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Charge Controllers - Why do you Need Them

Solar Charge Controllers

Lead Acid batteries will be damaged if overcharged, or their voltage pushed too high by continuing to charge with a high charge voltage.

Image of a typical controller

Therefore it will be necessary to connect a controller between the solar panels and the battery. The photograph to the right shows a typical controller for use in solar power systems.

When charging from the battery from a low state, it will effectively connect to the solar panels to the battery until a certain pre-set voltage is achieved. This voltage may be something approaching 30 volts for a 24 system.
The graph here shows that a fully charged 24volt battery may be around 25.5 volts, but this is for a battery that has been rested, not one that is being charged.
Therefore, although the controller may disconnect the solar panels before a voltage of 30 volts is reached in order to avoid battery damage, the batteries are unlikely to be fully charged at this point.
For this reason the controller will then connect the charge in pulses, to fully charge the battery.
Charging voltages should be lower for gel type lead acid batteries, and should also be lower for batteries at a higher temperature. A charge controller may be able to be programmed for the particular type of battery and may also have the facility for a battery temperature sensor.

Types of Charge Controller

Charge controllers will vary in their sophistication but the main choices to be made are system voltage (though the controller may be designed for more than one voltage) and the maximum charge current (maximum current to be produced by the solar panels).

Charge controllers may also have a controlled DC output. This may be of use in a system without an inverter and will cut off the DC supply when the battery voltage is too low.
This facility is unlikely to be able to supply the current required for an inverter and as an inverter would have it's own control, would not be neccessary - an inverter would be connected direct to the battery.

The simplest type of charge controller would simply stop charging when the battery voltage reaches a certain level, and start again when the voltage drops. Most controllers however will use Pulse Width Modulation (short pulses of varying duration) to continue charging a battery to near it's maximum, after a certain voltage has been reached.

A further development of the charge controller is the Maximum Power Point Tracker Controller which gives improved efficiency.