I was going to suggest the multimeter test as well. Let us know what you find out as I was considering one of these kits for similar purpose.
I'm not sure what I am doing wrong, but when I connect the multimeter, its reading around 13.64 Volts in full sunlight. By comparison, when plugged into my truck, its reading around 11 Volts. The charger charges while plugged into the truck, but not when using solar power.
I tried to read the amps, but it kept reading 0 for both my truck and solar charger.