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SECTION 4 |
i
InFo
For background information on batteries and
charging process, please read White Paper titled
"batteries, Chargers and Alternators" available
online at: www.samlexamerica.com (Home >
Support > White Papers).
All battery charging voltages are specified at battery
temperature of 25˚C / 77˚F.
Principle of operation
G4 Series is a Bi-directional Inverter / Charger with a Transfer Relay that
operates either as an inverter OR as a battery charger. It uses a common
Converter Section that can work in two directions – in one direction it
converts external AC power to DC power to charge the batteries (Utility
Mode / Generator Mode) and in the other direction, it converts the DC
power from the battery to AC power (Inverter Mode). Please note that
it cannot work in both the directions at the same time (i.e. it can
not work as an inverter and as a charger at the same time).
When AC input power from utility / generator is available within the
specified limits of voltage and frequency, the internal Transfer Relay
passes through the AC input to the AC loads and at the same time, the
AC input is fed to the Battery Charger Section.
First, the AC input voltage is stepped down by the Low Frequency
Isolation Transformer and is then rectified by the 4 sets of H-Bridge
Mosfets and fed to the batteries for charging. AC line current will flow
only during the period when the rectified instantaneous voltage is higher
than the battery voltage.
Micro-controller based PWM technique is used to accomplish 3 or 4
stage charging. During charging, the top 2 sets of H-Bridge Mosfets are
kept OFF (their Body Diodes are used for conduction). The bottom 2 sets
of Mosfets are switched at PWM frequency of 32 KHz. When charging
starts, the current does not rise sharply, but ramps up slowly (starts at
5.3% Duty Cycle).
The Battery Charger Section of these units is a powerful, micro-controller
based, 3 Stage or a 4 Stage Charger - 80A for the 12v, 2000vA version
and 70A for the 24v, 2500vA and 3500vA versions.
3 Stage Charging Algorithm is used for normal day to day charging. The
3 stages are – Bulk, Boost (Absorption) and Float. The charging voltages
and currents are programmable within the ranges given in the Table 4.1
to take care of a wide range of battery types like flooded, AGM, Gel Cell,
Lead Calcium etc.
4 Stage Charging Algorithm is used in the Equalization Mode.
Equalization Mode is activated manually when desired and is used only
for Flooded or Wet Cell batteries. The 4 stages will be - Bulk, Boost
(Absorption), Equalization and Float. Equalization Mode is desirable
for the proper health of Wet Cell Batteries. Equalization voltage is
14 | SAMLEX AMERICA INC.
battery Charging in
G4 Series
programmable. Equalization current and Equalization time are computed
automatically. Please see Table 4.1 for details.
over Temperature Protection
During Charging
The main Bi-directional Transformer is protected against over temperature
during Battery Charging Mode by reducing the charging current to
50% if the transformer temperature ≥ 120°C. Hence, during charging
process, the internal charging current being displayed on the LCD
display might drop to half the value when the temp is
> 120°C. once the transformer cools down, the charging current
will resume to the full value. This cycling may repeat a number of
times till the transformer temperature does not rise > 120°C.
Automatic Interruption of battery
Charger Current In Generator Mode
When Generator Frequency is outside
the Range of 45 Hz To 65 Hz
The magnitude of voltage distortion due to non-linear current draw
is a function of the impedance of the AC source. Standby generators
are normally rated at < 10 KvA and have much higher impedance as
compared to utility transformers that may be rated at hundreds of KvA.
Thus, effects of distortion will be much higher in small size generators.
When powered from lower capacity standby generators, the following
adverse effects are possible in a Bidirectional Inverter Charger, which may
result in damage the Power Mosfets in the battery charging circuit:
-
Distorted voltages will create harmonics of the fundamental
frequency leading to spurious zero crossings that may be sensed as
higher / lower frequency and will adversely affect the timing signals
for switching of the power Mosfets.
-
voltage spikes with peak of > 200v may be generated in the
secondary of the transformer feeding the Mosfets
In G4 Series, protection is provided to limit the voltage distortion to
acceptable limits. When the batteries are being charged at higher
charging current, the generator has to deliver proportionately higher
current that produces higher frequency deviation and waveform
distortion in the output waveform of the generator. When the
frequency of the generator is fluctuating excessively and is
not within the range of 45 Hz to 65 Hz, the control logic will
automatically stop charging to limit voltage distortion to
acceptable limits. Please see details under Section 8 - Operation,
Page 51.
Adaptive Charging Control For
Complete Charging And Prevention
of over Charging / boiling
An automatic Adaptive Charging Algorithm is used that ensures that the
battery is completely charged in a safe manner for longer battery life. In
this algorithm, the time the battery remains in Boost (Absorption) and
Equalization Stages is proportional to the time the battery remains in
the Bulk Charge Stage. A battery that is deeply discharged will remain in
Bulk Stage for a longer duration and will require longer time in the Boost
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