Batteries

A battery is an electrochemical storage device.

Rechargeable batteries can have their charge restored after using.

Battery Array

Battery Components

  • Cells
  • Interconnections
  • Discharge leads
  • Charge leads
  • Battery Management System
  • Thermal Management

Ideal Voltage Source

We often model a battery as an ideal voltage source.

For any current demanded, the battery delivers the same voltage.

Simple Battery Model

We can model a battery as an ideal voltage source in series with a resistance.

This resistance is called the internal resistance.

This model shows a linear decrease in the voltage with increasing current from the battery.

Battery Capacity

The amount of charge that can be delivered from a cell before it is depleted is measured in amp-hours.

The Samsung 30Q has a capacity of 3000 mAh or 3 Ah.

Battery Maximum Current

Batteries also have a maximum current they can deliver for a given time before overheating measured in amps.

For example, the Samsung 30Q has a maximum continuous current (can be delivered indefinitely) rating of 15 amps.

Combining Batteries

By Kirchoff’s Laws, if we place two batteries in series, we can model the combination as a single battery with the voltages added and the same current capacity.

If we place two batteries in parallel, we can model the combination as a single battery with the same voltage, and with the combined current capacity.

Battery Charge

Since current is the charge per time, the charge that a battery can store is given by

Q = I t

Q, the charge is usually measured in coulombs but batteries use amp-hours (Ah) or milliamp-hours (mAh).

Battery Energy

The power from the battery is given by

P = IV

The energy is this power multiplied by time (we will use a simple battery model where the voltage does not decrease as the battery discharges)

E = P t = I V t = Q V

So the energy of the battery is given by the charge it can deliver multiplied by the voltage of the battery.