Charge Controllers

What is a charge controller, and do I need one.

The primary purpose of a charge controller is to prevent battery overcharge. For all but the smallest of systems, a charge controller is a very important piece of equipment. Unless your charge sources are quite small, or your battery bank is very large, you need a charge controller. You may elect not to use a controller if your incoming energy is in the range of 2 watts / 50 amp-hours of battery capacity. Basically this is just a trickle charge for maintenance purposes (perhaps maintenance of a set of emergency backup batteries). Generally speaking, you need a charge controller or you can expect to replace your batteries often as you risk the constant chance of them being overcharged.

Second in importance for the charge controller is to perform additional charging algorithms to help insure battery longevity and health. These types of charging modes include but are not limited to, bulk, absorption, float, equalize. . Many if not most solid state controllers are capable of these types of charging modes.

Third in importance of the controller is to maximize the energy your charge sources are creating and matching them to your batteries. This will be discussed in detail below.

For the most part charge controllers fall into the following categories. We will present them in order of complexity and capability.

1 or 2 stage controllers
3 stage or PWM controllers
MPPT (Maximum-Power point tracking) controllers

1 or 2 Stage controllers. Also referred to single or dual voltage charge/diversion controllers. These controllers are by far the simplest in design as they generally rely on a relay or set of relays to disconnect or divert an energy source or load when the battery voltage reaches a certain point. They can range from a few amps to hundreds of amps capacity and are often limited only by the capacity of the relays they are switching. They do not offer battery maintenance charging (such as equalize charging) as they simply monitor the battery voltage and “trip” at a certain level. They can be very economical and very reliable as they are simpler in nature and therefor have fewer components to break. Newer model single stage controllers offer microprocessor controlled circuitry allowing for more precise and stable operation.

Who should consider this type of controller:

You have or will have a wind or hydro energy source. This type of controller can be wired as a “Diversion Controller”. In this mode, when the batteries reach their high voltage trip point, incoming energy is diverted away from the batteries to a diversion or dummy load, while allowing the charge source (wind turbine) to stay fully loaded. It is very important that wind turbines remain under load, particularly in high wind events or there is a real chance they will over-speed and be destroyed. Although some PWM controllers can also be used in a diversion mode, when they are utilized in this fashion, they, for all practical purposes have been set to mimic a single stage controller. If you have a wind turbine, it must have a load on it at all times, so all of the other technologies cannot be fully incorporated for a wind turbine. There are indications that some of the larger controller/inverter manufactures are working on more advanced MPPT controllers for wind energy, but at this time there really is nothing better (in my opinion), for wind energy then a single stage, diversion controller.
You have a solar only system but your budget does not allow you to purchase a more advanced technology controller. Single stage controllers are often very economical and generally offer much more capacity per dollar then PWM or MPPT controllers. Keep in mind, this type of controller does not offer the level of battery maintenance charging that the PWM/MPPT controllers do.
You have a more advanced controller for normal conditions, but you also have a wind generator and need to have an insurance policy against overcharge in high wind conditions. In this situation, your solar panels are routed though your PWM/MPPT controller. Under nominal conditions, the turbines power is seen by the solar charge controller (due to and increase battery voltage) and and when necessary the solar energy that is allowed to flow into your batteries is reduced or cut off as required; however, the turbine is allowed to stay fully loaded. This is a very good compliment of controllers and works well, even in the largest of solar systems.
You have a backup generator and want to insure that when it is running, your batteries remain fully protected from overcharge. Most inverters designed specifically for alternate energy systems have a generator start /stop capability. If this is the case, the inverter itself is the charge controller, and you will not need this additional protection from a single stage controller. However, if you have a generator that is manually started and not controlled by the inverter, then using a single state charge controller to insure against battery overcharge is a good choice.

Who should NOT consider this type of controller:

You do not have a wind or hydro turbine and have a sufficient budget to purchase a more advanced controller.
All of your charge sources can be routed through your PWM/MPPT controller or otherwise controlled.

Multistage, 3 stage or PWM controllers.

PWM (Pulse Width Modulated) controllers are solid state controllers that offer more advanced charging algorithms than single stage controllers are capable of. First lets talk about the PWM aspect. As the battery voltage increases, the charge controllers primary responsibility is to restrict the incoming voltage (and current) to insure the battery does not over charge. The simplest way to do this would be to open the circuit (turn off the internal switch), when the battery gets “full”, but of course as soon as you do this, the battery voltage will start to drop and the controller will need to turn the switch on again. This on and off switching, then would attempt to keep the batteries at a full charge state and everything would be fine. Turning on and off repeatedly is no problem for a solid state device, but it really is not that simple. It is better if the charge voltage/current to the battery varies based on its state of charge and where the battery is in its overall charge cycle (Was it nearly depleted, or has it been mostly charged now for awhile and just needs to be maintained?)

