Solar panel powers two-stage lead-acid battery charger
A solar-powered lead-acid battery charger can ensure that the battery remains fully charged over a wide temperature range. The ideal charging circuit compensates for temperature and sunlight variations, including recovery from shading.
Ramesh Khanna and Frank Edrada, National Semiconductor -- EDN, June 24, 2010
At A Glance
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Solar or photovoltaic panels comprise multiple solar cells that connect in series. An ideal solar cell appears as a current source that connects in parallel with a rectifying diode. The photovoltaic current, IPH, depends on the sunlight falling on the solar cell. In the dark, a solar cell is simply a diode. A solar cell residing in shade has limited currentgenerating and -carrying capability, resulting in limited current-carrying ability for the entire solar panel.
The current-output ability of a solar cell falls with a reduction in sunlight (Figure 1). Figure 2 shows the effect of temperature on a solar cell’s output voltage;
that is, solar-cell output voltage decreases with an increase in temperature.
This design uses a solar panel comprising a series of solar cells to charge a
lead-acid battery when the solar panel is operating at the maximum power
point. A number of design steps are necessary to ensure that the cell
operates continuously at the “knee” of the IV curve, thus providing
maximum power to the load (figures 1 and 2).
The design in Figure 3 uses an 18-cell, 3W SunWize Technologies SC3-6V solar panel as the input
source. The design comprises one stage that monitors
the solar-panel voltages using a SEPIC (single-ended-primary-inductor-converter) topology that employs a National
Semiconductor LM5001
controller. The LM5001 provides the output voltage
that tracks the solar panel’s output voltage
over the temperature range. A simple, cost-effective
thermal-monitor circuit using a string
of BAT54 diodes tracks the solar panel’s
voltage over a temperature range of 25 to
100°C.
The second stage of the design
takes the output of the first stage
and boosts the input voltage to
a nominal 13.3V at 25°C and
14.4V at 100°C. The second
stage is configured as a constant-current controller to charge
the 12V lead-acid battery that matches
the optimum charge technique for a
lead-acid battery. Figures 3a and 3b are detailed
schematics for the two stages.
The charging current into the battery also varies to ensure that the solar panel does not exceed its maximum power at high temperatures. You accomplish this task by reducing the input current to the battery at higher temperatures. Again, a cost-effective thermal-monitor circuit using BAT54 diodes provides feedback to adjust the output voltage and output current over the temperature range.
The design incorporates recovery from shading, overvoltage protection, dual-stage current regulation, and operation over a wide temperature range. Shading reduces the solar panel’s output current and can force the solar panel, which has limited output-power capability, into an overload condition to the right of the knee shown in Figure 1. A National Semiconductor LM4041monitors the input voltage, and, if the input voltage decreases due to shading on the solar panel, the unit will undergo restart mode once you remove the shading-induced false condition.
Under full sunlight, the solar panel
delivers 198 mA of current under
loaded conditions at 25°C (Table 1).
A shaded solar panel’s current decreases
to 30 mA, depending on the extent
of the shading. This decrease forces the
solar panel into an overload condition
because the battery requires a higher
current than the solar panel can deliver,
and the solar panel’s output voltage
decreases from approximately 11V to
less than 4V. The design incorporates
dual-stage current regulation to provide
optimum current under various temperature
conditions.
Stage 1 of the design uses a SEPIC topology to take the output of the solar panel, which varies from 9V at 25°C to 6V at 100°C, as the input to the SEPIC. The SEPIC design uses a National Semiconductor LM5001 operating at 780 kHz. The solar panel has a negative- temperature coefficient of −2.2 mV/°C per cell. For a panel comprising 18 cells, that coefficient amounts to −39.6 mV/°C for an unloaded panel. This coefficient implies that the solar panel’s voltage varies by −40 mV/°C. That is, if you get 9V at 25°C, then a variation of 75°C (40 mV/°C), or 3V will occur at 100°C.
A SEPIC is a dc/dc converter that
allows the output voltage to be greater
than, less than, or equal to the input
voltage and provides an output voltage
that is of the same polarity as the input
voltage controlling the duty cycle
of the control transistor (Figure 4).
In continuous mode—that is, when the input inductor current, IL1—never falls to 0A—the average voltage across VC1 equals the input voltage, provided that the value of C1 is large enough. You can easily visualize this operation because the average voltage across inductors L1 and L2 is 0V; the loop comprising VIN, L1, C1, and L2 highlights the fact that VC1 equals the input voltage. Because C1 blocks dc current, the average current through capacitor C1 and IC1 is 0A. Thus, the average current through L2 is the average load current and independent of input current.
Replacing inductor L2 with a transformer
yields an isolated version of a
SEPIC. Using a coupled inductor—
that is, a 1-to-1 transformer—in place
of L1 and L2 makes the design more
cost-effective and allows you to replace
inductors L1 and L2 with one magnetic
element. You can then redraw the schematic
(Figure 5). Turning on Q1 holds
the positive C1 terminal that connects
to the drain of Q1 at ground level, and
1-to-1 transformer T1 induces a voltage
equal to the input voltage at the
junction of D1 and C1. Thus, the voltage
across capacitor C1 equals the input
voltage. The SEPIC can provide
an output voltage that is greater than
or less than the input voltage, according
to the follow equation: (VOUT/VIN)=
D/(1−D), where VOUT is the output
voltage, VIN is the input voltage, and D
is the duty cycle of the main FET, Q1.
