Simplifying solar-based battery charging
Background
Solar power is green and abundantly ‘free’, but can often be less than reliable. Varying temperature effects that shift the solar panel’s optimal power delivery point, in addition to device aging, partial shading, the sun going down, animal waste, etc. can all impede a panel’s performance. Due to these reliability and variability concerns, nearly all solar-powered devices feature rechargeable batteries for backup power purposes.
Once just lead-acid based, these batteries have now expanded to include lithium-based chemistries too. The goal of the solar-based recharging system is to extract as much of the solar power as possible to charge the batteries quickly, as well as maintaining their state of charge. Furthermore, drain on the battery when the panel is lightly, or not illuminated, is important and should be minimized whenever possible.
Clearly, solar powered applications are on the rise. Solar panels of various sizes now power a variety of innovative applications from crosswalk marker lights to trash compactors to marine buoy lights. Some batteries used in solar powered applications are a type of deep cycle battery capable of surviving prolonged, repeated charge cycles, in addition to deep discharges. These type of batteries are commonly found in ‘off grid’ (i.e., disconnected from the electric utility company) renewable energy systems such as solar or wind power generation. System up time is paramount for off-grid installations due to proximity access difficulties.
Solar panel basics
For a given amount of light energy and operating conditions, a solar panel has a certain output voltage at which peak output power is produced. Figure 1 shows the characteristics of a 72 cell panel at a panel temperature of 60°C. The blue line shows the I-V curve of the panel with the x-axis being the panel voltage. The dashed red line shows the resulting output power of the panel as the panel voltage is swept from 0 V to the open circuit voltage of the panel using a simple load box to accomplish the sweep. For this particular case of conditions, the maximum power point is at 32 V and the panel can deliver 140 W. Once the panel temperature is allowed to vary, which it certainly will in a real world setting, the maximum power point can vary between 28 V on a hot day to 44 V on a cold winter’s day.
Figure 1: With no partial shading, a simpler power curve can exist for a given solar panel
Many simpler solar charging systems set the panel voltage operating point to a fixed level. In the case of this particular panel, these simpler systems would set the operating point of the panel to be 32 V in order to extract the most power at a given temperature, 60°C in this case. However, when the panel temperature changes, significant power is wasted because the panel is no longer operated at its true maximum power point. Upwards of 20% to 30% of the available power can be wasted in these cases.
To make matters worse, most panels are required, by safety standards set in place, to have bypass diodes built into the solar cell array. The reason for this has to do with what occurs when only portions of the panel are shaded from sunlight, while other areas get full sun. When this occurs, the solar cells that are shaded become reverse biased but still have high currents flowing through them because the other illuminated cells are providing the current. High temperatures in the shaded cells can occur and this can pose a fire hazard. To help lower the risk of fire, manufacturers place bypass diodes throughout the panel. Figure 2 shows how bypass diodes can be placed in the 72 cell panel.
Figure 2: 3 bypass diodes placed in a 72 cell solar panel for safety considerations
With bypass diodes in the panel, complex power versus voltage characteristics can occur when partial shading is present. Figure 3 shows such a scenario where two local power maxima are present, one at 21 V and the other at 37 V. If the above 32 V simple power point method were used, 79.4 W of power would be available compared to 90.1 W available at the true maximum power point of 21 V. This represents a significant loss of 13.5% in this case. Clearly, a system that can operate and track the true maximum power point would be a superior approach.
Figure 3: With partial shading, more complex power curves exist for a solar panel
Design challenges of a solar powered rechargeable battery system
Typical solar panel efficiencies range from about 5% to 15% Combined with the fact that larger (i.e., more powerful) panels cost more, solar powered designs must maximize efficiency to minimize total system cost.
To effectively harvest energy from the sun in a solar based product, the design must manage a widely varying input while also finding a way to operate the solar panel at or near its maximum power point. Furthermore, the design must safely charge the battery chemistry of choice used in the product.
There are also other design problems encountered with solar charging systems. For any given solar-powered system, firmware development and debug can take a huge amount of time. A more complex buck-boost topology is needed if the panel’s optimal power delivery point can be below, equal to, or above the battery voltage (this is a very common scenario). A buck-boost topology allows true isolation in both directions (when compared to a step-down or ‘buck’ topology only, if the panel is dark it might drain the battery through the body diode of the NMOS through the inductor). Proper voltage termination is needed to protect the battery. Finally, since the panel is not a reliable source of power, in-situ charging of the battery (where the charger feeds the battery, and a load is connected to battery) is needed – the battery is the power source but also acts as a ‘buffer’ in this case.
What is Maximum Power Point Tracking (MPPT) and why is it needed?
Maximum Power Point Tracking is a technique that helps extract the highest amount of power from the panel under all operating conditions. Some of these non-ideal operating conditions include:
- Panel is partially shaded (leaves, bird droppings, shadows, snow, etc)
- Panel’s temperature variation
- Panel’s aging
For example, in off-grid solar panel systems, failure of the power system is costly. Customers want to extract the most power they can from the panel. Further, they want to maximize the time interval needed between maintenance visits to the solar installation.
True active MPPT will seek out the optimum operating point under all conditions. This results in less overall system cost because the smallest panel or smallest battery can be used, reducing the need to over design the system. True MPPT will find the best peak power point and reject false local maximum common in partially shaded panels (note: partial shading power patterns are determined by the number and arrangement of bypass diodes inside the panel).
