A “12 V 18 Ah” battery means nominal voltage of 12 volts, and a storage capacity of about 18 ampere‑hours (at the 20‑hour discharge rate). 
Specifications
Parameter / Feature
Typical / Nominal Value
Nominal Voltage
12 V
Capacity (20‑hour rate)
18 Ah @ 25 °C
Capacity (other rates)
~16.8 Ah at 10‑hour rate; ~15.5 Ah at 5‑hour rate; ~12–15 Ah at 1‑hour (depending on discharge current)
Cells per Unit
6 cells (standard for 12 V SLA battery)
Case / Construction
Sealed valve‑regulated lead acid (VRLA) / AGM; non‑spillable; maintenance‑free.
Self‑discharge (when stored)
Typically low — many 12 V SLA/AGM batteries lose < 3% capacity per month at ~25 °C storage.
Maximum Discharge Current (short term)
Can deliver high short bursts — e.g. 5‑second discharge currents of up to ~180 A–270 A (depending on design) are common specs.
Dimensions (typical)
~181 mm (length) × ~77 mm (width) × ~167 mm (height) (common size for 12 V 18 Ah SLA)
Weight (typical)
Around 5.0 – 5.6 kg (varies slightly by manufacturer)
Use Case / Operating Conditions
Suitable for both “standby/floating” use (UPS, alarm, emergency lighting) and cycling use (solar backup, inverter support, small renewable systems)
