Learn about charging your batteries from renewable resources and what it costs.
As we stress mother earth to the breaking point by burning fossil fuel, concerned folks are gravitating towards using renewable energy. The sun provides peak power of about 1,000 watts per square meter (93W/sq.ft.) and a solar panel transforms this power into roughly 130W per square meter (12W/sq.ft.). These conditions correspond to a clear day with the solar panel facing the sun that is 42° or better. Surface dust on the solar panels and high heat reduce the overall efficiency.
Generating electricity by sunlight goes back to 1839 when Edmond Becquerel (1820–1891) first discovered the photovoltaic effect. It took another century before researchers understood the process on an atomic level that works similar to a solid-state device with n-type and p-type silicon bonded together.
Commercial photovoltaic (PV) systems are 10 to 20 percent efficient. Of these, the flexible panels are only in the 10 percent range and the solid panels are about 20 percent efficient. Multi-junction cell technologies are being tested that achieve efficiencies of 40 percent and higher.
A solar cell produces an open circuit voltage of 0.50–0.65V. Like batteries, solar cells can be connected in series and parallel to achieve higher voltages and currents (See BU-302: Series and Parallel Battery Configurations)
At 25°C (77°F), a high quality monocrystalline silicon solar panel produces about 0.60V open-circuit (OCV). The surface temperature in full sunlight will likely rise to 45°C (113°F), reducing the open-circuit voltage to 0.55 V per cell due to lower efficiency. Solar cells become more efficient at low temperatures, but caution must be exercised with respect to the batteries when charging below freezing temperature. (See BU-410: Charging at High and Low Temperatures)
A solar charging system is not complete without a charge controller. The charge controller takes the energy from the solar panels or wind turbine and converts the voltage to a level that is suitable to charge the battery. For a 12V battery bank, the supply voltage is about 15V. This allows charging lead acid to 14.40V (6 x 2.40V/cell) and Li-ion to 12.60 (3 x 4.20V/cell). Note that 2.40V/cell and 4.20V/cell are the respective full charge voltage thresholds for lead acid and lithium-ion.
Charge controllers are also available for lithium-ion to charge 10.8V packs (3 cells in series). When acquiring a charge controller for lithium-ion, observe the voltage requirements. The standard Li-ion family has a nominal voltage of 3.6V/cell; lithium iron phosphate is 3.30V/cell nominal. Connect the correct batteries for which the charge controller is designed. Do not connect a lead acid battery to a charge controller designed for Li-ion and vice-versa. Mismatch could compromise the safety and longevity of the batteries as the charge algorithm between lead and lithium-based batteries differs.
A lower-cost charge controller only produces an output voltage when sufficient light is available. With a diminishing light source, the charge controller simply turns off and resumes when sufficient levels of light are restored. Most of these devices cannot utilize fringe power present at dawn and dusk and this limits them to applications with only ideal lighting conditions.
An advanced charge controller tracks the power by continually measuring the voltage to dynamically adjust the current. It enables maximum power transfer with available light conditions, and this is made possible with maximum power point tracking (MPPT). Figure 1 illustrates the voltage and current source from a solar cell with varying sunlight. The optimal power is available at the voltage knee where the dropping voltage line meets the vertical power line. MPPT determines this point.
Figure 1: Voltage and current from source a solar cell at varying sunlight.
MPPT finds the best power point where the vertical power line meets the dropping voltage curves. (VxA=W)
It should be noted that not all charge controllers with MPPT function equally well. Some systems are coarse and do not respond immediately to light changes, causing the output to fall if a shadow falls on the panel. Other systems drop off too early and do not fully utilize low light conditions.
A common MPPT method is perturb and observe (P&O). The controller increases the voltage by a small amount and measures power. If the power increases by the equal amount, further voltage increases are applied until the optimal setting is reached. P&O achieves good efficiency but it can be sluggish and result in oscillations.
Other methods are incremental conductance that computes the maximum power point by comparing current and voltage deltas. This requires more computation but has improved tracking ability over P&O. Current sweep is a method that observes the current and voltage characteristics of the PV array to calculate the maximum power point.
Solar panels are normally connected in series, each providing about 20V on a sunny day. The controller reads the overall string voltage but if one panel gets shaded, the MPPT loses effectiveness because the average calculation. Advanced systems process each panel, or a group thereof, individually. This allows low voltage tracking of shaded panels down to 5V. The negative is higher system cost.
You may ask: “Why can I not simply plug a 12V solar panel directly into my laptop or mobile phone?” This should work in principle, but it is not recommended. The charge controller unit transforms the incoming DC voltage from the solar panel or wind turbine to the correct voltage range. In bright sunlight, the voltage of a 12V solar panel can go up to 40V, and this could damage the device.
From 1998 to 2011, the price of commercial Photovoltaic (PV) systems has dropped by 5–7 percent annually and analysis suggest that the price-drop will continue. It now costs between $4 and $5 per watt for a typical residential solar installation delivering 5kW. Larger installations cost $3 to $4 per watt with further reductions for megawatt systems.
A maintenance charger is usually powered by a small solar cell and provides a trickle charge on a sunny day. These devices help to prevent sulfation of a lead acid battery when not used for a while. Even a small float charge will keep the battery at full charge.
Choose a maintenance charger that switches from trickle charge to float charge when the battery is fully charged. A prolonged trickle charge, even at a low current, could overcharge the battery and promote internal corrosion. A float charge that is correctly adjusted only replenishes what the battery loses through self-discharge. (See also BU-403: Charging Lead Acid)
Last updated: 2015-11-12
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