I spent quite a bit a time looking through the Full Spectrum Power website. Terrible, terrible lack of information. Whatever you get here (perhaps someone will advise) the documentation page states that you can send an e-mail request for missing documents:
support@fullspectrumpower.com
Is the device labelled anywhere, perhaps in a 4-pt font as is typical, detailing the light status? It helps to have my magnifying glass out nowadays and that is not necessary due to my age.
The solid red light is standard for charging on the Deltran Battery Tenders so it would be no surprise if that unit works in an identical fashion, but it is always good to have a definitive reference, which also benefits us all.