Lead acid battery types
Lead acid batteries have been around for a long time and exist in three main varieties. Simple flooded lead acid and sealed or AGM/GEL batteries are very common in off grid systems. Especially the 2 Volt or OPZs variety. 2 Volt batteries are used because of their physical size and power ratio. Every 12 volt battery is made up of six 2 volt cells. When they are low power the six cells can be easily contained within the same plastic enclosure.
When high power batteries are required they are simply too heavy if say six 2 Volt cells each weighing 60kg are joined together. Flooded batteries are relatively simple in construction with lead and copper plates that are immersed in a strong acid.
Sealed or AGM batteries are essentially sealed versions of the above. They have the advantage that they don’t release as much gas (if any) into the room when charging but have this disadvantage that you can’t top them up with electrolyte.
GEL Batteries use a GEL based acid electrolyte and are also sealed. They are more difficult to manufacture and consequently more expensive. Both sealed AGM and GEL batteries can be transported more easily because there is less chance of the acid spilling out.