An electronic load is a test instrument to simulate electric loads. Loads can be resistive, inductive, or capacitive, and real loads often are a mixture of these and can behave differently under different conditions.
Put simple, with an electronic load you can simulate the power consumption of electric and electronic devices. This enables you to test power sources, i.e. batteries and power supplies.
Common test scenarios include measuring the true battery capacity, and testing power supplies and their stability under varying loads.
Electronic loads support four different test modes to test specific aspects of a device.
Constant Current (CC)
This is a power consumption test and is the most frequently used mode in electronic loads. Use it to test the stability of power supplies under load.
It can also be used to discharge a battery in a controlled way, either to safely dispose it, or to measure its capacity. In CC mode, the battery is discharged at a constant current regardless of the voltage which is typically dropping over time.
Technically, the electronic load uses power transistors that act as variable resistors to ensure that regardless of voltage, a constant current flows.
Constant Current and Constant Resistance both measure fundamentally the same properties: they simulate a power consuming load. Constant Resistance (see below) is simulating true device behavior (the current falls when the voltage falls), whereas Constant Current focuses more on dynamic loads (like adaptive power regulators) or to easily calculate battery capacity by keeping the current constant: when the voltage falls (i.e. battery is discharging), in CC mode the electronic load decreases its resistance to keep the current constant.
Constant Resistance (CR)
This is a resistor simulation test (the electronic load acts like a typical electrical consumer). This is useful for testing power supplies under load and to find out their operating range and how they behave when they reach their load limits. You can also use this mode to test battery capacity.
Provided you are testing resistive components (and not inductive components), Ohms Law applies. When the electronic load is i.e. set to 5 Ohms, in a circuitry powered by 3.3V, this causes a current of 0.66A to flow (I = U/R, thus 3.3V/5Ohm = 0.66A). Powered with 5V, the same test setup causes a current of 5V/5Ohm = 1A.
In Constant Resistance mode, the absorbed power is directly related to the voltage. When the voltage changes, so does the absorbed power. To measure power independently from voltage, see Constant Power below.
Constant Resistance can for example be used to discharge a battery and create a specific battery profile.
Constant Power (CP/CW)
In this mode, the electronic load absorbs a defined constant power, regardless of voltage. It senses the voltage, calculates the appropriate current, and then draws this current to consume the set power.
This mode can be used to discharge a battery with constant power, i.e. to simulate the behavior of a DC-DC-converter. It can also be used to test DC-DC converters and power supplies to ensure they are able to supply a given power at a set voltage.
Note the fundamental difference compared to constant resistance: when a constant resistance is applied to a battery, it will draw a certain current that depends on the voltage. Over time, when the battery gets discharged, its voltage will fall. In constant resistance mode, according to Ohms Law, the current will also fall.
Not so in constant power mode: when the voltage of the power source falls, the electronic load will automatically increase the current to keep the power constant. With falling voltage, the current will rise.
Constant Voltage (CV)
In this mode, the electronic load tries to eliminate enough energy from the load in order to let the voltage drop to a given voltage. Since an electronic load can extract only a certain amount of energy (i.e. 400W), this mode has limitations.
- When the supplied voltage is lower than the CV set at the electronic load, nothing happens.
- When the supplied voltage is higher than the CV that you set at the electronic load, it will try and eliminate enough energy to drop the voltage to the desired level: it will start to sink current. If the voltage source can provide more energy than the electronic load can absorb, then the voltage will not drop fully to the set constant voltage: the voltage will be higher than the set CV because the electronic load could not eliminate enough energy.
Analyzing Battery Capacity
To better understand CV and its limitations, here is an example: the capacity of a rechargeable battery needs to be examined.
To achieve this, you could set CV to the safe discharge voltage of the battery (i.e. 12V for a 13.8V LiFePo4 battery). The electronic load would now try and eliminate energy from the battery until its voltage drops below 12V, preventing over-discharge.
CV Battery Testing Can Be Dangerous
CV does not provide any control over the discharge currents. These currents solely depend on the maximum sink capacity of the electronic load. Small batteries can be destroyed and can explode under test. Here is why:
With a typical 400W electronic load, when connecting a battery in CV mode and setting CV to the desired “fully-discharged” voltage, the electronic load would try to eliminate as much energy from the battery as needed to immediately drop the voltage to the set CV.
The maximum energy the electronic load can extract is limited, though, so with large capacity batteries, the electronic load would extract its maximum energy instead (i.e. 400W), and the voltage would only gradually fall to the desired CV as the battery discharges.
For smaller batteries, the maximum discharge power of 400W can easily exceed its specifications by multitudes. If you i.e. test a 3.3V 1000mA LiPo cell via CV with a 400W electronic load, the discharge current would be 121.2A: 400W / 3.3V = 121.2A: the resulting discharge rate of 121C (121.2A / 1Ah = 121.2C) would most likely lead to fire and explosion.
That’s why CV can only be used for battery testing when the battery is using a relatively high voltage (i.e. 12, 24, or 48V), and/or when its total capacity is high.
A 48V 400Ah solar panel battery pack can easily be analyzed using CV: 400W / 48V = 8.3A, discharge rate = 8.3A/400Ah = 0.02C.
A safe way of battery testing is using CC (Constant Current) mode: now you can individually set a safe current tailored to a specific particular battery under test. Just make sure the electronic load turns off once the CC can no longer be delivered in order to not over-discharge batteries.
Battery Test
Many electronic loads offer specific battery test modes to test battery capacity.
These modes base on the Constant Current (CC) mode and add these features:
- Two Or More Discharge Currents: once the set constant current can no longer be delivered by the battery under test, the electronic load can optionally continue discharging the battery until a lower second current threshold is reached.
- Calculating Capacity: in battery test mode, the electronic load automatically calculates the total battery capacity by summing up the energy extracted by the load.
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(content created Mar 30, 2024)