A vehicle battery stores chemical energy and delivers it as electrical energy. It supplies the starter’s very high current for a short time, powers loads when the engine is off and stabilises the system voltage while the alternator is running. The alternator, driven by the engine belt, generates alternating current that is rectified and regulated to recharge the battery and run the vehicle loads.
Stored energy
Energy in a 60 Ah battery
Problem. A battery has an open-circuit voltage of 12.6 V and a capacity of 60 Ah. Estimate its stored energy.
Solution. E = 12.6 × 60 = 756 Wh, approximately 0.76 kWh. This is a nominal estimate, not the usable energy at every discharge rate.
Charging voltage is regulated
The voltage regulator sets the charging voltage according to temperature, battery type and the vehicle’s energy management strategy. A single “correct” charging voltage cannot be applied to every vehicle; the manufacturer’s specification is the reference.
- The battery supplies cranking current and buffers the electrical system.
- The alternator recharges the battery and powers running loads.
- The regulator controls charging voltage; its target is vehicle-specific.
- Open-circuit voltage is a rough state-of-charge indicator, not a health test.
- Capacity falls with age, temperature and repeated deep discharge.
- E = V × Ah gives a nominal energy estimate.
- The alternator and regulator recharge the battery.
- Charging voltage targets are vehicle-specific.
- Open-circuit voltage is not a full health assessment.
Worked example
A 12 V, 80 Ah battery stores approximately 12 × 80 = 960 Wh nominally. At a 5 A discharge the same battery would deliver less usable energy than the nominal figure suggests, because capacity depends on the discharge rate.