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battery can deliver for 20 Hours at 80ºF (26.7ºC) till the voltage drops to 1.75V / Cell. i.e. 10.5V for 12V battery or
21V for 24V battery. For example, a 100 Ah battery will deliver 5A for 20 Hours.
1.4.8 Rated Capacity Specied in Reserve Capacity (RC)
Battery capacity may also be expressed as Reserve Capacity (RC) in minutes typically for automotive SLI (Starting,
Lighting and Ignition) batteries. It is the time in minutes a vehicle can be driven after the charging system fails.
This is roughly equivalent to the conditions after the alternator fails while the vehicle is being driven at night with
the headlights on. The battery alone must supply current to the headlights and the computer/ignition system. The
assumed battery load is a constant discharge current of 25A.
Reserve capacity is the time in minutes for which the battery can deliver 25 Amperes at 80ºF (26.7ºC) till the voltage
drops to 1.75V / Cell i.e. 10.5V for 12V battery or 21V for 24V battery.
Approximate relationship between the two units is: Capacity “C” in Ah = Reserve Capacity in RC minutes x 0.6
1.4.9 Typical Battery Sizes
Table 1.4.1 shows details of some popular battery sizes:
TABLE 1.4.1 POPULAR BATTERY SIZES
BCI* Group Battery Voltage, V Battery Capacity, Ah
27 / 31 12 105
4D 12 160
8D 12 225
GC2** 6 220
* Battery Council International; ** Golf Cart
1.4.10 C-rate of Charge / Discharge
1.4.10.1 The rate of charge / discharge of a battery is normally expressed in “C-rate” which is a multiple of the
numerical value of the battery’s Ampere Hour (Ah) Capacity (C) (See Section 1.4.7 for information on Ampere Hour
Capacity). Few examples of C-rates (2C, 1C, 0.2C etc.) for 100Ah capacity battery (C=100 Ah) are given below:
• 2C = (2x100) A = 200A (As the battery capacity is 100 Ah, the battery will be completely discharged in 0.5 Hrs.)
• 1C = (1x 100) A = 100A (As the battery capacity is 100 Ah, the battery will be completely discharged in 1 Hr.)
• 0.2C (or C/5) = (0.2 x 100) A = 20A (As the battery capacity is 100 Ah, the battery will be completely discharged
in 5 Hrs.)
1.4.10.2 Example for Determining C-rate of Charge for Particular Value of Charge Current:
• Determine the Ah capacity (C) of the battery – say 100 Ah (C=100 Ah)
• Determine the value of charge current – say 10 Amperes
• C-rate of charge at 10A = Multiple of numerical value Ampere Hour Capacity (C) = (10 ÷ 100) C = 1/10 C or 0.1C
1.4.10.3 Example for Determining C-rate of Discharge for Particular Value of Discharge Current:
• Determine the Ah capacity (C) of the battery – say 100 Ah (C=100)
• Determine the value of discharge current – say 20 Amperes
• C-rate of discharge at 20A = Multiple of numerical value of Ah Capacity (C) = (20 ÷ 100) C = 1/5 C
SECTION 1.4 | General Information – Battery Related
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