Photovoltaic charge and discharge controllers can be basically divided into six types: parallel type, series type, pulse width modulation (PWM) type, multi-channel controller, intelligent type and maximum power tracking type.
1. Parallel controller
When the battery is fully charged, the output of the photovoltaic array is shunted to a parallel resistor or power module using electronic components, and then dissipated in the form of heat. Because this method consumes thermal energy, it is generally used in small, low-power systems, such as systems with voltages within 12V and 20A. This type of controller is very reliable, with no voltage drops in series loops and no mechanical parts like relays. Although this control method is simple and easy to implement, due to the bypass method, if individual cells in the solar cell module are blocked or stained, it is easy to cause a hot spot effect.
2. Serial controller
Use the switch contacts of mechanical relays or solid-state switching devices to control the charging process, the switch is connected in series between the solar cell array and the battery, and the charging circuit is disconnected when the battery is fully charged; the series switch can also be used to cut off the photovoltaic array at night, replacing the anti-reverse charging diode.
3. PWM controller
It switches the input of the photovoltaic array in PWM pulses. When the battery tends to be full, the width of the pulse narrows and the charging current decreases, and when the battery voltage drops, the pulse width becomes wider, which meets the charging requirements of the battery. The switching device used to realize the pulse width modulation function can be connected in series between the solar cell array and the storage battery, or can be connected in parallel with the solar cell array to form bypass control. According to Sandia National Laboratory research and Florida Solar Energy Research Center test results, the charging efficiency of the PWM controller is 30% higher than that of the simple disconnect/recovery (or two-point) controller, which is more conducive to the rapid recovery of battery capacity and the improvement of the total cycle life of the battery.
4. Multi-channel controller
For large-scale photovoltaic power stations above 10kW, multi-channel control technology is generally used, that is, the solar cell array is divided into multiple groups to charge the batteries, when the battery is nearly full, the controller will disconnect the solar cell array one by one, and when the battery voltage drops, the controller will turn on the solar cell array one by one. It achieves the purpose of decreasing the battery’s full current and increasing the battery’s depletion current. It can fully achieve the effect of the PWM controller and meet the charging requirements of the battery.
5. Intelligent controller
The single-chip microcomputer with CPU (central processing unit) is used to collect the operating parameters of the photovoltaic power system in real time at high speed, and the single-channel or multi-channel photovoltaic array is controlled by software program to switch off/on according to certain control rules. The biggest advantage of the intelligent controller is that it can have the functions of data collection and remote data transmission for the operation of the photovoltaic system.
6. Maximum power tracking controller
Multiply the voltage U and current I of the solar cell array to obtain the power P, and then judge whether the output power of the solar cell array reaches the maximum at this time. If it is not running at the maximum power point, adjust the pulse width, modulate the output duty cycle D, change the operating point of the solar cell array, perform real-time sampling again, and make a judgment on whether to change the duty cycle. Through such an optimization process, the solar cell array can always run at the maximum power point. This type of controller can keep the solar cell array always in the maximum power point state to make full use of the output energy of the solar cell array. At the same time, the PWM modulation method is used to make the charging current into a pulse current to reduce the polarization of the battery and improve the charging efficiency. The maximum power tracking controller is more used in photovoltaic water pump systems and grid-connected power generation systems without batteries.