Solar electric power systems are essentially very simple. You have a device which converts sunlight into electrical energy (a photovoltaic panel or 'solar panel') and this is used to charge up a battery. The battery allows you to collect energy when the sun is shining and store it until needed.
This simple picture is complicated by the limitations of the battery. If you leave the photovoltaic panel connected to a battery, the battery will slowly charge up. If you are putting in more energy than you are taking out, then the battery will eventually become fully charged. If this continues, the battery will become overcharged and be damaged by corrosion of it's plates and loss of electrolyte. To avoid this, some sort of charge regulator is used to stop the charging when the battery is full.
A charge regulator is an electronic switch which senses when the battery is full and either disconnects the panel from the battery or diverts the energy away from it. Most modern regulators are of the type which disconnect the panel with a switch in series with it. These are called series regulators.
Almost all regulators use the battery's terminal voltage to determine how fully charged it is. As a battery gets charged up, its terminal voltage increases. Single stage regulators simply allow the battery voltage to increase until it reaches a set limit (in the range 14.5-15 Volts for a 12V lead acid battery) and then turn the charge current off until the battery voltage drops below a lower limit. The charge current is then turned back on and the cycle repeats.
Two stage regulators improve on this by allowing the voltage to rise to a high voltage initially (typically 15V) and then change to a second mode which maintains the battery voltage at a lower level. The first stage is referred to as a boost charge and the second as floating the battery.
Both these single and two stage regulators use only the immediate battery voltage to determine battery state of charge. They make a decision about the state of charge based on only one piece of information - the battery voltage at the time. Because they rely on this scanty information, they are easily fooled and will make the wrong decision in various circumstances. They are really quite simple regulators, having little built in intelligence.
Problems with simple regulators
Based on our experience manufacturing these regulators over the last ten years, we have observed that simple regulators tend to make the following mistakes:
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- They undercharge batteries with high internal resistance (due to sulphation or bad connections).
- They can boost charge a battery too often if an occasional heavy load is applied.
- Under some conditions the charge current can oscillate rapidly on and off causing annoying flickering if any lighting or other voltage sensitive equipment is connected.
- This fast switching can cause annoying radio interference.
- They waste energy in the afternoon when the batteries are full.
- Voltage transients due to inverters or large inductive loads such as motors can cause incorrect changes in regulator state.
Imagine a human battery technician was controlling the charging. An experienced operator would not make a decision about battery condition based on only one piece of information (ie. the current battery voltage). The operator would ask such questions as how much was the battery used last night, how quickly did the battery voltage rise, how much charging has taken place today, are the users on holiday, how many days since I last did a boost charge and so on. Having put all the information together, an intelligent decision could then be made. Can we distill the intelligence of a good operator and build it into a regulator? This is what we have tried to do with our Smart regulator range.
Our aim was to find a simple, reliable and inexpensive way to implement intelligent care of the battery.
We decided to base the design around a small microcontroller integrated circuit. This single chip microprocessor controls all the operations of the regulator. It allows all the pieces of information about the battery state and charge history to be easily gathered, stored and interpreted. The controller is programmed with a sophisticated set of rules to help it to decide what to do.