Solar Regulators and Controllers
The most important function of solar regulators and controllers is to sense when the batteries are fully charged (invariably done by measuring the voltage of the batteries), and disconnect the power source (e.g. solar panels or a wind turbine), when the batteries are full.
It sounds simple, but in fact keeping lead-acid batteries operating at their maximum potential is quite complicated. They don’t like being discharged (sulphate forms on the lead plates) and they don’t like being overcharged (they warp and lose water). However, over time, because they are made up of six separate cells per 12V battery, some cells begin to lose more charge than others, and so an occasional brief overcharging, an ‘equalizing charge’, can be used to bring them back in line. Thus the more sophisticated charge controllers do far more than merely act as as an on/off switch to the power source.
Basic Solar Regulators
The simplest ‘on-off’ solar regulators feed the full charge of the power source into the batteries, while measuring the battery voltage, and switch off the charge from the solar panels/wind turbine at a predetermined voltage level. When the voltage drops below another, preset level, it reconnects the charging source. However, the battery won’t actually be fully charged when the disconnection occurs: with the full current from the charging source running into the battery, the battery voltage is raised artificially high, so regulators of this type typically only achieve about 90% capacity before disconnection. Also, because the battery voltage is allowed to drop before reconnection, the battery may discharge to just 70% capacity before the charging source is switched back on. In brief, if you use a simple, cheap, regulator, don’t expect your battery to be better than 80% full on average.
While it is fine for a charge controller to feed the full current of a power source into a well-discharged battery, it is actually better not to do so, because the battery reaches capacity, but to start to taper the charge current as the voltage reaches the fully charged threshold. This gentle ‘trickle charge’ towards the maximum battery voltage will be far more effective at filling the last 10% of battery capacity. The tapering is normally done by ‘pulse-width-modulation’, which is a fancy term for switching the charge current on and off very rapidly (several thousand times a second), thus gradually lengthening the period of the ‘off’ pulse and decreasing the ‘on’ pulse as the battery fills up.
More advanced Solar Regulators and Controllers
After a long period of charging, some advanced solar regulators will determine that the battery is completely full, and reduce the voltage to a ‘float’ level that will keep the battery at the optimum voltage to maintain charge. Every so often they will allow the voltage to rise briefly, to give an equalizing charge. This is normally only available on very high-end controllers, such as the ProStar solar models.
There are two ways that regulators spill the excess energy once batteries are fully charged. One is by simply disconnecting the charge source. This is fine for solar panels, but not necessarily the best method for wind turbines: In a high wind, disconnecting the turbine allows it to freewheel, when it can spin dangerously fast. So the alternative is to feed the excess electricity into a load – normally just a big resistor – which keeps the turbine working, and slows it down. These are known as ‘shunt’ regulators. As a bonus, you may be able to arrange the system so that the heat produced by the resistors goes to heating up water, or some other use, rather than simply being wasted.
This is one reason why it is always best to use regulators that are designed for wind turbines, for wind turbines, and regulators that are designed for solar panels, for solar panels. Wind turbines can also produce AC electricity, and although this is generally rectified to DC before being fed to the regulator, it may not be very smooth, so it is better to use a regulator designed for wind turbines that will be built to cope with the input.
Maximum Power Point Tracking
Maximum power point tracking solar regulators and controllers are sophisticated charge regulators that can deliver considerably more power into your battery bank than conventional charge controllers.
Solar cells work most efficiently at a particular voltage, which changes slightly as the amount of light on the panel changes. Conventional charge controllers work at a voltage which is determined simply by the state of charge of the battery bank which is connected to the panel – but that is not always the voltage at which the panel would be giving it’s maximum power output.
By employing a control circuit to search for the voltage which gives the maximum power output, and a voltage convertor, an MPPT controller can be drawing power from the solar panels at the most efficient voltage for them, but delivering it to the batteries at a different voltage. For example, in the diagram below, the Maximum Power Point for the solar panel occurs at around 17 volts, but if connected to a fairly flat battery, a conventional solar controller would pull the solar panel voltage down to around 12V. An MPPT controller could keep the panel operating at the Maximum Power Point of 17V.
The result is more efficient transfer of power to your battery – often about 25% more delivered energy! Although MPPT trackers are more expensive than conventional solar regulators, they have come down in price significantly in recent years, and will soon repay their purchase price on larger arrays. The benefits of MPPT regulators are greatest during cold weather, on cloudy or hazy days or when the battery is deeply discharged.
Another helpful feature of MPPT trackers is that they can use a solar panel which has a nominal rating of 24V or more to charge a 12V battery.
The right solar regulator for you
Different types of battery (sealed; gel; flooded) have different optimum regulation voltages, so it may be better to get a controller that allows for this selection. Some controllers have low voltage disconnects built in, to allow loads to be switched off if battery voltages get too low – undercharging being in general, even worse for lead acid batteries than overcharging.
Most battery charging systems are 12V, and so most of the regulators we sell are rated for 12V. To calculate the current capacity, divide the panel or wind turbine power rating (in Watts) by the battery voltage (usually 12). So a 120W solar panel would need a 10A regulator, and a 400W wind turbine would need a controller of a least 33A capacity.