Learn about charging your batteries from renewable resources and what it costs.
Can I charge cellular phones, laptops or power tools with solar power or a wind turbine? Absolutely. This is an exciting way to step outside the four walls and use 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 higher above the horizon. Surface dust on the solar panels and high heat reduce the overall efficiency.
Commercial photovoltaic 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 achieves 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 (VOC). 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 and attention is needed when charging batteries below freezing temperature. (See BU-410: Charging at High and Low Temperatures)
A solar charging system is not complete without the inverter and charge controller. Most units include the inverter and charge controller to charge 12-volt lead acid batteries. Charge controllers are also available for lithium-ion to charge 10.8V packs (3 sells 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 vise-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. These devices cannot utilize fringe power present at dawn and dusk and this limits the low-cost charge controller to applications with ideal lighting conditions.
An advanced charge controller tracks the power by continually measuring the voltage to dynamically adjust the current. This enables maximum power transfer with available light conditions that 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 find this point.
It should be noted that not all charge controllers with MPPT function equally well. Some systems are course 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.
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)
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 inverter is responsible to transform 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 host.
From 1998–2011, 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 to $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 power by a small solar cell and provides a trickle charge on a sunny day. These devices are useful to prevent sulfation of a lead acid battery when not used for a while.
When shipping maintenance charger, look for a unit 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. Float charge, correctly adjusted, only replenishes what the battery loses through self-discharge. (See also BU-403: Charging Lead Acid)
Last updated: 3/16/2015
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