There are many different philosophies used, including 3 stage charging, constant voltage and tapered charging, but for the most part, to promote better battery health, batteries are not just brought up to full charge then shut off by a multistage solid state controller. The controller examines the batteries state of charge and as it progresses through the charge cycle, reduces or increases the current as required. Once the battery charge cycle is complete, the charge controller will reduce the current even more to hold the battery at the “Full State”. So this is what the controller does, but it does it via pulses of energy that may last from less than a second to several minutes. The term pulse width modulation is really pretty simple to understand. If you had a garden house with a hand held nozzle that you can turn on and off by squeezing the trigger, you could easily control the water volume to say 50% by simply squeezing the trigger half way, or conversely, you could, by repeatedly squeezing the trigger once every two seconds for a full second on, and a full second off accomplish the same thing. Each time you squeeze the trigger it’s a pulse, the length of time you hold the trigger is the width of the pulse. Basically, if you turn on the water on every second, and off the next second, then you are letting 50% of the water that is available to pass though the nozzle. If you where to hold the trigger for 1.5 seconds on and .5 seconds off, then you would be allowing 3/4ths of the volume to pass. This is what PWM controllers do. This pulsing action is much easier on the charger (most current carrying electronic switches heat up if they are half way on, but they don’t mind at all if they are full on and full off many times a second). But there’s an even bigger reason why this is done — The batteries love it. These pulses of energy are much better on the plates of the batteries then a steady high voltage D/C charge. So PWM controllers allow an electrical pulse from the charge source to pass, that can be better for battery life then steady state charge currents. Some PWM controllers will incorporate high frequency switching. The same basic on/off cycles are used, but much much faster (Many times a second), sending very fast short burst to the batteries. These short burst of higher currents can be very effective on keeping the plates of the batteries in good health. Not all PWM controllers are the same, some use high frequency pulses, others use pulses only as the batteries reach a higher state of charge, but all PWM controllers incorporate pulses of energy of varying frequency to control the actual current to the battery.

The main advantage to solid state controllers is that they offer more advanced charging modes than single stage controllers. We will briefly discuss the three stage charging mode.

Bulk Charge
Absorption Charge
Float Charge
The 1st stage in a 3 stage charging mode is the bulk charge: In this mode, most of the available current is sent to the batteries until they reach about 80% of the capacity. Generally the bulk charge rate is set to between 13.5 to 15 volts. There is really is no perfect voltage setting here as there are many factors involved. The ambient temperature, the size of the energy sources vs battery size. The desired length of time in this mode, the cost of the energy if it is supplemented by the grid or generator, etc. — Simply stated, the bulk charge gets the battery up to a mostly full state in a quick but healthy rate. Common low cost automotive charger uses the bulk charge mode as their only charge stage. The voltage is simply set to perhaps 14 volts by the charger, and that’s that. This really is not very good for long term life of the battery.

The 2nd stage of the 3 stage charging mode is the the Absorption Charge: As the battery gets full, it, due to internal resistance accepts less charge current. During the Absorption charge, the charger will hold a slightly higher voltage to overcome this. During this stage, the current flowing into the batteries will still be less, but the voltage will be higher. Via this stage, the the battery will be brought up to its maximum capacity (Fully charged). Again, the voltage setting varies from controller to controller, but is more dependent on the battery type and temperature, then the bulk charge is. For a 12 volt lead acid battery, you would expect an absorption charge voltage of between 14.2 and 15.5 volts. During this stage, battery gassing is normal and required in order to complete the chemical reactions to obtain a fully charged battery.

The final stage is the Float Charge: This mode is the charge mode that the battery is under most of the time for a properly designed system. Once the batteries are brought to a full state of charge, the float charge mode maintains the batteries at a voltage level of about 13 to 13.2 volts (for a flooded, 12 volt lead acid battery), by applying pulses of current as required. These pulses may last less than a second or be several seconds long. By applying the required amount of charge current to offset any load the battery might be powering, as well as overcoming the batteries natural self discharge, the batteries longevity is greatly increased. Allowing a battery to set in a depleted state of charge for long periods of time significantly reduces battery life. A decent battery, kept at is proper float setting, will last many years.

Another charge mode incorporated by many chargers is the equalize charge. This mode is not a part of the normal charge cycle, but is instead initiated manually to help mix the electrolytes of the battery. During normal use, a batteries chemical mix becomes stratified. (Separated from top to bottom). An equalize charge uses an approximately 10 percent higher voltage, to help mix these elements in your battery. Equalize charging also helps bring all of the batteries in a multi-battery bank to an equal state. Most people agree that an equalize charge should be run once every 10 to 40 days, for 2 to 16 hours. During this charge cycle quite a lot of gassing will occur, which causes the fluids to be mixed and the plates to be “Cleaned”

Well made, three stage (multistage) charge controllers are recommended for solar only installations or solar and wind installations if the controller is also capable of running in a diversion mode. Multistage stage controllers are not recommended as the only charger for systems incorporating wind turbines, unless the controller can be configured as a diversion controller. Some of the more advanced 3 stage controllers can be used as either a solar (disconnect or shunt) or diversion controller. If you have a wind turbine, insure the controller you are considering is capable of running in a diversion mode, as wind turbines must have a load at all times. Some 3 stage controllers may not allow the turbine to see a full load at all times which may allow damage to occur to the turbine from over speed in high wind conditions.

