Lab 1: Use of DMM, Power Supplies (CC/CV), Batteries

by Phatham Loahavilai · January 9, 2023

Objectives

Practice the use of digital multimeter (DMM) to measure voltage.
Understand the operation and limitation of power supplies in constant current (CC) and constant voltage (CV) modes.
Understand lithium-ion polymer battery characteristics, including charging and discharging methodologies.

Introduction

Lithium-ion polymer batteries are (currently, as of 2023) commonly used in portable devices (such as phones, laptops, etc.) for more than decade. Therefore, knowing their characteristics is an important basic toward advanced applications. Examples include protection circuits, electric vehicles, battery backup solutions, and solar panel energy storage solutions.

Moreover, this laboratory will introduce a technique called “shunt resistor” to measure the current flowing through a system by measuring voltage drop across a small-value, or shunt, resistor without having to connect an ammeter across the current-flowing path. This technique is extremely useful, especially in case of continuous current supply with limited tools, because the devices does not need to be disconnected to measure the flowing current.

Apparatus

Resistors: 0.1Ω and 50Ω
Lithium-ion Polymer Battery
Programmable DC Power Supply
Digital Multimeter
Breadboard

Figure 1: Lithium-ion polymer battery connected via. a special tool (green printed circuit board, to provide reliable breadboard connection) on a breadboard.

Methodology

This laboratory will explore the characteristics of lithium-ion polymer battery. We will first try to charge the battery to 4.2 volt for 1 hour (Figure 2a), then discharge them using a 50-ohm resistor for another hour (Figure 2b). The current measurements will be taken from the shunt resistor $R_\mathrm{sense}=0.1\Omega$ .

The charging or discharging current is usually determined by a constant $C$ , which is equals to battery capacity in $\mathrm{mA\cdot h}$ or $\mathrm{A\cdot h}$ , divided by 1 hour. In this case, our battery has a capacity of $110\mathrm{mAh}$ , so $C=110 \mathrm{mA}$ . Therefore, if we charge at 55mA, we can say that we are charging at 0.5C.

Procedures

Measure the Resistances

Use the multimeter in resistance measurement mode to measure the values of 0.1Ω and 50Ω resistors. Note down the actual values. Calculate the percentage error.

Click here to read Digital Multimeter Guidelines

Measure the Initial Voltage of Lithium-ion Polymer Battery

Insert the battery on top of the breadboard. Use the multimeter in voltage mode to measure the voltage of battery. The nominal voltage of lithium-ion polymer batteries is 3.7V. Values higher than this means that the battery has been charged to some extent.

Charge the Battery

Refer to Figure 2a.

Set the DC power source $V_\mathrm{CHG}$ to be 4.2V and 80mA (or 0.08A). Do not connect the wires yet.
Connect the circuit according to Figure 2a, with $R_\mathrm{sense}=0.1\Omega$ .
Let one of the TAs or the instructor inspect your circuit, then turn on the channel and connect the wires to power supply.
Record the voltages of $V_\mathrm{BATT}$ and $V_\mathrm{sense}$ , including power supply mode for 1 hour.

Swapping the positive and negative terminals can create a short-circuit and leads to fire!

Click here to read Power Supply Guidelines

Notice that the power supply first acts as constant current supply with rising voltage, then it changes to constant voltage supply with dropping current.

Discharge the Battery

Refer to Figure 2b.

Remove the DC power source and record the battery voltage.
Connect $R_L = 50 \Omega$ instead of the DC power supply.
Record the voltages of $V_\mathrm{BATT}$ and $V_\mathrm{sense}$ for 1 hour.

Do not connect 0.1Ω resistor directly to the battery, as we can create a short-circuit and leads to fire! Double-check the values!

Lab Worksheet

Complete the lab worksheet and submit by Monday.