With an AC Multiplus system it's easy - when AC input is ok then that is your output, when it's not ok then the inverter and battery take over. When the battery is charged it enters a resting state.
How do you do this with a DC output system where Phoenix is a DC power supply and battery is the backup supply?
My concern is that to my knowledge LiFePo4 batteries do not like 24/7 charging so should the (Phoenix?) charger output only to load when charging is completed? How?
What model Phoenix is it as far as I know these work the same as the multiplus except there is no charger,
if there is a charger then its a very old model. and as such the latest firmware shoul;d be installed. and it would still work the same as a multiplus but would be restricted in using assistants.
controlling the charge state.- reduce the charge voltage so that it equates to the level you want it to charge to.
Hope the above helps, but I really don't know what model of unit you have so this answer maybe completely wrong
