Sine June last year I've been using a solar panel which claims to have a built in voltage regulator capping output at 18 volts DC, to charge Lithium-Polymer batteries that have an input range of 15V to 19V DC. Both use DC barrel plugs for input and output.
Recently I've been using a multimeter to test the panel's output in different locations and different times of day to optimise my setup, and today, I was surprised to see a reading of 21 volts DC.
I've read here in this answer to 'How to keep solar panel voltage at 24V?' that (if I understood it right) devices drawing from solar panels can usually draw only the voltage they need and not exceed it, meaning the voltage measured when not under load is (I believe) only the open circuit voltage i.e. max available and doesn't mean that this is the voltage a device will actually receive – but I'd like to be sure that this is what's happening in my setup, since sending too high a voltage to a li-po battery I rely on could be quite a big problem.
How can I measure the panel's actual output under load, when connected to my battery?
I've read a few seemingly related questions here, including How to measure the current in a solar powered circuit? and Current output of a solar panel when connected to battery and load, and while the question titles sound similar, the actual question content and specific circumstances are different enough that I can't see how to use their answers to solve my problem.
I'm currently in Sierra Leone which doesn't have many well stocked electronics shops, and even basic things like my digital multimeter or fuses of specific sizes and ampage have proved quite tricky to find here: if it's possible, I'd appreciate answers with basic requirements in terms of equipment (but if it's not there are ways to have things delivered from overseas, it's just very difficult).
Best Answer
If I understand you correctly, you have a solar panel connected to a battery (technically, a whole battery/charging circuit combination) and you wish to measure the voltage while it is receiving sunlight.
This is as simple as setting the multimeter to measure volts and attaching the meter in parallel with the load.
After looking at the solar panel and that power bank, it seems that the challenge may be finding points to probe while the unit is operating. You may have to make a short passthrough cable (female to male DC jack/plug with cable in the middle) with probe points so that you can measure voltage without destroying the existing cable. This may also prove handy if you decide you want to measure current (you can cut the cable and put your meter in series with the power bank).
Edit: in more detail, here's how I would do it. Electrically, you have the schematic below. The schematic editor makes it brutally difficult to show connectors and cable assemblies though, so I'll have to describe the setup. Basically, get yourself a female DC power jack that mates with any of the plugs that come with the solar charger, and a male power plug that mates with the power bank. Solder a cable between them, making sure to get the correct polarity (you can check with your multimeter!), and leave the terminals on at least one end somewhat exposed. Then you can insert that cable between the charger and power bank, and probe the voltage while the solar panel is charging the power bank.
simulate this circuit – Schematic created using CircuitLab