What liquid is in solar batteries?

Well first there is no such thing as a ‘Solar Battery’ and technically any secondary battery can be used in conjunction with Solar PV. There’s NiCd, Li-ion, Lead Acid, …

The electrolyte in these are typically liquid.

Lead Acid has been the commonly used in the past for off grid, because of cost, though life is pretty bad in comparison to Li-ion, which is really high cost, good life, and high energy density (low space for the same capacity.) And we’ve got Flow batteries that are up in coming – which have much lower price potential than Li-ion, good life, and decent energy density.

Lead Acid – Sulfuric Acid

Li-ion – Lithium Salt solution in an organic solvent

NiCd – Ionic Salt solution in an organic solvent (such as Potassium Hydroxide)

Flow battery – Various salt solutions in water.