Any autonomous power supply system containing rechargeable batteries must contain means for monitoring the charge and discharge of the batteries. Charge controllers are used in autonomous photovoltaic systems to properly charge storage batteries, to protect overcharging when the battery is charged and the solar panel is generating excess electricity. Some Solar Charge Controllers also have connectors for connecting a DC load and protect the battery from deep discharge.
The use of solar charge controller is highly recommended. The fact is that the lead-acid batteries are afraid of both deep discharge and overcharge. In the event of over-discharge, the battery life will be drastically reduced or even damaged. If the battery is charged, but the charging current continues to flow through it, then this can lead to boiling of the electrolyte and violent gas evolution (in the case of flooded batteries) or to swelling and even explosion of sealed batteries.
Alkaline batteries, although they are not afraid of deep discharge, also do not tolerate overcharging. For lithium batteries, in addition to protection against overcharging and over discharge, it is imperative to install a voltage balancing system between the elements of the series chain.
Therefore, devices are introduced into the autonomous power supply system that disconnect the load from the storage batteries if they are unacceptably discharged, and also turn off the energy source (photovoltaic battery, wind turbine, etc.) if the batteries are charged.
The discharge controller cuts off the load when the battery is unusually discharged. Usually photovoltaic solar kits are equipped with a charge-discharge controller. Never to connect the load directly to the AB bypassing the charge controller in order to get "the last portion" of the battery power. This way you can bring your AB out of standing.
Load cut-off voltages for lead-acid batteries are typically in the range of 10.5 to 11.5 V. For 12 V batteries with more than 10 hours of discharge, this means using 100% to 20% of the rated capacity. With faster discharges, the amount of withdrawn capacity decreases.
The cut-off voltage of the power source is usually 14-14.3 V. This prevents gassing when charging the batteries. There are charge controllers that provide an "equalize" mode. This mode is necessary periodically for flooded batteries, the charge voltage should be about 15 V. For sealed batteries, this mode is prohibited.
Often the cut-off voltages can be adjusted during manufacture or customization. But, in general, charge controllers are sold with "typical" cut-off voltage levels already set and can have built in power inverter or uninterruptible power supplies.
Calculating the state of charge of the battery
The controllers also differ in the control algorithm. Most controllers provide regulation by voltage, or by the state of charge of the battery ( SOC - state of charge). SOC can only be read by advanced controllers. Many inexpensive controllers that display the degree of charge of the BATTERY in%, in fact, cannot calculate the SOC and give an approximate figure depending on the voltage on the BATTERY and, at best, its rate of change. SOC regulation is believed to provide better battery performance and longer battery life.