OMD10078 rev F
42
5.1.3 Two-level charge “Boost Charge”
The function values of the two-level charge “BOOST CHARGE” can be enabled and
set via the Test Software.
This type of charge, according to DIN41773, is used with vented lead acid (open
type) or Ni-Cd batteries that have a wide voltage range (Pb: 2,2÷2,7 V/cell.).
Picture 32 – BOOST charge diagram
In order to design the correct number of cells, the following table shows the voltage
limits of the UPS. The maximum capacity is a function of the maximum recharging
current, that can be found in the technical specification.
Minimum battery voltage
495Vdc
Maximum rectifier voltage
750Vdc
As soon as the charging current exceeds a certain threshold (generally 0.08C
10
) the
rectifier switches to BOOST charge (2.4 V/cell for lead acid batteries, 1.55 V/cell for Ni-
Cd batteries) and starts a charging cycle with the first part at constant current and
increasing voltage. When the voltage reaches the boost charge level the current begins
to decrease until it reaches the second threshold (generally 0.03C
10
) and the rectifier is
switched back to FLOATING charge.
During the charging phase, the battery emits hydrogen owing to the chemical
reaction that also causes the heating of the elements. In order to avoid the over-heating
or excessive consumption of the electrolyte, the microprocessor is equipped with a
safety timer that provides to stop the rectifier in case the boost charge exceeds 12
hours. An alarm indication on the display informs about the failure.