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ITGlossary · Batteries
Ampere-hour (Ah)
The unit of battery capacity, showing how much current a battery can deliver and for how long, e.g. 9 Ah means 0.45 A for 20 hours.
Technical level:Basic
Ampere-hours (Ah) tell you how much current a battery can deliver and for how long. A 12 V 9 Ah battery, one of the most common types in small UPS units, can in theory supply 0.45 A for 20 hours, since capacity is usually rated at a slow 20-hour discharge. Under the heavy discharge of a UPS, the energy actually available is noticeably lower.
To compare batteries of different voltages, calculate watt-hours: voltage × Ah. So 12 V × 9 Ah gives about 108 Wh.
A common mistake: two 12 V 9 Ah batteries in series give 24 V 9 Ah, not 18 Ah. Ah add up only in a parallel connection; a series connection raises the voltage.
Related terms
- C-rateThe charge or discharge current expressed as a multiple of battery capacity, e.g. 1C for a 100 Ah battery means 100 A.
- Depth of Discharge (DoD)The share of a battery's capacity that has been drawn in a discharge, where 100% means fully discharged.
- Fast ChargingA charging method that restores battery capacity quickly without damage by controlling current, voltage and temperature.
- State of Charge (SoC)A battery's current charge level as a percentage of its capacity, from 0% (empty) to 100% (fully charged).
- Battery BankA group of batteries connected together to increase total backup time or capacity.
- Battery Charging CurrentThe amount of current supplied to charge a battery, regulated by the UPS system.