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I have a 24V system with both LiFePO4 and flooded batteries, in separate banks. I am planning to install a 20A dipole breaker between each bank and the 20A charge controller. This will both protect the controller and let me easily switch charging between one type of battery to the other. I don't discharge batteries that often, so normally the flooded battery bank will be connected to the controller, getting a trickle charge every day. The LiFePO4 bank will be disconnected once fully charged.
Is this a good plan? Have I missed anything?
The main thing to keep in mind is that a lot of MPPTs (even some of the name-brand expensive ones) will self-destruct if the battery is disconnected WHILE solar is still connected. As long as there is no way to leave the MPPT "floating high" with no battery connected, you likely will be OK.
Yes, I have read that you always need batteries connected to the charge controller. The flooded battery bank will normally be the one connected, as I have also read that LiFePO4 batteries shouldn't be overcharged, and will hold a resting charge better than flooded. My two battery banks, together, store less than 5 kilowatts.