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).