MPPT Controllers: Maximum Power Point Tracking controllers.

These controllers are the most advanced controllers available on the market at this time and may offer 10 to 20 (and some test claim 30) percent more efficiency over the other controllers mentioned in this article. This efficiency does come at a heavy financial price and therefor often the debate that is raised is; it is simply better to add 20% more in solar panels and use a more traditional controller, or pay the price for the MPPT controller? We will leave that up to you. One of the first things we would like to reiterate is that MPPT controllers are designed for solar, and at the time of this writing, there does not seem to be a MPPT controller designed specifically for wind power; although, there is talk that some of the larger controller manufactures are looking into a MPPT controller for wind.

So how do MPPT controllers work. In order to charge a battery (any battery), the charge voltage must exceed the battery voltage. If the charge voltage is equal to or less than the battery voltage, there will be no positive current flow and the battery will either not charge or be slowly drained (if no blocking diode is present). Since a standard 12 volt battery (lead acid), requires a charge voltage of around 15 volts (to finish the charge process), the solar panel must have an output of least 15 volts.

There are a couple of factors however, that must also be considered.

The rated output of a solar panel is generally measured at 25 degrees Celsius. All solar panels are more efficient when it is cooler, therefor on cold sunny days, the panel will put out more power (watts/hour) than on a hot sunny day. Due to this fact, solar panels must therefor put out more voltage to accommodate the lower voltage output that will occur on hot days.
There a multiple loses that are suffered from the solar panel to the battery. These include resistance in the wire, as well as voltage drops across blocking diodes and connectors.
Semi-cloudy or hazy days can reduce voltage output.. This is also considered to some degree to allow charging in less than perfect sun.
All of these factors require the solar panels start out with a open circuit voltage of at least 18 volts, and perhaps as high as 24 volts, with a maximum power voltage generally in the range of 17 to 18 volts. This assures that after any and all voltage drops occur, there will still be a high enough voltage at the battery to allow for a positive charge current. The maximum power point of a panel is measured in watts; (remember watts = volts x amps), this point is where product of the voltage and amperage of the solar panel is at the highest point (producing the most amount of useable power). The problem is that as soon as you hook up the solar panel to the battery, the battery “Drags down” the voltage at the solar panel. Immediately, the solar panel is below its maximum power point (unless it is a very hot day, there is considerable voltage loss in the copper and connections, and the battery is mostly charged.). More often than not, the solar panels are operating below their maximum capability. This is especially exasperated when the batteries are low or under a large load (perhaps measuring 12 volts or lower), this drags the solar panel voltage down even lower, causing a great deal of power to be lost due to the mismatch of the battery voltage and the solar panels maximum power point..

The solution which is well provided for by MPPT controllers, is to prevent the solar panel voltage from dropping so dramatically though the use of very sophisticated electronics which include transformers and highly intelligent tracking algorithms that monitor both the solar panel voltage as well as the battery voltage. The MPPT controller basically downshifts the incoming voltage from the solar panel by converting the D/C current into an A/C current, reducing the A/C voltage via a variable transformer, then rectifying it back to D/C at a voltage level that properly matches the best charge voltage for the battery. The solar panel voltage is dropped somewhat (this is unavoidable, since any current draw will cause both loss and reduction in available voltage), but the drop at the panels, is considerably less than in conventional controllers. Basically, the MPPT controller converts excess voltage to amperage. The point that allows the most amount charge current is determined by the microprocessor in the controller and is continuously updated (tracked.). The controller constantly tracks this point and responds as required, thus the name, Maximum Power Point Tracking.

MPPT controllers are recommended for solar systems in cold climates as well as in environments where you find you are drawing down your batteries due to constant loading, and you do not wish to add additional solar panels for whatever reason. The greater the disparity between the battery voltage and the solar panel’s maximum power point, the more efficient MPPT controller are. MPPT controllers are not recommended for use with wind turbines; however, they may be used in systems that include a diversion controller for the wind/hydro energy, and a MPPT controller for the solar energy.

Conclusion: If you have a wind based system, then you will want to consider either the very economical relay based single stage controllers, or the more advanced PWM controller designed for both wind and/or wind/solar. Solar only systems should if at all possible consider PWM controllers and if required by your pannels a MPPT controller (some panels have a very high VOC and simply will not perform well without a MPPT controller). PWM controllers are by far the most popular and prevelant controllers being used today for alternate energy systems.