PWM or MPPT? Choosing the right regulator for off-
Charge controllers, or regulators, are an important part of any renewable energy, battery-charging system. There are two different types that you may have heard of: Pulse Width Modulation (PWM) controllers, and Maximum Power Point Tracking (MPPT) controllers. The following article will discuss the safety aspects of using a regulator, brief explanation of how they operate, and the differences between the types of controller.
Although lead-acid batteries like to be kept fully charged, they don't like being overcharged: feeding excess electricity into them can cause them to overheat and warp the lead plates, and will electrolyse the water in the acid, gradually boiling them dry and producing potentially dangerous hydrogen gas.The most important function of a charge controller 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 a science. 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. There are a bewildering variety of regulators out there, so let's look at what makes the difference between a good one and a passable one.
Keeping things simple - PWM controllers
The simplest 'on-off' 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. These are called PWM solar controllers and are often cheaper than their MPPT cousins. 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.
A little more sophistication
However, it is possible to be even more sophisticated than that: After a long period of charging, some advanced 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 Morningstar 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 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 (MPPT)
If you are feeling really rich, and have a big enough battery bank to make it worthwhile, there are a few Maximum Power Point Tracker regulators on the market now. The clever bit about these, is not the battery charging side, but how they draw electricity from the power source. The voltage that solar panels give out, and the current you can draw from them, depends on the load they are connected to. Power = voltage x current, so by finding the optimum load, you can maximize the amount of power drawn from the panels. This isn't necessarily the same voltage that is wanted to feed the batteries however, so the regulators convert whatever voltage they have decided is best to draw from the panel, into the correct voltage for charging the batteries.
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 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 regulator for you
There are a few other little features you might like to look for when selecting the right 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.
Other than that, the only other thing you need to get right is the current and voltage rating! 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.