However, power available from a solar panel is heavily dependent on the operating environment. This includes such things as light intensity, time and location, etc. Therefore, batteries typically are used as energy storage elements. They can be charged when extra power is available from the solar panel, as well as to power the system when the available power from the solar panel is insufficient. How do we design a Li-Ion battery charger to get the most out of the solar cells and efficiently charge the Li-Ion battery? First, we'll discuss the operating principle and electrical output characteristics of a solar cell. Then, we'll cover battery charging system requirements and system solutions for matching the solar cell characteristics to get the maximum power from the solar cells.

Solar cell I-V characteristics
Basically, a solar cell comprises a p-n junction in which light energy (photons) causes electrons and holes to recombine, generating an electric current. Because the characteristics of a p-n junction are similar to those of a diode, the electrical circuit shown in the figure below is often used as a simplified model of the solar cell's characteristics.

Simplified Circuit Model of a Solar Cell

The current source I_PH generates a current proportional to the amount of light falling upon the cell. With no load connected, nearly all the current generated flows through diode D, whose forward voltage determines the solar cell's open circuit voltage (V_OC. This voltage varies somewhat with the exact properties of each type of solar cell. But for most silicon cells, it is in the range between 0.5V and 0.6V which is the normal forward voltage of a p-n junction diode.

The parallel resistor (R_P) represents a small leakage current that occurs in practical cells, while R_s represents the connection losses. As the load current increases, more of the current generated by the solar cell is diverted away from the diode and into the load. For most values of load current, this has only a small effect on the output voltage. Solar output
The figure below shows the output characteristics of a solar cell. There is a small change due to the diode's I-V characteristic. There is also a small voltage drop due to the series resistor (R_S), but the output voltage remains largely constant. At some point, however, the current flowing through the internal diode becomes so small that it becomes insufficiently biased, and the voltage across it decreases rapidly with increasing load current. Finally, when all the current generated flows through the load and none through the diode, the output voltage is zero. This current is known as the solar cell's short circuit current (I_SC). Together with V_OC, it is one of the primary parameters defining its operating performance. Therefore, the solar cell is considered a "current-limited" power source. When the output current increases, its output voltage drops until it finally reduces to zero, when the load current reaches its short circuit current.

Typical Solar Cell I-V Characteristic

In most applications, it is desirable to get as much power as possible out of the solar cell. Since output power is the product of output voltage and current, it is necessary to determine which part of the cell's operating region yields the maximum value of the product of output voltage and current. This is known as the maximum power point (MPP). At one extreme, the output voltage is at its maximum value (V_OC), but output current is zero. At the other extreme, the output current is at its maximum value (I_SC), but output voltage is zero. In both cases, the product of output voltage and current is zero. Hence, the MPP must lie somewhere between the two extremes.

It can be proved easily (or experimentally observed) that in any application, the MPP actually occurs somewhere on the knee of the solar cell's output characteristic (see figure below). The problem in practice is that the exact location of a solar cell's MPP varies with light and ambient temperature. Systems designed to maximize solar power generation, therefore, must dynamically scale the current drawn from the solar cell so that it operates at or near the MPP under the actual operating conditions.

Solar Cell Output Characteristics

The dynamic power path management (DPPM) technology can meet this design challenge for tracking MPP. The figure below shows the Lithium Ion (Li-Ion) battery charge application circuit to maximize available power from the solar panel, where MOSFET Q₂ is used to regulate battery charge current, charge voltage or system bus voltage. A solar panel is used as a power source to recharge a single Li-Ion cell. The solar panel comprises a number of strings, each with 11 silicon cells in series. It behaves like a current-limited voltage source in which the current limit is determined by the size of the panel, and the amount of light falling on it.

DPPM monitors the system bus voltage (V_OUT) drop due to the current limiting power source. The capacitor (C_O) connected across the system bus starts to discharge, causing the system bus voltage to start dropping once the current required by the system and battery charger is greater than the current available from the solar panel. Once the system bus voltage falls to the pre-set DPPM threshold, the battery charge control system regulates the system bus voltage at the DPPM threshold. This is accomplished by reducing the battery charging current, thereby achieving maximum power from the solar panel. The DPPM control circuit tries to reach a steady-state condition where the system gets its needed power, and the battery is charged with the remaining power. This maximizes the use of the power available from the solar panel, and improves system reliability.

Using a Solar Panel to Charge One Cell Li-Ion Battery

Assuming a voltage drop of 300mV across MOSFET Q₁, the voltage across each cell will be equal to 436mV, which maximizes the solar panel's power output. If V_OUT is greater than 4.5V, the DPPM function does nothing — and the solar panel moves away from its MPP. But this only happens if less power is needed by the system and the battery charger than the solar panel can supply. In which case, a reduction in efficiency is not so important. Figure 3 shows that the output power curve is quite flat as it approaches the MPP, then falls off sharply. Therefore, it is better to set V_DPPM slightly high rather than slightly low. This will minimize the effect of an incorrect operating point on output power. If the power available from the solar panel is insufficient to power the system, even when the battery charge current has been reduced to zero, MOSFET Q₂ turns on, V_OUT drops to just below the battery voltage V_BAT, and the battery provides whatever current the solar panel is unable to provide.

The internal safety timer is automatically extended if the charger is operated in DPPM. Thus, when considering special operating conditions such as low-light or no-light conditions, battery charging is very slow, or the battery could even operate in discharge mode. It is nearly impossible to set the proper charge safety timer to cover all applications. Otherwise, it may generate a false safety timer fault. Therefore, disabling the safety timer is one option to solve this issue.

Summary
The power source from a solar panel is considered a "current-limited" voltage source. The maximum power from the solar panel to charge a Li-Ion battery can be achieved by regulating the system bus voltage around the MPP through charge current reduction when the total current demand from the system and battery charging exceeds the output current capability from the solar panel. System power and battery charging power control architecture are critical elements for designing a reliable solar panel powered system.

About the author
Jinrong Qian is an Applications Engineering Manager and Distinguished Member of the Technical Staff for the Portable Power Battery Management group at Texas Instruments. He has published more than 40 peer-reviewed power electronics transactions and power management articles, and holds 19 U.S. patents. He earned a Bachelor of Science degree in Electrical Engineering from Zhejiang University in 1985, and a Ph. D. from Virginia Polytechnic Institute and State University in 1997. Jinrong can be reached at ti_jinrongqian@list.ti.com.

Nigel Smith is a system engineer specializing in power systems for portable applications at Texas Instruments. Before joining TI, Nigel worked for several years in the space industry where, amongst other things, he designed a number of NiCd and Li-Ion-based satellite power systems. He earned a Bachelor of Science degree from Electronics from Salford University (UK). You can reach Nigel at ti_nigelsmith@list.ti.com.