Design skills of lithium battery solar charger circuit

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In recent years, small devices powered by batteries have developed rapidly, such as tablets, handheld game consoles, video players, digital photo frames, and the like. In general, these devices use a rechargeable lithium ion (Li-Ion) battery as the power source. Some common charging solutions include wall adapter class chargers and universal serial bus (USB) class chargers. Although these charger solutions are a low-cost solution for charging lithium-ion batteries, these chargers all share a common disadvantage: they rely on the main power supply to operate. This dependence on the main power supply increases the user's electricity bill and increases greenhouse gas emissions. And because of the dependence on the main power supply, the portability of these charging solutions is also greatly reduced. A solar charger that uses solar panels to collect natural light energy may be an ideal solution to extend battery life in an environmentally friendly way. Another benefit of the solar charger is that it provides a mobile charging solution.

In this article, we will explain some of the important considerations in the development of a solar charging solution. The main reason to consider these factors is that as the lighting environment changes and the voltage and current change, the solar panel becomes a high output impedance power supply. The wall power adapter or USB power supply is a low output impedance power supply with a pre-defined output voltage and current. Among the factors we need to discuss in our solar charging solutions are: maximum power point tracking (MPPT), reverse leakage protection, charge termination method tips, and solar panel crash protection.

The maximum power point tracking maximum power point (MPP) is the solar cell working area that can obtain the maximum power [1]. The graph in Figure 1 shows the region. The graph shows the typical output current vs. output power versus the MPP two-section solar panel voltage curve. The MPP on the curve is obvious because it is the voltage and current corresponding to the maximum power output of the solar panel. MPP is related to ambient temperature and light and therefore will change over time. This suggests that chargers that utilize solar power must have appropriate circuitry to keep track of the MPP as environmental conditions change. MPPT solutions range from simple open-loop technology (cell voltage is maintained at a fixed open circuit voltage) and complex microcontroller-like technologies (measuring input and output power and then properly adjusting panel voltage).

Figure 1 Output current and output power as a function of two-section solar panel voltage

Proper selection of an MPPT solution for a charging solution requires a trade-off between cost and efficiency, and should depend on the application.

Reverse Leakage Protection Reverse leakage is a phenomenon in which the charge stored in the battery is lost and returned to the power supply. Reverse leakage occurs when the battery voltage is higher than the power supply. When this happens, the power supply becomes the load on the battery and the battery is no longer charged. This state does not occur when using a wall power adapter or USB power supply because the voltage output of both power supplies is always above the Li-Ion supply voltage. When using solar panels, the voltage of the solar panel will drop below the battery voltage in the absence of light. Figure 2a shows a schematic of a USB power charger connected to the battery. When switch S1 is turned off, the power is disconnected from the battery and the battery has no current. When using solar panels, if the same layout is used, the switch body diode turns on if the solar panel voltage drops below the battery voltage. One common way to solve this problem is to use a back-to-back switch, as shown in Figure 2b.

Figure

2a Schematic diagram of USB type charger showing power switch

Figure 2b shows the schematic of a solar panel charger for a back-to-back power switch

Charge termination Lithium-ion battery charging requires precise current and voltage control of the battery to ensure that the battery is fully charged, to prevent battery life, and to prevent dangerous conditions during charging. The common process of charging a Li-Ion battery (see Figure 3) can be divided into three phases: pre-regulation, constant current charging, and constant voltage charging.

Figure 3 Battery voltage and current graphs for different stages of Li-Ion battery charging

During the pre-regulation phase, the battery is charged with a constant current of 0.1C (usually) to slowly increase the battery voltage to approximately 2.5V. This stage is only used for deep discharge batteries. Once the battery voltage rises above ~2.5V, constant current charging is used. During the constant current charging phase, the battery is charged with a constant current of 1C (normally) until the battery voltage reaches ~4.2V. Once the battery voltage reaches ~4.2V, the battery is charged with a constant 4.2V voltage. At this stage, the current entering the battery needs to be monitored. When the battery current drops to 0.1C, the charge is terminated. During the constant voltage charging phase, the current entering the battery is reduced because the battery impedance increases as the battery is fully charged. Once the current is reduced below 0.1C, the charging source must be completely disconnected from the power source. If it is not completely disconnected, metallic lithium plating will occur, which will make the battery unstable and dangerous. We must stop charging the Li-Ion battery based on the current entering the power supply to ensure that the battery is just full to its maximum charge.
Chargers that use solar charging must follow the charging process described above. Most of the problems occur during the constant voltage charging phase that monitors the battery current. The current entering the battery may be reduced, but not because of the increase in battery power, but because of changes in the lighting environment that result in a decrease in solar panel output. Therefore, the battery may never be fully charged to its maximum charge, and the solar panel may always be connected to the battery. To solve this problem, we can use a long time constant timer. At the end of the timer, the solar panel is disconnected from the charger, regardless of battery level, which prevents battery damage.

Solar panel crash protection In some traditional chargers, we know the current and voltage of the power supply in advance. Therefore, the charger circuit is specifically designed to operate within the specified range of power supply. When using solar panel output, the current magnitude and open circuit voltage are dynamic, depending on the surrounding environment. Therefore, designing a control loop for a solar charger is more challenging than a wall power adapter.

The system that uses solar energy to charge lithium-ion batteries, while trying to maintain the charging process of lithium-ion batteries, must not allow the solar panels to collapse unexpectedly. Because if the solar panel voltage drops sharply, it is impossible to obtain useful electrical energy from the solar panel. There is a high probability of solar panel collapse during the constant current charging phase. At this stage, the solar panel may not provide the current required to charge the battery. When this happens, the solar panel voltage begins to collapse rapidly. Therefore, the charger must be able to detect a rapid drop in the solar panel voltage and immediately reduce the current drawn from the solar panel to prevent the solar panel from collapsing.

Summary Solar chargers provide a mobile, environmentally friendly charging method for lithium-ion batteries. When designing a solar charger, you will encounter many problems that are not encountered when designing a wall power adapter charger. If the designer uses his brain, he can design a charger that can be input using solar, USB and wall power adapters to achieve a perfect charge of the lithium-ion battery.