Build a Programmable DC Electronic Load for Battery and Power Supply Testing

This site already covers building a variable bench power supply and testing battery capacity, but neither answers a question that comes up constantly once you're building your own battery packs, chargers, or power supplies: how do you actually verify a source can deliver what it claims under real load, and how do you discharge a pack to a known state in a controlled, repeatable way? A commercial electronic load solves this — it's a device that draws a precisely controlled amount of current, power, or resistance from whatever you connect it to, standing in for the real-world load your project will eventually see. This build wraps a low-cost programmable DC electronic load module in a proper bench enclosure with adequate cooling, turning it into a permanent piece of test equipment instead of a bare board on the bench.

What an Electronic Load Actually Does

Where a power supply sources current, an electronic load sinks it — it presents itself to whatever's connected as a controllable resistance, dissipating the incoming power as heat through a bank of MOSFETs. Most programmable modules support four modes: constant current (CC, draws a fixed amperage regardless of voltage), constant voltage (CV, useful for testing a supply's regulation), constant power (CP, draws a fixed wattage as current adjusts with voltage), and constant resistance (CR, behaves like a fixed resistive load). For maker use, CC mode covers the majority of real testing: discharging an 18650 pack at a known rate to verify its actual capacity against the datasheet, load-testing a bench supply or LiFePO4 station build to confirm it holds voltage under real current draw, or characterizing a solar charge controller's output curve.

Choosing the Load Module

Complete pre-built modules (rather than discrete MOSFET-and-op-amp designs) are the practical starting point here — they integrate the current-sense circuitry, PWM control loop, and often a small display and rotary encoder for setting targets, for a fraction of the cost and build time of a from-scratch design. Look for a module rated for at least 150W continuous dissipation and 20A, which comfortably covers single-cell and small-pack battery testing plus most bench power supply verification a maker shop will do. Confirm the module includes reverse-polarity protection and over-temperature shutdown before buying — both matter more here than on most builds, since the whole point of this device is to run at its thermal limit on purpose.

Assembly

1. Mount the electronic load module's MOSFET/heatsink assembly to the larger external heatsink using thermal paste and secure standoffs — the module's stock heatsink is typically undersized for sustained full-power operation, which is the main reason to rehouse it rather than run it bare.
2. Mount the 12V fan directly over the heatsink fins, wired to run continuously whenever the load is powered (or through the module's built-in temperature-controlled fan output if it has one) rather than relying on passive convection.
3. Drill and mount the enclosure with ventilation slots aligned to the heatsink and fan, binding posts for the load input on the front panel, and a cutout for the module's display/control interface if it isn't a separate remote panel.
4. Wire the binding posts to the load module's input terminals with heavy-gauge silicone wire — undersized wire here is a real fire risk given the currents involved, so don't reuse thin hookup wire from other projects.
5. Wire the 12V control-circuit supply to the module's power input, keeping it electrically separate from the high-current load path per the module's documentation.
6. Add the enable switch in line with the module's load-enable input if it has a dedicated one, giving you a physical way to disconnect the load without touching the device under test.

Verifying It Actually Works

Before trusting readings for real testing, cross-check the load against a known-good source. Connect a bench power supply set to a known voltage and current limit, set the load to a CC target below that current limit, and confirm the load's displayed current matches both the power supply's display and an inline multimeter reading. A discrepancy of more than a few percent usually points to a current-sense calibration issue that most modules let you adjust through a calibration menu or trimmer.

Practical Uses on This Site's Other Projects

Use caseHow the load helps 18650/LiFePO4 capacity testingDischarge at a fixed CC rate (e.g. 0.5C) while logging voltage vs. time to calculate real-world mAh, cross-referencing against this site's battery capacity testing guide Bench power supply verificationLoad the supply's output to its rated max current and confirm voltage sag stays within spec — catches an undersized transformer or regulator before it fails in a real project LiFePO4 power station buildVerify the inverter and BMS actually deliver rated continuous wattage under sustained load, not just peak Solar charge controller testingSimulate a fixed-current battery draw to characterize the controller's charging curve without needing an actual depleted battery on hand USB-C PD trigger board testingConfirm a negotiated voltage holds under the current the board claims to support

Safety Notes

An electronic load intentionally converts electrical power into heat at its rated maximum, which means the heatsink gets genuinely hot during sustained testing — treat it the same as any other high-power resistive device and keep it clear of anything flammable, on a non-combustible surface, and within sight while running a sustained discharge test. Never exceed the module's rated voltage or current input, and never connect it directly across a source with no current limiting of its own (like an unprotected large LiPo pack) without first setting a conservative CC target, since a fault in CV or CR mode could otherwise pull far more current than intended.

Between this and the bench power supply project already on the site, you end up with a complete source-and-sink pair of test equipment — the supply to push power into a project, and the load to pull it back out in a controlled, measurable way, which covers most of what a maker electronics bench actually needs beyond a multimeter and scope.

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