Battery Capacity and Discharge Rate: 10-Hour vs 20-Hour Ratings
The ampere-hour (Ah) is the usual unit of battery capacity, but a given battery delivers different amounts of charge at different discharge currents: the higher the current, the lower the usable capacity. A capacity figure is therefore only comparable when the discharge rate is stated as well.
The 10-hour rate is the discharge condition based on complete discharge over 10 hours. For a 100Ah (10-hour rate) battery, the reference discharge current is about 10A. The 20-hour rate is based on discharge over 20 hours; the same battery carries a larger nominal capacity figure because the current is lower and the active material is used more fully.
In the YUASA series data sheets these two figures correspond to different end voltages: the 10-hour rate is measured at 1.80V/cell (25℃) and the 20-hour rate at 1.75V/cell (25℃). High-rate series additionally list a 15-minute power rating at 1.67V/cell for short-duration high-current duty.
In practice, determine the load current and required backup time first, then convert using the manufacturer's discharge-rate data rather than simply multiplying load current by time to obtain an Ah figure. The higher the current and the shorter the backup time, the more important it is to consult high-rate discharge data instead of extrapolating linearly from the 10-hour capacity.