A Simple IC Solution
An IC charging solution that solves the problems outlined above needs to possess many, if not all, of the following attributes:
• Minimal software and firmware development time
• Flexible buck/boost topology
• Active MPPT algorithm
• Simple, autonomous operation (no µP needed)
• Termination algorithms for various battery chemistries
• In-situ charging – to power a load while the battery is being charged
• Wide input voltage range to accommodate various power sources
• Wide output voltage range to address multiple battery stacks
• High output/charging current
• Small, low profile solution footprints
• Advanced packaging for improved thermal performance and space efficiency
• Cost-effective solution
Typical convoluted competing solar battery charging systems consist of a DC-DC switching battery charger, a microprocessor plus several ICs and discrete components in an attempt to replicate maximum power point control / tracking functionality. An alternative solution could be a solar module; however these are costly, not simple to design in (require software, firmware, etc.) and tend to lock on to false solar panel maxima and therefore do not operate as efficiently as possible. Fortunately, a simpler solution is at hand, thanks to a buck-boost solar powered battery charging controller.
An efficient solar-powered solution
Linear Technology has developed a simple, innovative high voltage buck-boost charging controller IC specifically for solar applications, one which requires neither software nor firmware development, thus greatly reducing time-to-market.
The LT8490, shown in Figure 4, is a synchronous buck-boost battery charging controller for lead acid and Lithium batteries, featuring automatic maximum power point tracking and temperature compensation. The device operates from input voltages above, below or equal to the regulated battery float voltage. The device’s full-featured battery charger offers many selectable constant-current constant-voltage (CC-CV) charging profiles, making it ideal for charging a variety of Lithium or lead acid chemistry types, including sealed lead acid, gel cells and flooded cells. All charge termination algorithms are provided onchip, eliminating the need for software or firmware development, thus reducing design cycle time.
The device operates over a wide 6 V to 80 V input voltage range and can produce a 1.3 V to 80 V battery float voltage output using a single inductor with 4-switch synchronous rectification. The device is capable of charging currents as high as 10 A depending on the choice of external FETs. The LT8490’s MPPT circuit enables a sweep of the full operating range of a solar panel, finding the true maximum power point, even in the presence of local maxima points caused by partial shading of the panel. Once the true maximum power point is found, the device will operate at that point while using a dithering technique to quickly track changes in the local maximum power point. With this methodology, the LT8490 fully utilizes the power generated by a solar panel even in non-ideal operating environments.
A global MPPT sweep is shown in Figure 5. The yellow trace shows the panel output voltage. The device commands the panel voltage to go to the open circuit level then subsequently commands the panel to ramp down linearly to the minimum level. The red trace shows the panel current as the panel voltage changes. The current is measured by the LT8490 and the power is calculated inside the IC. Once the sweep is completed, the panel voltage is returned to the point at which maximum power was measured.
Figure 5: Global MPPT sweep from the LT8490 (Yellow – Panel Voltage, Red – Panel Current, Green – Control Signal from LT8490)
The dithering technique is used to track smaller changes in the maximum power point between global sweeps. This is shown in Figure 6. About midway through the scope shot, a change to the power point is applied to the panel to simulate a cloud moving in the sky thus changing the amount of sunlight striking the panel. The device continuallly moves the panel voltage a small amount above and then below the current MPPT point to check if a better operating point exists. If it finds one, it properly tracks to the new point and repeats the process. In this way, the device is able to track changes without having to do a global sweep too often.
The charging controller performs automatic temperature compensation of the battery charge voltage by sensing an external thermistor on the battery. The STATUS and FAULT pins can be used to drive LED indicator lamps. Charging current limits can be adjusted by changing as few as 1 or 2 resistors, and a charging time scale can be selected with the appropriate resistor divider. Other features of the device include: input and charge current limit pins, a 3.3 V regulated LDO output, status pins and a synchronizable fixed switching frequency.
The charging controller is available in a low profile (0.75 mm) 64-pin 7 mm x 11 mm QFN package. The device is guaranteed for operation from -40°C to 125°C.
Key device features include:
• VIN Range 6 V to 80 V
• VBAT Range 1.3 V to 80 V
• Single Inductor Allows VIN Above, Below, or Equal to VBAT
• Automatic Sweeps to Find True MPPT Point
• Support of Many Types of Lead Acid and Lithium Battery Types
• LED Drivers for Status Indication
• Battery Charging Algorithms Built-in
• Automatic Float Voltage Temperature Compensation
• Input & Output Current Monitor Pins
• Synchronizable Fixed Frequency: 100 kHz to 400 kHz
• 64-Pin 7 mm x 11 mm x 0.75 mm QFN Package
Conclusion
Solar power is green and plentiful, but can be less than reliable. Varying temperature effects moving the MPPT point, aging, partial shading, the sun going down, bird waste, etc. can all reduce panel performance. Linear Technology has developed the LT8490, a buck-boost switching regulator battery charger that implements a constant-current constant-voltage (CC-CV) charging profile used for most battery types, including lead acid (sealed SLA, flooded, gel) and Lithium. It also provides automatic and efficient true maximum power point tracking for solar powered applications. No software or firmware development is required, thus greatly reducing system time-to-market. The device is also a cost-effective and simpler solution than large and complex convoluted competing charging systems. It greatly simplifies what was historically a difficult design task.
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