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BATTERY DISCHARGER at constant current
16
D01470-03
Introduction
The ZIVAN battery discharger at constant current is an indispensable portable device to control the
efficiency state of a battery, whether used or new.
It has been designed to replace in a much preferable manner, the classical and bulky power resistor
blocks, retaining its reliability, but with the flexibility of electronic equipment.
In the recent years the type of battery present on the market have multiplied, and each battery type has
its own charging and discharging features.
Anyway, due to the importance of the electric traction today, which allows both a rational employ of the
power resources and a minor disturb of the ecological balance, it is proper to forward some explanation
concerning the traction batteries.
A traction battery is usually employed as main power source for the propulsion of industrial vehicles.
The lead-acid accumulator is an electrochemical device which accumulates, as chemical power, the
electrical power supplied during the charge, in order to feed it again, during the discharge.
One battery is characterized from two size: voltage and capacity.
Voltage:
Each lead-acid cell has a 2 Volt nominal voltage (regardless of their sizes)
In order to have higher voltages you have to connect in series more cells, forming in that a “BATTERY” of
cells.
The nominal voltage of a battery is obtained multiplying by 2 the number of cells in series.
The voltage at open circuit, of a cell at rest, depends on the electrolyte density:
Vo
=
ρ
ρ
ρ
ρ
+
0,84
where:
Vo
= cell voltage in V (Volt)
ρ
ρ
ρ
ρ
= electrolyte density in kg/dm³
Example: with a density 1,26 kg/dm³ you have Vo = 2,1V.
The sulphuric acid concentration decreases during the discharge and consequently the electrolyte density
decreases, becoming therefore an indicator of state of charge.
In a state of full charge, the voltage of a single cell is actually included between 2,07V and 2,12V,
according to the electrolyte temperature and density. The discharging process causes a decrease of the
voltage value, more or less sensible in conformity with the discharging rate. This voltage drop becomes
rapid as the voltage approaches the end of discharge, beyond which it is not recommended to proceed.
Capacity:
It is the quantity of electricity which can be supplied from the accumulators to an external circuit, before
the voltage decreases under the final limit value and it is obtained multiplying the intensity of discharging
current
I
by the discharging time
t
expressed in hour:
C
=
I
×
t
where:
C
= capacity in Ah (amperhour)
I
= discharging current in A (ampere)
t
= discharging time in h (hours)
The capacity of traction battery is usually referred to a discharging rate of 5 hours (C5), because the real
battery exploitation is supposed to be in a 8 hours working day comparable to a constant discharge at a 5
hours rating:
In this case you have the correlations:
C5
=
Inom
×
5h
;
Inom
C5
5h
=
which finds the nominal current of a traction battery.
