Lithium-ion Battery Solar Charger Design Tips

In recent years, the development of small battery-powered devices has been rapid, such as tablet computers, handheld game consoles, video players, digital photo frames, etc. Generally speaking, these devices use rechargeable lithium-ion (Li-Ion) batteries as power sources. Some common charging solutions include wall adapter type chargers and universal serial bus (USB) type chargers. Although these charger solutions are a low-cost solution for charging lithium-ion batteries, these chargers also have a common disadvantage: they rely on main power to operate. This dependence on main power increases users' electricity bills and also increases greenhouse gas emissions. And because of the dependence on main power, the portability of these charging solutions is also greatly reduced. To extend the battery life in an environmentally friendly way, solar chargers that use solar panels to collect natural light energy may be an ideal solution. Another benefit of solar chargers is that they provide a mobile charging solution.

In this article, we will explain some important considerations in the development of solar charging solutions. The main reason for these considerations is that the solar panel becomes a high output impedance power source as the voltage and current change with different lighting conditions. The wall power adapter or USB power supply is a low output impedance power source with a predetermined output voltage and current. The key factors to focus on in the solar charging solution we are going to discuss include: maximum power point tracking (MPPT), reverse leakage protection, charging termination method techniques, and solar panel collapse protection.

Maximum Power Point Tracking
The maximum power point (MPP) is the region of operation of a solar cell where maximum power can be obtained [1]. This region is illustrated by the graph in Figure 1. The graph shows a typical output current vs. output power versus the voltage curve of a dual-cell solar panel with an MPP. The MPP is obvious on the curve because it is the voltage and current that corresponds to the maximum power output of the solar panel. The MPP is dependent on ambient temperature and light, and therefore varies over time. This means that chargers that utilize solar power must have circuitry to continuously track the MPP as environmental conditions change. There are many different MPPT schemes, ranging from simple open-loop techniques where the panel voltage is maintained at a fixed open-circuit voltage to complex microcontroller-based techniques that measure input and output power and then adjust the panel voltage appropriately.

Figure 1 Output current and output power as a function of voltage for a dual-cell solar panel

Correctly selecting the MPPT scheme for a charging solution requires a compromise between cost and efficiency and should be application specific.

Reverse leakage protection

Reverse leakage is a phenomenon where the charge stored in the battery is lost and returned to the power source. Reverse leakage occurs when the battery voltage is higher than the power source. When this occurs, the power source becomes a load on the battery and no longer charges the battery. This condition does not occur when using a wall power adapter or USB power source because the voltage output of these two power sources always remains above the lithium-ion power supply voltage. When using a solar panel, the voltage of the solar panel can drop below the battery voltage when there is insufficient light. Figure 2a shows a schematic of a USB power charger connected to a battery. When switch S1 is closed, the power source is disconnected from the battery and no current is drawn from the battery. When using a solar panel, if the same layout is used, the switch body diode turns on if the solar panel voltage drops below the battery voltage. A common way to solve this problem is to use back-to-back switches, as shown in Figure 2b.

Figure 2a Schematic diagram of a USB charger showing the power switch

Figure 2b shows the schematic diagram of a solar panel charger with back-to-back power switches

Charge Termination
Charging Li-ion batteries requires precise current and voltage control of the battery to ensure full charge, prevent shortening of battery life, and prevent hazardous conditions during charging. The common process of charging Li-ion batteries (see Figure 3) can be divided into three stages: pre-regulation, constant current charging, and constant voltage charging.

Figure 3. Battery voltage and current curves at different stages of lithium-ion battery charging

During the pre-regulation phase, the battery is charged with a 0.1C constant current (typically) to slowly raise the battery voltage to around 2.5V. This phase is only used for deeply discharged 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 1C constant current (typically) until the battery voltage reaches ~4.2V. Once the battery voltage reaches ~4.2V, the battery is charged with a 4.2V constant voltage. During this phase, the current entering the battery is monitored. When the battery current drops to 0.1C, charging is terminated. During the constant voltage charging phase, the current entering the battery is reduced because the battery impedance increases as the battery is filled. Once the current decreases below 0.1C, the charging source must be completely disconnected from the power supply. If it is not completely disconnected, metallic lithium plating will occur, making the battery unstable and a dangerous state. Li-ion battery charging must be terminated based on the current entering the power supply to ensure that the battery is just charged to its maximum capacity.

Chargers that use solar charging must follow the charging process described above. Problems mostly occur during the constant voltage charging phase where the battery current is monitored. The current going into the battery may decrease, not because the battery charge has increased, but because the solar panel output has decreased due to changes in the light environment. Therefore, the battery may never be charged to its maximum capacity, and the solar panel may always be connected to the battery. To solve this problem, we can use a long constant timer. When the timer ends, the solar panel is disconnected from the charger regardless of the battery charge, thus preventing battery damage.

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

Systems that use solar energy to charge lithium-ion batteries must not allow the solar panel to unexpectedly collapse while trying to maintain the lithium-ion battery charging process. Because if the solar panel voltage drops sharply, no useful power can be obtained from the solar panel. The probability of solar panel collapse is higher during the constant current charging stage. During this stage, the solar panel may not be able to 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 the rapid drop in solar panel voltage and immediately reduce the current obtained from the solar panel to prevent the solar panel from collapsing.

Summary
Solar chargers can provide a mobile, environmentally friendly way to charge lithium-ion batteries. When designing solar chargers, many problems are encountered that are not encountered when designing wall power adapter chargers. If designers use their brains, they can design chargers that can use solar, USB, and wall power adapter inputs to achieve perfect charging of lithium-ion batteries.

References
1. Energy Harvesting: Solar, Wind and Ocean Energy Conversion Systems, by Alireza Khaligh and Omer C. Ona, published by CRC Press, Boca Raton, FL, USA, December 1, 2009, 978-1-4398150-8-3

2. “800mA, Single-Input, Single-Cell Li-Ion Solar Battery Charger,” TI BQ24210 Data Sheet, March 2011 (SLUSA76).

3. Harman Grewal, “Li-Ion Battery Charger Solution Using MSP430,” TI SLAA287 Application Report, December 2005.