Battery Battery charger switch over system

[Ajax123]

I have a 4kW solar pv installation that has a 6kW lead acid battery storage. In the summer I am constantly exporting and in the winter not much is generated. I was wondering if its possible let go install a second battery store which runs from solenoid of some sort such that when I e battery is full it switches over to the second and then when one is discharges it switches back. Is such a system available? We cannot get the feed on tarric so I don't like exporting.