Managing Battery Charging - Using Diversion Loads
Tcharge regulation can be relatively simple: When the I-'1/batteries are full, the controller disconnects the PV r array. Adding wind or microhydro to the system makes charge regulation more complicated, since turbines may overspecd if unloaded. Often, diversion control is the solution. Off-grid 'hybrid' combinations of solar, wind, or microhydro have been around for decades. These renewable electricity systems can power buildings and other sites far from the electrical grid. But hybrid systems can present unique design challenges, since multiple sources of generation—for example, a PV array, wind turbine, and a backup generator— increase the complexity of how to control batter}' charging so your batteries don't get overcharged, and damaged.
Details
In a PV system, the charge controller is placed between the energy source and the battery. Its job is to regulate the voltage and current coming from the energy source to charge the battery and protect the battery from overcharge (and damage). Modern charge controllers have a three-stage charge cycle. During the "bulk" phase, the voltage rises to the bulk level while the batteries draw maximum current. Once this level is reached, the absorption phase begins: the voltage is maintained at the bulk phase level for a specified time, while the current tapers off as the battery reaches a full charge. Once the battery is fully charged, the voltage drops to the float level and the batter}` only draws a small current until the next cycle.
Today`s charge controllers mostly use pulse width modulation (PWM) to control current into the batter)`. Pulses of current in rapid succession are allowed to pass from the energy source to the battery (or from the battery to the diversion load—more on this to follow). The controller cannot limit the size of this current but instead controls the duration so that it can achieve the correct average current in the circuit. By modulating the width of the pulses, the controller regulates the batter)` charge rate. The target is to maintain the correct battery voltage for the prevailing stage of the charging cycle.
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