Samlex America PST-200S-24E - Manual (Page 17)

16 | SAMLEX AMERICA INC. SAMLEX AMERICA INC. | 17
seCtIOn 7 | General Information on Batteries for
Powering Inverters
an electrical conductor in one second. The Capacity “C” in Ah relates to the ability of the
battery to provide a constant specified value of discharge current (also called “C-Rate”) over
a specified time in hours before the battery reaches a specified discharged terminal voltage
(Also called “End Point Voltage”) at a specified temperature of the electrolyte. As a bench-
mark, the automotive battery industry rates batteries at a “Discharge Rate” C/20 Amperes
corresponding to 20 Hour discharge period. The rated capacity “C” in Ah in this case will be
the number of Amperes of current the 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, 21V for 24V battery and 42V for a
48V battery. for example, a 100 Ah battery will deliver 5A for 20 Hours.
Rated capacity 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 com-
puter/ignition system. The assumed battery load is a constant discharge current of 25 A.
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, 21V for 24V
battery and 42V for 48V battery. Approximate relationship between the two units is:
Capacity “C” in Ah = Reserve Capacity in RC minutes x 0.6
typical Battery Sizes
The Table 7.1 below shows details of some popular battery sizes:
tABLe 7.1: 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
Specifying charging / Discharging currents: c-Rate
Electrical energy is stored in a cell / battery in the form of DC power. The value of the
stored energy is related to the amount of the active materials pasted on the battery
plates, the surface area of the plates and the amount of electrolyte covering the plates.
As explained above, the amount of stored electrical energy is also called the Capacity of
the battery and is designated by the symbol “C”.
The time in Hours over which the battery is discharged to the “End Point Voltage” for
purposes of specifying Ah capacity depends upon the type of application. Let us denote
this discharge time in hours by “T”. Let us denote the discharge current of the battery