Battery knowledge · Units and electrical basics
Watt
The SI unit of power: one joule per second, or in a circuit, volts times amperes. A cell delivering 20 A at 3.6 V is supplying 72 W. [1]
- Abbreviation
- W
- Unit
- W
- Relationship
- P = V × I
Example
Calculate electrical power
At a particular instant, a terminal voltage of 12 V and current magnitude of 3 A correspond to 36 W. State whether the battery is receiving or supplying that power.
Explore the graph
Voltage connects capacity in ampere-hours to nominal energy in watt-hours.
5 Ah × 12 V = 60 Wh nominal energy.
E_nominal = Q_nominal V_nominal; actual energy = ∫ VI dt
Illustrative model. Specific energy divides energy by mass; energy density divides it by volume.
Use in Lemonergy
Lemonergy Studio blocks
Compare grid and battery energy
Separate energy and loss signals make efficiency visible.
Blocks to use
- Regulated CC/CV charger
Regulates current and voltage
Inputs & settings
- currentA
- voltageV
- cutoffCurrentA / cutoffHoldS
- Electrical ports p and n
- Battery with SOC-dependent OCV
Stores charge and supplies terminal voltage
Inputs & settings
- capacityAh
- initialSoc
- emptyVoltageV / fullVoltageV
- resistanceOhm
- Electrical ports p and n
This overview identifies the components to use. Follow the example in Lemonergy for their connections and settings.
- Keep battery settings fixed.
- Change converter efficiency.
Example. 7 kW battery-side power at 95% efficiency requires about 7.37 kW grid power and produces about 0.37 kW converter loss.
Observe. Battery I²R loss differs from converter loss.
A loss signal is not a predicted temperature. This network takes external temperature.
Open the Studio example →References & further reading
Technical references
- The International System of Units (SI), 9th edition
Bureau International des Poids et Mesures · 2026
Technical reference · Definitions of SI units and coherent derived units; 2026 revision.