GEL (Gel battery)
Lead-acid with gelled electrolyte. Specialised for deep cycle use: boats, mobility scooters, solar systems, caravans.
A GEL battery is a lead-acid variant in which the electrolyte is mixed with silica to form an immobilised gel. It is a sealed, spill-proof battery that tolerates deep cycling well (discharges down to 80% of capacity without significant permanent damage), at the cost of lower instant cranking current (reduced CCA compared to an equivalent AGM). It is mainly used in deep-cycle applications: leisure marine, electric wheelchairs, solar systems, UPS back-up and caravans. Typically tolerates several hundred cycles at 50% depth of discharge. NOT recommended for pure starting use in automotive (use AGM or EFB instead).
Reference GEL brands: Sonnenschein (Exide), Victron Energy GEL, Trojan, Mastervolt.