Page 354 - Battery Reference Book

Discharge curves 31/3
31 .I Discharge curves Capacity realized = (say) 0.XCzo = 16 Ah
xAx5h=16Ah
A classical method of representing a discharge curve
for a secondary battery is to plot terminal voltage x = 3.2A
against time into the discharge at a stipulated battery
temperature and constant current discharge rate. Such i.e. current of 3.2A for 5h at VI volts (VI < VI.
curves can be prepared at several different battery tem- Similarly at the 1 h rate the capacity returned
peratures in order to establish the effect of temperature (10C20) might be only 50% of the nominal capacity
on the voltage-time relationship. obtained at the 20 h rate, i.e.
When stating the capacity obtained for a battery
10
during continuous discharge, it is common practice to - x CZO = 50% of 1 x Czo
state the capacity available at a particular discharge 1
rate and battery temperature when the battery is dis- = 50% of 20Ah = 1OAh
charged to a particular end-point voltage per cell. The
20h rate at 20°C is commonly used. Thus, if a bat- y=Axlh=lOAh
tery is discharged continuously €or 20 h at 20"C, the i.e. current of 10A for 1 h at V2 volts (Vz < VI).
nominal capacity is available, referred to as 1 x C20 To obtain the capacity of a battery at the nominal
capacity. If the same battery were continuously dis- (20 h) rate, i.e. C20 at 20°C, it will be necessary to con-
charged at a higher discharge current to the end-point dition the battery to 20°C then prepare constant current
voltage over, STY, 10 h instead of 20 h. the capacity is discharge voltage-time plots at a range of discharge
referred to as 2Czo; i.e.
currents such that the time taken for discharge to the
end-point voltage ranges from appreciably below 20 h
20 h discharge x Czo = 2c to appreciably above 20 h. Select the end-point volt-
10 h discharge ages, Le. the voltage at which the voltage starts to
decrease rapidly, appropriate to each discharge cur-
Similarly, when complete discharges are performed in rent, and read off for each discharge current the cor-
40, 50, 100, 200 and 400 h, the capacity obtained is responding end-point voltage and discharge time. From
referred to as O.SC, 0.4C, 0.2C, 0.1C and 0.05C. the products of discharge current and discharge time,
If the same battery is continuously discharged during calculate the corresponding capacities. Plot capacity
a shorter period of time than 20h at 20°C, the end- against discharge time and read off the nominal capa-
point voltage per cell will reduce and the full nominal city obtained during a 20 h discharge, Le. C2o.
capacity (1 x C~O) will not be obtained, i.e. 2C (10h Knowing the nominal capacity (Cz0) and the nomi-
discharge) is less than 1Czo. Conversely, if the battery nal discharge time (20 h), it is possible to calculate the
is discharged during a longer period of time than 20 h, nominal discharge current (Zzo), the discharge current
the end-point voltage per cell will increase and the full that will deliver the nominal battery capacity (Czo Ah)
nominal capacity (1 x C20) will be obtained. In fact, in 20 h continuous discharge at 20°C. It is then pos-
with some batteries, slightly more than the nominal sible to plot end-point voltages against discharge time
capacity will be obtained in these circumstances, i.e. (to each end-point voltage) and read off the voltage
0.05C (400 h discharge) > lC20. obtained during the 20h discharge, and then to say
In a typical case, if a battery with a nominal capacity whether a discharge at 20°C at ZzOA for 20h to a
of 20 Ah at the 20 h rate (I x C20) is continuously stated end-point voltage will deliver the nominal capa-
discharged at a higher discharge current for 5 h, i.e. city of C20 Ah. It is also possible from such curves
at the 5 h discharge rate, 4C might be 80% of 1C20. to obtain these parameters for any other stated rate
That is, the capacity obtained would be 16Ah, i.e. of constant discharge of the battery. If a battery is
4C = 16Ah. continuously discharged to the end-point voltage at
A higher discharge current will, of course, be 20°C (or any other stipulated temperature) for 20 h at
obtained during this discharge in return for this loss a discharge current of ZzoA it will deliver its nom-
of capacity. Conversely, the voltages obtained during inal capacity of CZO Ah. If the continuous discharge
the 5h discharge will be lower than those obtained current is increased say to 2f20 or lOI20 the discharge
during the 20 h (discharge. time and end-point voltage both decrease and the capa-
At the 2Qh (1C20) rate: city returned will be only a percentage of the nominal
capacity, C2o.
Current (IA) x Time (th) = C20 This whole sequence of measurements is designed
to determine the nominal capacity (C~O) of a battery at
IA x 20h = 20Ah the nominal discharge rate (Izo) (or the capacity of a
battery of any other discharge rate) at 20°C. it can be
i.e. current of Z amps for 20 h at V volts. repeated at a variety of battery temperatures to obtain
At the 5 h (4C) rate: capacity data over a range of temperatures.