1. single stage voltage sensing controller
Single-stage voltage sensing is often referred to as a hysteresis controller or dual voltage point controller, which operates with reference to the battery terminal voltage. When the voltage rises to a set value, the controller disconnects the power supply; when the voltage drops to a set value, the power supply reconnects. This cycle can be very rapid, or it can be controlled by setting a minimum period with a timer. Rapid cycling will cause radio frequency interference, and battery terminal voltage will fluctuate, potentially causing unreliable operation of lighting and some equipment. This type of controller performs poorly when the battery is aged and in harsh conditions, as the terminal voltage may not accurately represent the battery’s state of charge and may lead to increased battery sulfation.
Hysteresis is usually controlled by a series of voltage settings, such as boost and float voltages, rather than just one voltage variable. As shown in Figure 1.
The controller operates as follows:
(1) When charging starts, the controller is in the strong charging mode, so it continues to provide charge to the battery until it reaches the strong charging voltage.
(2) Once the strong charging voltage is reached, the charging current will be completely disconnected (cut off) from the battery.
(3) The voltage will then drop until the float voltage point is reached. The power supply will then be reconnected and the voltage will rise until it reaches the float voltage.
(4) At this point the current will be disconnected and the voltage will drop until the float voltage point is reached again and then reconnected to the power supply again.
(5) The cycle will last all day unless the battery drops to a strong charging point. If this happens, the controller will return to its initial state and charge the battery until its strong charge voltage is reached.
2. multi-level switching controllers
Multi-level switching controllers require that the PV array be divided into several sections, each section is connected to a controller, and the controller controls each stage as if each section was connected to a single-stage controller. The switching voltage of each stage can be set to a different value, so that when the voltage rises, it can be removed step by step, and when the voltage drops, it can be added step by step, rather than a fully open and fully closed situation. This type of controller works well in larger arrays, where a full array can cause the battery’s terminal voltage to rise rapidly, increasing the risk of overcharging. This controller is similar to the hysteretic controller described earlier in that it switches the stages on and off based on the voltage setting. Such controllers should not be confused with switch mode controllers.
3. switch mode controller (similar to linear controller)
Over the years, controllers have evolved from simple “lag” charging methods (which are not efficient enough to fully charge the battery) to modern microprocessor-based methods based on more complex charging methods to more efficiently charge the battery to charge. Such controllers can also be programmed to accommodate the different types of batteries on the market, and they are often referred to as “smart” controllers. A recent advance in this technology is a pulse-width modulation (PWM)-based controller that uses a small microprocessor to adjust the controller’s charging voltage to better match the voltage needed to charge the battery.
One of the most commonly used controllers in Australia is the “PL” series produced by Plasmatronics. The following gives a description of the charging method and the principle of ensuring the effective charging of the battery. Other brands on the market follow a similar path. (The following is taken from the PL Reference Manual)
1) Control cycle
The PL controller is designed to keep the battery fully charged without overcharging.
To achieve this, it uses a charge control consisting of three main states: strong charge, absorption, and float charge. There is also an equalization state, which occurs when the cycle is set (for example, charging every 14 days or 30 days). as shown in Figure 2.
2) Strong charge
When in a strong charge state, all available solar current directly powers the battery pack. The battery continues to charge until the voltage reaches the maximum strong charge voltage. The voltage setting is determined by the battery manufacturer.
After the battery voltage maintains this voltage for more than 3 minutes, the controller will automatically switch to the absorption state. This voltage can be adjusted between 2.25~2.75V per cell, for typical lead-acid batteries, the voltage value is usually set to 2.5V per cell.
3) Absorb
In this state, when the battery charging is at the end stage, the controller will keep the battery voltage constant at the absorption voltage, which is achieved by rapidly switching the charging current on and off to control the charging pulse of the battery. It tries to extend the duration of the on-pulse to be longer than the off-pulse, but the width of the on-pulse also varies according to the actual sink voltage (i.e. PWM). Its schematic diagram is shown in Figure 3.
