity: 1C discharges a 100 Ah battery at 100 A. Higher C-rates deliver less than the nameplate capacity (Peukert). Full explanation →
- Peukert effect
- The capacity penalty at high currents: internal losses grow with load, so heavy drains deliver fewer usable Ah than the C20 nameplate. Full explanation →
- Internal resistance
- The battery’s own opposition to current, in milliohms. It sets the CCA ceiling, heats the battery under load, and climbs with age. Full explanation →
- Specific gravity
- The electrolyte’s density — a flooded lead-acid battery’s honest fuel gauge: ~1.265 full, ~1.190 at 50%. Full explanation →
- Nominal voltage
- The chemistry’s nameplate voltage: 2.0V per lead-acid cell, 3.2V for LFP, 3.6–3.7V for NMC. Packs are multiples of the cell. Full explanation →
Battery types
- SLI battery
- Starting-Lighting-Ignition: the thin-plate battery built for short, strong cranking bursts and immediate recharge. Full explanation →
- AGM (absorbent glass mat)
- A sealed lead-acid battery whose electrolyte is held in a fibreglass mat ~90–95% saturated — spill-proof, vibration-tolerant, recombination-capable. Full explanation →
- EFB (enhanced flooded battery)
- A flooded battery with carbon added to the negative plate — roughly 2–3× the cycling of standard flooded, the entry start-stop battery. Full explanation →
- Gel battery
- A sealed lead-acid whose electrolyte is a silica gel. Strong deep-cycling at moderate rates, but the lowest charge-voltage ceiling (14.1–14.4V). Full explanation →
- VRLA (valve-regulated lead-acid)
- The sealed family — AGM and gel — where oxygen recombines inside and a valve vents only on overpressure. Full explanation →
- Deep-cycle battery
- The thick-plate design built to be discharged deeply and often — the opposite of the SLI sprint. Full explanation →
- LFP (lithium iron phosphate)
- The lithium chemistry of stability: ~3.2V nominal, 2,000–5,000 cycles, no nickel or cobalt, the storage and entry-EV standard. Full explanation →
- NMC (nickel-manganese-cobalt)
- The lithium chemistry of density: ~3.7V nominal, high energy per kilogram, nickel and cobalt dependent — the premium EV choice. Full explanation →
- LTO (lithium titanate)
- The lithium chemistry of endurance: lower voltage and density, but ~10,000+ cycles and extreme-rate tolerance — niche industrial use. Full explanation →
Safety, testing and the industry
- BMS (battery management system)
- The electronics that protect every lithium cell — over/undervoltage, overcurrent, temperature — and estimate SOC/SOH. Lithium’s safety layer. Full explanation →
- Thermal runaway
- The self-feeding failure chain: heat breaks down the SEI, the cathode follows, and the cell vents or ignites. The lithium safety worst case. Full explanation →
- Equalization
- A deliberate overcharge of a flooded lead-acid bank that remixes the acid and rebalances cells — flooded only, and watched. Full explanation →
- Conductance testing
- An AC-signal health screen that estimates CCA from the battery’s internal conductance — fast, non-invasive, an estimate rather than a load test. Full explanation →
- Gigafactory
- A cell plant built at multi-GWh scale — the unit of the battery industry’s capacity race. Full explanation →
- BESS (battery energy storage system)
- A containerised or building-scale battery bank serving the grid — the fastest-growing layer of the battery market. Full explanation →
- CAGR (compound annual growth rate)
- The constant annual rate that carries a value from start to finish: (end ÷ start)^(1/years) − 1. Battery demand’s ~20% CAGR doubles it every ~3.8 years. Full explanation →
- Black mass
- The shredded, crushed output of battery recycling — the mixed cathode/anode powder that smelters and leachers recover metals from. Full explanation →
- Calendar aging
- Battery aging that runs with time rather than use — heat roughly doubles its rate every 10°C, and it continues in storage. Full explanation →
Our Interpretation
My position: A glossary is a site’s honesty policy made visible — the place where Ah and Wh stop being interchangeable, where CCA names its standard, and where a cycle-life claim names its depth of discharge. Terminology discipline is content discipline.
Why: Battery errors usually begin as word errors — mixing power and energy, comparing CCA across standards, quoting cycle life without its depth. Definitions that carry their units, conditions and limits are the cheapest quality control a reader can buy, and this page exists to be exactly that.
My editorial view, not a purchasing guarantee.
Sources
Definitions cross-verified against the engineering references cited on the linked pages.
Related: battery calculators · lead-acid knowledge · lithium knowledge · Last reviewed: 2026-09-17