If the battery is overcharged, or if the voltage is too high by continuing to charge at a high charging voltage, the lead-acid battery will be damaged. Therefore, it is necessary to connect a solar charge controller between the solar panel and the battery. When the battery is charged from a low state, it will effectively connect the solar panel to the battery until a certain preset voltage is reached. For a 24 volt system, this voltage may be closer to 30 volts.
A fully charged 24-volt battery may be about 25.5 volts, but this is for batteries that have been rested, not batteries that are being charged. Therefore, although the controller may disconnect the solar panel before the voltage reaches 30 volts to avoid battery damage, the battery is unlikely to be fully charged at this point. The controller will be connected to charge in a pulse mode to fully charge the battery.
For gel-type lead-acid batteries, the charging voltage should be low, and the battery should also be low at higher temperatures. The charge controller can be programmed for a specific type of battery and can also have facilities for a battery temperature sensor.
The complexity of the charge controller varies, but the main choices are the system voltage (although the controller may be designed for multiple voltages) and the maximum charge current (the maximum current generated by the solar panel).
The solar charge controller can also have a controlled DC output. This is used in systems without solar inverters and will cut off DC power when the battery voltage is too low. The facility is unlikely to provide the current required by the solar inverter, and since the solar inverter has its controls, it can be connected directly to the battery.
The simplest type of solar charge controller will stop charging when the battery voltage reaches a certain level and restart charging when the voltage drops. However, most controllers will use pulse width modulation (short pulses of different durations) to continue charging the battery to near maximum after a certain voltage has been reached. A further development of the solar charge controller is the maximum power point tracking controller, which improves efficiency.
The basic solar charge controller performs only the necessary functions to ensure that the battery is not damaged by overcharging, effectively cutting off the current of the photovoltaic panel (or reducing it to a pulse) when the battery voltage reaches a certain level. The maximum power point tracker controller performs additional functions to increase system efficiency.
The loss of efficiency in the basic system is caused by a mismatch between the voltage generated by the photovoltaic panel and the voltage required to charge the battery under certain conditions. A 24-volt battery requires about 28 volts to be fully charged. When the battery being charged is in a fairly low state, its voltage (when charging) may only be 24 volts.
Photovoltaic panels need to be able to charge the battery on a clear day, so 24-volt photovoltaic panels generate at least 24 volts in these conditions. Hovering in bright sunlight, these solar panels could be cables that generate 40 volts. They may produce rated output power (volts x amps) at 32-34 volts.
When the battery voltage is 24 volts, it will pull the panel voltage down to 24 volts (assuming there is no voltage drop in the cable). This results in the output of the panel being significantly lower than its rated output, and therefore a loss of efficiency.
In addition to performing the functions of the basic controller, the MPPT controller includes a DC voltage converter that converts the voltage from the panel to the voltage required by the battery with little power loss. In other words, they try to keep the panel voltage at the maximum power point while meeting the different voltage requirements of the battery. In addition, a 24-volt system with an MPPT charge controller can connect the panels in series to produce 48 volts, maintaining the ability to provide some charging current in dim conditions where the standard system cannot provide any charging.