It's about where the battery is at 95-98% charged. Don't let it get over 100%. The 13.4v float will charge the battery a little, so should not be left at that for a long time, but the sun will go down after a few hours. With a light load, the battery will drop to around 13.28v, so 13.25 should not add to the charge. These values are aproximate, some tweeking could be used for your particular setup, but that's true with any lead-acid one as well and lead-acid will change faster as the batteries age. Some of the stuff you pick up on Amazon, supposedly designed for lead-acid, will kill your batteries -- so you need to know what you are doing there as well.