With the Smart Solar MPPT 250/70 I want to setup a custom battery configuration that uses 2 stage (no float). Is that possible?
To basically achieve the same effect as disabling the charger once in float stage you could simply set the float voltage to the lowest voltage allowed (should be 36v for a 48v system).
Just please also note that if you do this then the 're-bulk' voltage will automatically also drop - so if the batteries become depleted any time after the initial charge cycle, another charge cycle will NOT start again until the next day.
This is not ideal and I don't think that others running lithium totally disable float phase.
In saying that, the upcoming MPPT firmware release should allow the user to independently specify the re-bulk voltage. As long as there is no constraint that the re-bulk voltage specified must be under the float voltage, then this may work well for you.
I would just set float voltage a little lower than the battery resting voltage post charge. That way if the voltage drops to that level, the MPPT will start charging again and maintain the float voltage.
You'll have to provide more info:
-what kind of lithium batteries (and what BMS)
-what kind of system (ESS, off grid, backup, DC users, inverters, etc)
Usually it's easy: set the float voltage at a level below charged voltage, but depending on your system this might or might not be the right solution.