Because a standard 12v solar module operates at about 17vdc, a charge controller is needed to protect the battery from over-charging. Early charge controllers were shunt-type – they simply disconnected the module when the battery voltage reached a pre-set level. This keeps the battery from over-charging, but it doesn’t get it charged either! Those controllers can only get a battery about 80% charged because they lack an absorption cycle during which the battery can slowly absorb more energy.
Today’s good charge controllers feature staged charging systems which put the battery through a bulk, absorption and float cycle to get the battery completely “full”. In the bulk stage, current is held constant while voltage is allowed to rise. When the voltage has reached a “fully charged” level, then voltage is held constant and current is trickled into the battery until it can no longer absorb more energy. At this point, the controller “floats” the battery, charging it enough to counteract a battery’s tendency to self-discharge.
Most charge controllers will also provide load management. DC loads can be connected to the controller and it will monitor battery voltage, disconnecting when the battery drops below a pre-set level. We call this low voltage disconnect, or LVD. Some controllers have these LVD values pre-set and some can be adjusted by the user. Remember that inductive loads (like inverters or motors) should never be connected to the load terminals of a charge controller – they are very likely to damage the controller’s FETs. Inverters have their own LVD circuits and do not need to be connected to the charge controller in any event.
The biggest advance in charge controllers has been the arrival of maximum power point tracking (MPPT). This is a technology that was developed for grid-tie inverters, but has also been used very effectively in solar battery charging. The principle is that a standard solar module can produce more voltage than the battery can usually use. This is so that flooded batteries can be equalized even when the modules are operating in a very hot environment. For example, a 12v nominal solar module has a maximum voltage of 17 volts, while the battery has an average voltage of around 13 volts. The gap of 4 volts is lost during normal operation, which is about 99% of the time. An MPPT controller compensates for this loss by transforming that unused voltage into current and sending it to the battery. In ideal conditions, an MPPT controller can theoretically give you up to 30% more current in your battery. In Africa, we normally assume that an MPPT controller can add 10% to 15%, as modules are usually running hot.
Other features to consider in controllers include a display screen, equalization settings for flooded batteries, battery type selectors, temperature compensation (a battery can accept less current as it warms), and reverse polarity protection (in case you get confused). Also remember that only DC loads should be connected to the load ports of your controller – inverters are always connected directly to the battery.