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Battery (Li-ion / LiPo / LiFePO4)

Battery

A rechargeable lithium cell with high energy density. Powers phones, tools and packs - but needs protection and correct charging to stay safe.

Schematic symbol

Battery / cell

How battery (li-ion / lipo / lifepo4) appears in a circuit diagram.

Types & variants

Li-ion cylindrical (18650/21700)LiPo pouchLiFePO4 (LFP, safer/long-life)PrismaticCoin (rechargeable)

Key specs

Nominal voltage

V

~3.6-3.7V Li-ion/LiPo, 3.2V LiFePO4 per cell.

Capacity

mAh / Ah

Charge stored; runtime = capacity ÷ current.

C-rate

C

Safe charge/discharge current as a multiple of capacity.

Charge/cutoff voltage

V

4.2V full, ~3.0V empty (3.65/2.5V for LFP). Don't exceed.

Cycle life

cycles

Charges before notable fade; LFP lasts longest.

Laws that govern it

  • Faraday's first law of electrolysism = Z × I × t

    Charge shuttles lithium ions between electrodes, so amp-hours in and out track the ions moved — the reason coulomb counting works as a fuel gauge.

  • Ohm's lawV = I × R

    Milliohm internal resistance is what lets an 18650 deliver 10 A; it is also the sag you measure and mistake for a flat cell.

  • Joule's law of heatingH = I² × R × t

    That same resistance means I²R heating inside the cell, which is why high C-rate charging and discharging shortens life.

Each law is stated in full — with its diagram, variables and worked meaning — on the electrical laws page.

How it compares

Markings

Chemistry, capacity, voltage and sometimes a protection mark printed on the wrap. Polarity at the terminals.

Standard values

18650 (2000-3500mAh), 21700, 1S LiPo (3.7V), 3S/4S packs; LiFePO4 3.2V cells.

How to choose

Match voltage (cells in series) and capacity/C-rate to the load. Use a matching charger/BMS (CC-CV for Li-ion, LFP profile for LiFePO4). Choose LiFePO4 for safety/longevity, LiPo for light high-current.

Pinout & package

+ and - terminals; packs add a BMS and balance leads. ALWAYS use a BMS/protection circuit - never charge unprotected cells unattended.

Example circuits

  • 1S Li-ion + TP4056 charger + boost to 5V
  • Multi-cell pack with BMS
  • Solar + LiFePO4 storage

Common failures

Over-discharge kills capacity; overcharge/puncture/short -> swelling, fire or venting; cell imbalance in packs; capacity fade with age.

How to test

Measure resting voltage (≈3.6-3.9V healthy Li-ion). Swollen pouch = dispose safely. Capacity test with a charger/analyser under load.

Substitutes

Same chemistry and voltage; equal/greater capacity and C-rate; same size/protection. Never mix chemistries or old+new cells in a pack.

Where to buy

DigiKeyMouserLCSCRobuAmazon