Lead Acid battery SOC sensor

RCinFLA

Interesting product. Suppose to be reasonably priced. Uses fiber optics to measure index of refraction of electrolyte which changes with specific gravity level.

Side benefit it will warn you when electrolyte level drops below probe telling you to add water.

http://www.sensorsmag.com/sensors/electric-magnetic/fiber-sensing-measure-battery-state-charge-1484