What I do here, is just connect my battery to a normal solar charger, set to my normal full charging voltage.
That works pretty much as expected most of the time, but if its all grey and cloudy for several days in a row, the battery starts to get a bit low.
I have permanently connected to the battery (through a diode) a grid powered battery charger set to start supplying current when the battery gets down to a safe minimum battery voltage.
This normally does nothing, because the battery voltage will be higher than minimum, and the diode disconnects the charger.
But when things get desperate, the grid powered charger prevents any further fall in battery voltage. This charger does not need to be huge, any short high current surges can still be drawn from the battery, and the charger puts all that back over time.
When the sun returns, the battery voltage starts to rise, and the grid powered charger goes back to just sitting there doing nothing.