How To Choose The Best Solar Charge Controller

solar charge controllerHome solar inverter systems, as well as some PV hybrid systems, are controlled by a central charge controller that regulates overall energy flow. Therefore, the solar charge controller is a critical component that should be selected with care.

The solar charge controller is about 3 to 5% of the whole solar system cost. But investing in a quality controller ensures good battery management which extends battery life and reduces associated costs.

This solar charge controller performs crucial system functions such as low voltage disconnection (LVD) to protect the battery from deep discharge, and high voltage disconnection (HVD) to protect the battery from overcharging. A quality solar charge controller should have a good battery state of charge calculation (SOC) to monitor the battery status. Good battery management can be applied with the help of such functions.

Quality solar charge controllers have a low self consumption (< 4 mA) and come within a robust case with large connection terminals. In addition, quality controllers must have a user friendly display indicating all system values. For instance, symbols or icons may indicate the SOC, charging or load supply. This enables the user to easily understand the system, regardless of language.

Shunt Controllers
The most efficient and wide spread topology used in solar charge controllers is the shunt controller. A pulse-width modulated short circuit is applied to the solar module to regulate the energy flow. Due to the high inner resistance of solar modules, this can be done without any negative effects.

Serial Controllers
Serial controllers disconnect the battery from the solar module to regulate the system. Two antiseriell MosFets are used to prevent currents in both directions. If the circuit is open, the module is driven in open circuit mode and module current is zero.

DC-DC Converter
Controllers with integrated DC-DC converters can apply a different voltage to the controller input than to the controller output. This can be used to track the module voltage independently from the battery voltage and leads to a higher flexibility concerning input voltage range. Such controllers can charge e.g. a 12V battery not only from a 36-cell module but also from thin film modules and other crystalline modules with more than 36 cells. Also maximum power point algorithms (MPPT) can be applied to such controllers which allow to drive the solar module always in the point of maximum power supply.

Topology Overview Application Advice
Shunt controllers, serial controllers and MPP trackers dominate the world market. Below is a comparison among all three.

Efficiency is a crucial issue in all PV battery systems. Shunt controllers show the highest charging efficiency, so they are good for all standard systems. Shunt controllers are also appropriate for telecom applications due to their good EMC performance. If the battery is also charged from a fixed voltage source a serial controller is a good choice. In regions with low irradiation and temperature, an MPP tracker can bring more energy to the battery than other controllers. If the number of cells within the solar module does not fit to the battery voltage or thin-film modules are used then an MPP tracker is necessary.

By: Michael Müller, Director of Off-Grid PV at Steca Solar