# Testing solar charger board with a bench power supply

I have a solar charger wired up to charge a battery that powers an inverter, all on one board.

I want to test the board before connecting it to the solar array that I have. I bought a bench supply that is rated for a higher power, voltage, and current than my solar array. I had tested the output from the solar array with a DMM and set the values for voltage and current on the bench supply to match.

However, it struggles to output a constant voltage and current simultaneously. Is this possibly the result of a cheap bench supply, or is it related to the relationship of current and voltage over a load?

Is there a circuit I could place inbetween the supply and the charger to remedy the issue?

• A programmable "electronic" resistor ... should be a "better" choice for testing. Commented Aug 17, 2022 at 18:30

TL;DR: No, you can't simulate a solar panel's behaviour well with just a bench power supply.

A bench power supply is normally used as an adjustable voltage source with an adjustable maximum current.

You can make it behave like a current source by forcing it into the current limiter, and some bench power supplies have a constant-current mode. In both cases you lose control of the power supply's output voltage.

You can't set voltage and current and expect a power supply to deliver both a fixed voltage and a fixed current at the same time, because this would imply the power supply is controlling the impedance of the load, as V = I·R.

A solar panel is a current source over most of its I/V characteristic, and delivers maximum power at its Maximum Power Point. An MPP Tracking solar charger can vary its input impedance to make the solar panel sit at or near its MPP.

All this means you can't simulate a solar panel's behaviour well with just a bench power supply. If your solar charger does MPPT then things are even worse, as the MPPT algorithm presupposes a solar panel's I/V characteristic that your power supply does not follow.

For example: when an MPPT charger changes its input impedance, your power supply will not react by changing its output voltage, but a solar panel will.

Solar panel simulators do exist, and they (including a few simple ones) are discussed here (and in other places on the interwebs): What is the simplest method to simulate a solar panel on a current/voltage regulated PSU?

• One link lost in the post? Commented Aug 17, 2022 at 18:29
• @Antonio51: It sometimed happens in older postings. If you mean the Agilent link: I replaced it with a working link just now. Keysight is an Agilent spin-off, by the way. Commented Aug 17, 2022 at 18:45
• Thank you. Interesting. Commented Aug 17, 2022 at 19:10