BeginnerTopic 14 of 106Electrical Engineering
Batteries and internal resistance
Real sources have internal resistance that drops terminal voltage under load.
In depth
A real source is an ideal EMF E in series with internal resistance r, so terminal voltage V=E-Ir drops under load. Maximum power transfers when R=r. Capacity is measured in Ah or Wh and degrades with age and cold.
Key points
- Internal resistance rises as batteries age
- C-rate describes discharge speed
- Series raises voltage, parallel raises capacity
ExampleA 12 V battery with r=0.1 Ω drops to 11.5 V when supplying 5 A.
Real sources have internal resistance, which reduces their useful voltage under load, so battery choice depends on it as well as capacity.
What is Batteries and internal resistance?
Real sources have internal resistance that drops terminal voltage under load.
Why is it important to understand batteries and internal resistance?
Real sources have internal resistance, which reduces their useful voltage under load, so battery choice depends on it as well as capacity.
How do I measure internal resistance?
Measure the open-circuit voltage and the voltage under a known load, then use the voltage drop divided by the load current.
Why do batteries heat up under heavy load?
The internal resistance dissipates power as heat, and the loss grows with the square of the current.
How is Batteries and internal resistance used in practice?
Typical applications include explaining why a battery voltage drops when a motor starts, choosing a battery for a high-current drone motor and predicting the runtime of a device that draws pulsed current.
What should I learn before Batteries and internal resistance?
Study "Resistor colour code and tolerance" first (topic 13 of 106 in Electrical Engineering). It covers the groundwork this topic builds on.
What are the key points of Batteries and internal resistance?
Internal resistance rises as batteries age. C-rate describes discharge speed. Series raises voltage, parallel raises capacity.
Can you give an example of Batteries and internal resistance?
A 12 V battery with r=0.1 Ω drops to 11.5 V when supplying 5 A.
What comes after Batteries and internal resistance?
The next topic is "Conductors, insulators and semiconductors", which builds directly on this one.
How difficult is Batteries and internal resistance?
It is classed as Beginner in Electrical Engineering (topic 14 of 106). It is an entry-level topic. It relies only on basic school mathematics and the ideas introduced earlier in this discipline.