Page 220 - Battery Reference Book

1814 Lead-acid secondary batteries
the variation in voltage of a motive power cell during by the rate at which acid is produced in the plates and
a 10- 12 h recharge. the rate of diffusion into the free electrolyte of the cell.
The size of a motive power cell is defined by its When the voltage on charge reaches approximately
dimensions and capacity. In other words, the amount 2.40V, there is a fairly sharp rise in voltage. At this
of active surface available in the plates of a given cell. stage there is almost complete conversion of lead
This can be obtained, for example, by 7 plates of height sulphate. Most of the charge is now being used in
A or 14 plates of height SA. dissociating the water of the sulphuric acid solution
Both cells will produce approximately the same into hydrogen and oxygen and the cell begins to gas
electrical storage capacity. The required height of a cell freely. When this happens the cell voltage rises, levels
is confined by the space available within the design of off and finally shows no further increase. Recharge
the vehicle it is to be fitted to. is considered to be complete when the voltage and
There follows a more detailed explanation of the relative density of the electrolyte remain constant for
chemical reaction which takes place while a battery about 3 h. Figure 18.1 shows the variation of voltage
is ‘cycling’; that is, changing from fully charged to of a motive power cell during a recharge period of
discharged and back again. 10-12h.

18.1.1 Discharging 18.1.5 Voltage on discharge
When a battery is delivering energy it is said to be The effect of discharge on voltage is the reverse of that
discharging. The energy is produced by the acid in on charge. The internal resistance of the cell creates
the electrolyte gradually combining with the active a voltage drop when a current is passing, causing the
material of the plates. This combination produces lead voltage during discharge to be less than it is on open
sulphate in both negative and positive plates. A cell circuit. This can be expressed as:
is completely discharged when both plates are entirely
sulphated, and as they are now composed of identical Voltage on discharge =
material the terminal voltage collapses. In practice, of Open circuit voltage - (Current x Internal resistance)
course, discharging would be stopped long before the
plates reached this condition. Figure 18.2 shows the effect on voltage of a discharge
at the 5 h rate, in a typical lead-acid cell.
18.1.2 Charging
18.1.6 Capacity
The object of charging is to drive all the acid out of the
plates and return it to the electrolyte. A direct current The capacity of the battery will vary according to the
is passed through the cell in the opposite direction current at which it is discharged. The higher the current
to that during discharge, liberating the acid from the being taken out of the battery, the lower the available
plates, i.e. the concentration of acid in the electrolyte capacity. For example, if a battery of 500 Ah capacity
increases. This reverses the action of the discharge and is discharged at the 5 h rate, it will give 100 A for 5 h.
restores the battery to its original charged condition. The same battery discharged at 200A, however, will
When the cell is fully charged, the active material give current for only 2 h, thereby providing a capacity
of the positive plates is lead dioxide, and that of the
negative plates is metallic lead in spongy form. The
concentration of acid in the electrolyte at this stage is
at its maximum. 2” I

18.1.3 Characteristic voltage
The nominal voltage of a lead-acid cell is 2V, which
remains unaltered by the number of plates or their
capacity. In practice, the voltage of a cell does vary
slightly according to the state of the charge, the cell 1.8
temperature, the charge or discharge current, and the
age of the cell. 1.7

18.1.4 Voltage on charge 1.6
0 1 2 3 4 5
When placed on charge there is an immediate rise in
the battery voltage, mainly due to the sudden increase Discharge time (h)
in density of electrolyte in the pores of the active Figure 18.2 Variation of voltage of lead-acid battery on discharge
material. The subsequent rise of voltage is governed (Courtesy of Chloride Batteries)