Last year we built a custom circuit to charge a car battery off a small solar panel we had.
It worked fine, but consumed alot of power itself.
Now we have simplified the entire circuit with just a by-pass diode: it allows power to flow to the battery from the solar panel, but prevents power travelling from the battery to the panel at night or low-light.
It is working well.
Soon we'll rig the solar panel (in the window) outside and angled for best light in the day.
The battery (for the backhoe) trickle-charges, and also powers our lighting in the garden house at night.
Works great, and simple... the way we like it.