It's stored chemical energy that's released in electric form. Charge it, keep water in it, and avoid corrosion, and you'll get full life from them. Hot weather dishcarges them faster. And the hotter it is the faster they'll recharge.
Thin plates are the storage parts.
It's still the best instrument to find out if battery is charged. The more discharged the battery, the more stratification will occur. But need more of them and more maintenance. They're 6 volts, so need two to get 12 volts. Issue is needs more connections, some loss in that, plus more chance of corrosion.