Application Notes

These four tips are an introduction to how to test with electronic loads and illustrate the flexibility and usefulness of a DC electronic load.

Introduction

Electronic loads began as a specialized product designed specifically to test power supplies. DC electronic loads characterize a power supply’s responses to various load conditions. Electronic loads use FET switches and non-reactive power electronics that minimize ringing and manage less than ideal behavior. Today, they are known as general-purpose instruments capable of testing most sources of DC power including DC-DC converters, LED drivers, batteries, solar cells, generators, and fuel cells.

These four tips are an introduction to how to test with electronic loads and illustrate the flexibility and usefulness of a DC electronic load.

• Battery Testing Using Constant Current Mode
• Testing Power Supply Transient Response
• Testing a Power Supply’s Ability to Limit Current
• Testing DC-DC Converters

Tip 1. Battery Testing Using Constant Current Mode

The current priority mode is the most common setting for an electronic load. A simple example is drawing a constant current from a battery to determine its total energy storage. As current draws from a battery, its voltage drops. Understanding the relationship between the voltage and the remaining energy allows devices to predict their remaining run time. Each battery chemistry has a unique voltage profile as the current flows from the battery.

It is important to stop pulling current when a battery reaches its low voltage limit. Continuing to draw current below the low current threshold damages the battery.

To illustrate battery testing using constant current mode, we will use an industrial Li-ion 18650 battery. Battery capacity (C) is measurable in mAh. The capacity is also necessary to determine the charging and discharging current. The charging current is limited to 0.5 C or 0.5 * 2500 mAh = 1250 mA. The charging process starts with a constant current (CC) of 1250 mA and needs to stop before the battery voltage reaches 4.2 V. Discharging is similar a constant current is drawn until the 2.5 V cutoff is achieved. The amount of current drawn can vary from 0 to 20 A, but high currents reduce the number of times it charges and discharges — limiting the life of the battery. Also discharging the battery below the cutoff voltage reduces the life of the battery. Run time is determined using the discharge plots, see Figure 2.

A battery can support a 1 C discharge for an hour, or five hours at a (1 C)/(5 h) = 0.2 C discharge rate. Li-ion batteries typically have slightly more capacity at a 0.2 C discharge rate. Finally, temperature affects both battery capacity and discharge voltages. Temperatures above 25 °C are ideal but in lower temperatures capacity and voltage drop. A multimeter monitoring the voltage is used to ensure the battery is not discharging below the 2.5 V cutoff.

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