Because the solar panel has limited
current-output capability, you must
consider the inrush-current capability
of the circuit. The LM5001 control element
operates at 780 kHz, which determines
the internal soft start. The
design incorporates an external softstart
circuit comprising D1/D1A, R, and
soft-start capacitor, CSS, to extend the
soft-start time and ensure that the solar
panel does not become overloaded during
turn-on (Figure 6).By changing the value of RB in Figure 7 to adjust the voltage divider in
the first stage of the SEPIC, you can adjust the SEPIC voltage’s setpoint to accommodate
different solar-panel voltages.
You can adjust a resistor divider comprising
RT and RB to match the SEPIC’s reference
voltage, typically 1.34V at 25°C,
which the thermal board generates. A
string of diodes generates a reference voltage
that varies with temperature, thus providing
temperature compensation to the
design. The output of the first stage tracks
the solar panel’s voltage and adjusts for
varying temperature. It is critical that the
reference circuit comprising the string of
diodes be close to the hot spot to track the
temperature variation.
Battery-charge current
The second stage of the design comprises
a SEPIC that operates in boost mode
with constant current-charge control.
This SEPIC charges the 12V battery (Figure 8). Adjusting
the value of RCA1 and RCA2 allows the adjustment of the battery-charging current to meet customer needs. You set the
battery-charging current, IBAT, by dividing the reference voltage,
which diode D1−VD1’s forward drop sets; a typical value
is 0.183V at 25°C. You then divide the result by RCA1+RCA2.
Because the reference voltage’s drop, due to D1, varies with
temperature, this calculation accounts for the battery’s
charging-current variation over temperature: IBAT=(VD1)/
(RCA1+RCA2). You set the two-stage current charging by bypassing
the RCA2 current-set resistor once the temperature
reaches 60°C. Adjusting the RTB and RBB resistor values in Figure 8 sets the second-stage converter’s boost voltage.
When the solar panel is in shade, its current capability
decreases, resulting in overloading of the solar panel because
of the fixed load on the solar panel; battery charging
requires this fixed load. This overloading reduces the solar
panel’s output voltage. The RBR/RTR divider that connects to
the LM4041 in the first stage senses the solar panel’s output
voltage, which initiates a restart mode by disabling the second
stage of the circuit by pulling down on the enable pin of
LM5001 (Figure 9).Once you remove the fault condition that shading causes, the solar panel’s voltage increases and allows the circuit to operate in the normal condition. The RBR/RTR divider senses when this voltage falls due to shading and rises again. The divider disables the second stage by pulling down the LM5001’s enable pin.
Boost overvoltage protection
If you accidentally disconnect the thermal board that
houses both diode D1 in Figure 8 and diode string VREF in Figure 7, the boost output voltage will clamp to a fixed output
voltage, thus preventing the output voltage from exceeding
a fixed, predetermined level and preventing any damage
to the battery.
The following equation determines the maximum boost voltage in case of an accidental disconnection of the thermal board: VBOOSTCLAMP=[(R0+RTB+RBB)/(RBB+R0)]VREFLM5001, where the LM5001’s reference voltage is nominally 1.26V, ensuring that the battery does not exceed its overvoltage limit.
Test data on the unit over temperature
in the lab using a dc source shows
the performance of the two-stage solarcharger
circuit. Figure 10 shows the
battery-charging current versus temperature,
and Figure 11 shows the
voltage from the solar panel versus
the overall charger efficiency. Figure
12 shows the typical waveforms of the
circuit, highlighting the battery-charging
current, the output ripple, and the
switch node’s waveform.At 60°C, the circuit moves into the
second-stage current regulation, in
which the current to the battery increases.
The bolded text in column 7
in Table 1 highlights this transition
point. This transition point provides
optimum efficiency for the application because 60°C is the
normal operating temperature for the application.
The solar-powered lead-acid battery charger connects to the battery using an industry-standard onboard-diagnosticconnector interface or another equivalent connection mechanism. You can adjust the design to accommodate various solar panels with different power and voltage ratings. You can also modify this cost-effective design to incorporate short-circuit protection with only three extra components.
Talkback
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Promised detailed schematic please...
Mario Craciun - 2010-22-8 09:34:47 PDT -
Where's the detailed schematic????????????????
Mike Hovenga - 2010-30-7 11:34:43 PDT -
This is a great article. But where can I find the schematics?
Dave D'Andrea
David D'Andrea - 2010-16-7 07:08:28 PDT -
The detailed schematic given has a input/output pin symbol called "Reset". Where does this connection going to? Also, is R10 (schematic 3b) supposed to be 0 Ohms? Where is the "thermoal sense board" schematic? There are three connections shown in Schematic 3b, but how is the thermal sense board schematic? OK article, but too much missing info about the detailed schematic...
Arild Kolsrud - 2010-15-7 14:16:27 PDT -
Where are the schematics that are supposed to be at www.edn.com/100624df?
Joel Kramer - 2010-11-7 05:43:07 PDT
