17
3
GENERAL INFORMATION: BATTERIES
Furthermore, it is to consider that these parameters depend also on the current
direction (charging or discharge). When the battery were at rest or under open-circuit
condition V
B
= V
E
. When current is drawn from the battery, the voltage will be lower
than V
E
. When current is owing into the battery, the terminal voltage will be higher
than V
E
. For example, at each moment during discharge phase the terminal voltage
can be derived as follow:
V
B
=V
E
– V
in
(3.1)
=V
E
– R
in
× I
B
(3.2)
where: V
B
= terminal voltage [V]
V
E
= equilibrium voltage [V]
V
in
= internal loss voltage [V]
R
in
= internal resistance [Ω]
I
B
= discharge current [A]
Obviously, higher discharge current results in reduction of the terminal voltage.
Therefore, to specify the state of the battery by the battery voltage, discharge current
should be also measured.
In case of discharge, the minimum voltage level acceptable for a Lead-Acid battery
is dened as discharge voltage threshold. Falling below this threshold is called deep
discharge, with which the battery may suffer damage. In case that the battery is
left longer after deep discharge, lead of the support structure is converted to lead-
sulphate in rough-crystalline form, which during charging can be only bad or cannot
be converted again anymore. As a result, the battery loses a part of its storage
capacity; besides loss of support structure arises as well.
In practice, harmful deep discharge is to be prevented: the loads will be compulsory
disconnected from battery as soon as the discharge voltage threshold is reached
i.e. with the help of a so-called deep discharge protection (DDP). This threshold is
basically given in the data sheets by the manufacturer for different discharge currents.
Preferably, the value of this threshold should depend on the discharge current. The
relation between the discharge current and the voltage during discharge for the Lead-
Acid battery is presented in Figure 3.3.
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