Battery emulation and power consumption testing

What is battery emulation?

Battery emulation is when a test instrument reproduces the electrical behavior of a real battery. You connect the DUT to the test instruments - such as a battery emulating power supply - instead of to a real cell.

With battery emulation, you can test a design early, before choosing a final battery. You can also repeat the same discharge conditions across multiple test runs. That means you can compare device performance under various battery conditions in a controlled, repeatable way.

What battery parameters are emulated?

There are the parameters that have the most significant effects on battery performance:

• Open circuit voltage
• Stage of charge
• Temperature
• Internal resistance
• Durability and battery life cycle

Open circuit voltage

The open circuit voltage (OCV) is the voltage across the battery terminals when there is no current flow. It depends on the battery chemistry and changes with the state of charge. A fully charged battery typically has an open circuit voltage 5 % to 7 % higher than its nominal voltage.

State of charge (SoC)

The state of charge (SoC) quantifies the amount of charge in a battery, as a percentage of total capacity. A fully charged battery has a state of charge of 100 %. As a battery discharges, its open circuit voltage and internal resistance both change.

Temperature

Temperature strongly affects battery performance. At high temperatures, self-discharge increases. At low temperatures, internal resistance increases and performance drops. Zinc-carbon and alkaline batteries are not normally used below 0 °C. Lithium cells can operate at temperatures as low as –40 °C, but the performance is significantly reduced.

Internal resistance

Battery internal resistance, also called ESR, stays fairly constant during most of the discharge cycle. However, it rises sharply as the battery approaches full discharge. This rise generates heat, which causes power loss, which drops the state of charge. With the dropping state of charge, the open circuit voltage decreases and the internal resistance increases.

Durability and battery life cycle

Durability describes how long a battery can operate before its performance drops below a predefined threshold. This threshold is based on the device that uses the battery, and it is commonly defined as 80% of the battery’s original rated value.

The battery’s life cycle depends on:

• Battery type
• State of charge
• Ambient temperature
• Number of charge cycles
• Depth of discharge

Through battery emulation, you can test the battery life cycle under real operating conditions - all without aging a physical cell through repeated battery life testing.

Battery modeling for emulation

To emulate a specific battery, you need a battery model: a data set describing open circuit voltage and internal resistance across the full state of charge range. Rohde & Schwarz R&S®NGM200 and DC power supplies can build this model with the or battery simulation option.

The model-building process has three steps:

1. Discharge the battery: Use a load profile that alternates between a higher discharge current and a brief phase of very low current. The low-current phase lets the instrument capture the open circuit voltage during discharge.

2. Log the data: Recording the voltage and current throughout the discharge produces a data set of several thousand measurement points.

3. Calculate the model: A dedicated battery modeling tool processes the log file. It calculates state of charge, internal resistance and voltage. The tool can also measure relaxation, constant-current discharge and pulsed discharge directly. It outputs a CSV file containing the complete battery model.

You can load the resulting file into the instrument's built-in battery model editor. There, you set the state of charge and battery capacity for the test. Preconfigured data sets already cover common battery types, including lead-acid, lithium-ion, nickel-cadmium and nickel-metal hydride. You can adjust these to your own requirements. You can also create battery model files directly with a spreadsheet or a text editor.

Power consumption testing

Battery emulation and power consumption testing go hand-in-hand. Together, they show how long a battery-powered device will run on a single charge. This matters a great deal for devices that are hard to recharge or replace. Implanted medical devices, for example, may need to operate continuously for 10 years or more.

A typical battery-powered device spends more than 99 % of its time in standby, sleep or deep-sleep mode. In this state, it draws very little current. The device then switches briefly into an active or transmit mode, which draws significantly more current. Reducing standby current, even from 100 µA to 10 µA, can meaningfully extend battery life. Accurate standby current measurement is therefore an important part of any power consumption test.

Testing this behavior requires an instrument with a wide current measurement range. It must measure from nano amps up to several amps. It must also react quickly to load changes without causing a voltage drop.