# Tiny dual-battery solar panel charges batteries in compact and off-grid devices

introduction
Today, developments in low-power electronics allow battery-powered sensors and other devices to be placed away from the grid. Ideally, to truly be free from the grid, exempt The need to replace the battery and instead recharge the battery with a renewable energy source such as solar energy provided by the local environment. This design point shows how to build a solar panel that relies on a small dual battery” title=”solar panel”> A compact battery charger that works with solar panels. Unique to this design is that the DC/DC converter uses power point control to draw maximum power from the solar panel.

Importance of Maximum Power Point Control
Although solar cells or solar panels are classified according to their power output” title=”power output” > power output, the available power of a panel is rarely constant. Its output power is highly dependent on light, temperature and the load current drawn from the panel. To illustrate this, Figure 1 shows the VI characteristic curve of a two-cell solar panel under constant light conditions. The IV curve ranges from short circuit (far left) to approximately 550mA load current It has a relatively constant current characteristic inside the battery, then it follows a constant voltage characteristic at lower current conditions and approaches the maximum voltage at open circuit (far right). The power output curve of the panel shows that the power output is at There is a distinct peak at about 750mV/530mA (the inflection point of the IV curve). If the load current increases beyond the power peak, the power curve drops rapidly to zero (far left). Likewise, light loads tend to trend the power to zero (far right), although this is often less of a problem.

Figure 1: Output Voltage, Current, and Power of Solar Panels

Of course, the lighting conditions of the panel will affect the available power – less light means lower power output; more light means higher power output. While illumination directly affects the magnitude of the peak power output, it does not have as much effect on the position of the peak on the voltage scale. That is, the output voltage of the panel where peak power occurs remains relatively constant, regardless of illumination. Therefore, it is wise to adjust the output current so that the voltage of the solar panel is at or above this peak power voltage (750mV here). This practice is called maximum power point control (MPPC).

Figure 2 shows the effect of changes in sunlight on the charging current with and without MPPT control. The intensity of the simulated sunlight drops from 100% to about 20% and then back up to 100%. Note that when the sunlight intensity drops to around 20%, the output voltage and current of the solar panel also drops, but the LTC®3105’s maximum power point control prevents the output voltage of the solar panel from dropping below the set point of 750mV. It does this by reducing the LTC3105’s output charge current to prevent the solar panel voltage from dropping close to 0V, as shown in the graph on the right in Figure 2. And without power point control, a small drop in sunlight intensity completely blocks the flow of charging current.

Figure 2: Changing sunlight intensity affects charging current

LTC3105 Boost Converter with Input Power Control
The LTC3105 is a synchronous boost DC/DC converter designed primarily to convert power from ambient energy sources such as low voltage solar cells and thermoelectric generators to battery charging power. The LTC3105 uses the MPPC to deliver the maximum available power from the energy source. It does this by reducing the LTC3105’s output current to prevent the solar panel’s voltage from dropping close to 0V. The LTC3105 can start up from input voltages as low as 250mV, allowing it to be powered by a single solar cell or up to 9 or 10 batteries connected in series.

The output disconnect function eliminates the blocking diodes often required in other solar powered DC/DC converters and allows the output voltage to be higher or lower than the input voltage. The 400mA switch current limit is reduced during start-up to operate from a relatively high impedance source, but still provides enough power for many low power solar applications once the converter is in normal operation. In addition, the LTC3105 features a 6mA adjustable output low dropout linear regulator, open drain power good output, shutdown input and Burst Mode® operation to improve efficiency in low power applications.

Solar Powered Lithium Ion Battery” title=”Lithium Ion Battery”>Lithium Ion Battery Charger
Figure 3 shows a compact solar-powered battery charger using the LTC3105 as a boost converter and the LTC4071 as a Li-Ion battery shunt charger. A dual-battery 400mW solar panel provides input power to the LTC3105 to generate over 60mA of charging current in full sunlight. As shown in Figure 1, the maximum power point control prevents the voltage of the solar panel from dropping below the maximum power point of 750mV. The output voltage of the converter is set to 4.35V, which is slightly higher than the 4.2V float voltage of the Li-Ion battery.

The LTC4071 shunt charger limits the voltage across the battery to 4.2V. Grounding the FBLDO pin sets the low dropout regulator to 2.2V, which is used to power the “charge” LED. The LED turns on when the battery is charging and turns off when the battery voltage is within 40mV of the float voltage (to indicate that the battery is nearly fully charged). An NTC thermistor senses the battery temperature and reduces the LTC4701’s float voltage to improve battery safety in high ambient temperature conditions. To protect the battery from over-discharge damage, the low battery disconnect feature disconnects the battery from the load when the battery voltage drops below 2.7V.

Figure 3: Dual Battery Solar Panel and Li-Ion Battery Charger

in conclusion
Although the circuit described in this article can produce only a few hundred mW of power, it can provide enough power to keep a 400mAhr Li-Ion battery fully charged in most weather conditions. The low input voltage combined with input power control makes the LTC3105 ideal for low power solar applications. In addition, the LTC4071 parallel charging system complements the LTC3105 by providing precise float voltage, state of charge and temperature safety features to ensure long battery life in outdoor environments.