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
Key specs
Nominal voltage
V~3.6-3.7V Li-ion/LiPo, 3.2V LiFePO4 per cell.
Capacity
mAh / AhCharge stored; runtime = capacity ÷ current.
C-rate
CSafe charge/discharge current as a multiple of capacity.
Charge/cutoff voltage
V4.2V full, ~3.0V empty (3.65/2.5V for LFP). Don't exceed.
Cycle life
cyclesCharges before notable fade; LFP lasts longest.
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.