For some batteries, the strong charge voltage and the sink voltage are the same value. This voltage can be adjusted between 2.25~2.75V per cell. For a typical lead-acid battery, the voltage value is usually set to 2.33V per cell.
The controller will hold the voltage of this battery until it reaches the absorption charge time, which is usually set to 2h (can be shortened). After the absorption time is completed, the controller will enter the float state.
4) Float charge
In this state, the battery is fully charged. The charging current is continuously pulse width modulated to maintain the battery voltage at the full charge level. This voltage should be below the gassing voltage to avoid excessive electrolyte loss. If charge is available from the battery, the controller will allow charging to resume until the battery returns to float voltage.
5) Balance
Many battery manufacturers recommend overcharging the battery pack from time to time, aiming to achieve charge equalization by fully charging all cells in the battery pack, while stirring the electrolyte in the electrolyte to reduce stratification. This is the role of equilibrium. After the battery is selected, the battery voltage needs to be raised to the highest equilibrium voltage every number of days (usually 30 to 60 days) between equalization charging, and then maintain the voltage within the set equalization charging time.
6) Return to the strong charging state
In order for the charge cycle to repeat, the controller must return to the high charge state. The following three methods can make the controller enter the forced charging state:
(1) The battery voltage drops below the return strong charging voltage for a period of 10 minutes.
(2) The controller will automatically start the strong charging cycle after the maximum interval between strong charging and charging, regardless of the state of the battery voltage at this time.
(3) Manual implementation through programming.
4. Switch-mode controller based on maximum power point tracking (MPPT)
This controller combines electronics with the controller to find the maximum power point of the photovoltaic array, converting excess available power to charge the battery.
The advantage of using MPPT is that under low light and unstable temperature conditions, the battery voltage is significantly lower than the maximum power point of the array, and it is more significant to use MPPT to charge the battery.
Such controllers are also often used in solar pumped water systems without battery storage, allowing for pumped storage earlier in the day.
[Example] Suppose there is a battery module with 4.45A charging current and 14V voltage, and its maximum power point is 17.1V, 4.38A (this data can be obtained from the solar module manufacturer).
Assuming the battery voltage is 14V, a standard controller will charge the battery at 4.45A.
Ignoring the effect of component temperature derating, if the efficiency of the MPPT is as high as 95%, the power input to the MPPT is 75W (17.1V × 4.38A), this is because the component can operate at the maximum power point, not the battery charging voltage.
The power input to the battery is
75×0.95=71.25(W)
So using a 14V battery, the charging current is
71.25/14=5.09(A)
If the MPPT is used instead of a standard controller, the battery will get an additional charge current of 0.64A.
| model | Battery voltage (DC)/V | Input voltage range/V | Maximum battery current (DC)/A | Maximum PV array power/W | Maximum load current/A |
| STECA Solarix MMP2010 | 12、24 | 17~100 | 20 | 250、500 | 10 |
| Phocus MMPT 100/20 -1 | 12、24 | ≤95 | 20 | 300、600 | 10 |
| Morningstar SS-MPPT-15 | 12、24 | ≤75 | 15 | 200、400 | 15 |
| Outback Flex Max 80 | 12、24、36、48、60 | ≤150 | 80 | 1250(12)、2550(24)、5000(48)、7500(60) | |
| Outback Flex Max60 | 12、24、36、48、60 | ≤150 | 60 | 900(12)、1800(24)、3600(48)、4500(60) |
While MPPT equipment is available from some small manufacturers, it has only become common in the last few years. There are now several manufacturers making MPPTs, including Morningstar, Steca, Outback and Phocus. Note that some controllers can allow components in series with voltages in excess of 120V (DC), but this level of voltage is lethal and should only be installed and serviced by a qualified electrician. Table 1 presents some MPPT-based switch-mode controller parameters.