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BATTERY CAPACITY
The capacity of a battery is the amount of electricity that it can supply to an external circuit before the
voltage falls below the final limit value; it is expressed in Ah (Ampere-hour)
The main factors that influence the capacity of lead-acid batteries are:
(1)Discharge rate
The capacity supplied at high discharge rates (high current intensity) is lower than that supplied at
low discharge rates (low current intensity).
The causes of the decrease in the capacity supplied at high rates are the following:
(a)Sulphation of the plate surface, which closes the pores of the active material;
(b)Limited time for electrolyte diffusion;
(c)Loss of voltage due to the accumulator internal resistance.
(2)Electrolyte specific gravity
The specific gravity of the electrolyte influences the voltage and the capacity for the following
reasons:
(d)It determines the potential of the plates;
(e)It influences the electrolyte viscosity and therefore its diffusion speed.
(3)Electrolyte temperature
Batteries operating in environments at low temperature undergo a temporary decrease of the
capacity available and of the voltage (the rated capacity and voltage values are restored when
the temperature returns to normal).
An increase in the battery temperature leads to an increase in its capacity.
The effect of the temperature on the capacity is due to the variations of the viscosity and
resistance of the electrolyte.
At low temperatures the viscosity increases and this reduces the rate of diffusion of the acid in the
pores of the active material.
The temperature effect is more sensitive at high discharge rates.
Due to the effect of the electrolyte temperature, the battery capacity varies as shown in the
diagram below.
(4)Battery age
Traction batteries give their rated capacity after a few work cycles (charges and discharges), then
maintain their performance for a considerable period of time (about 1500 cycles), depending on
battery .
Capacity (%)
Temperature (°C)
CAPACITY IN RELATION TO FLUID TEMPERATURE
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